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Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

The genetic and molecular basis of biofilm formation in staphylococci is multifaceted. The ability to form a biofilm affords at least two properties: the adherence of cells to a surface and accumulation to form multilayered cell clusters. A trademark is the production of the slime substance PIA, a polysaccharide composed of β-1,6-linked N-acetylglucosamines with partly deacetylated residues, in which the cells are embedded and protected against the host’s immune defence and antibiotic treatment. Mutations in the corresponding biosynthesis genes (ica operon) lead to a pleiotropic phenotype; the cells are biofilm and haemagglutination negative, less virulent and less adhesive on hydrophilic surfaces. ica expression is modulated by various environmental conditions, appears to be controlled by SigB and can be turned on and off by insertion sequence (IS) elements. A number of biofilm-negative mutants have been isolated in which polysaccharide intercellular adhesin (PIA) production appears to be unaffected. Two of the characterized mutants are affected in the major autolysin (atlE) and in D-alanine esterification of teichoic acids (dltA). Proteins have been identified that are also involved in biofilm formation, such as the accumulation-associated protein (AAP), the clumping factor A (ClfA), the staphylococcal surface protein (SSP1) and the biofilm-associated protein (Bap). Concepts for the prevention of obstinate polymer-associated infections include the search for new anti-infectives active in biofilms and new biocompatible materials that complicate biofilm formation and the development of vaccines.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

In numerous reports from the past two decades, it has been shown that especially biofilm-forming staphylococci, with Staphylococcus epidermidis as a leading species, cause a new infection that is best described as ‘chronic polymer-associated infection’. The skin bacterium S. epidermidis, once considered harmless, is now known to be an opportunistic pathogen and very frequently infects catheter materials. Scanning electron micrographs from early studies show that S. epidermidis forms multiple cell layers on the polymer surface; the cells in these layers are enveloped and protected by an amorphous slimy material (Peters et al., 1981; Christensen et al., 1982; Marrie et al., 1982). The slime substance is not a true capsule, but is loosely bound to the staphylococcal cells. In most natural environments, a biofilm consists of a multispecies microbial community. In contrast, with biomaterial-associated infections, nearly 80% of the cells involved are S. epidermidis. This phenomenon can be explained by the easy access of this skin inhabitant to wounds and implants (von Eiff et al., 1999).

Biofilms are formed on nearly all kinds of catheters and on many other medical devices and implants (reviewed by Götz and Peters, 2000). Microscopy of biofilm formation in vitro suggests that two steps are involved: (i) the attachment of the bacterial cells to the polymer surface, which may occur very rapidly; and (ii) the growth- dependent accumulation to form multilayered cell clusters surrounded by a slimy matrix.

The slimy material is eye-catching when bacteria are examined by microscopy and, therefore, is believed to play a crucial role in polymer-associated infections. The chemical composition of slime has been investigated intensively. Earlier studies had the disadvantage that the slime material was not purified and was contaminated with medium-derived sugar compounds (e.g. from agar or yeast extract), which led to false results and artifacts (Drewry et al., 1990). The slime substance purified from S. epidermidis RP62A (ATCC 35984), galactose-rich capsular polysaccharide adhesin (PS/A), has, in contrast to teichoic acid, some protective activity when used as a vaccine (Kojima et al., 1990). Later, a slime-associated antigen (SAA) mainly composed of N-acetylglucosamine was isolated from the same strain (Baldassarri et al., 1996). The strain also excretes considerable amounts of teichoic acids (Hussain et al., 1993), which may contribute to the highly viscous appearance of the colonies and the strong biofilm formation of this strain. Up to 1996, reports on the chemical composition of ‘slime’ were conflicting; the most reliable work was carried out by Mack et al. (1996a), who purified a specific polysaccharide antigen of the biofilm-producing S. epidermidis strains 1457 and RP62A and were able to distinguish two polysac- charide fractions. Polysaccharide I (>80%) is a linear homoglycan of at least 130 residues of β-1,6-linked N-acetylglucosamine, 15–20% of which are deacetylated and therefore positively charged. Polysaccharide II (<20%) is structurally related to polysaccharide I, but has a lower content of non-N-acetylated D-glucosaminyl residues and contains phosphate and ester-linked succinate, rendering it anionic. The structure of this polysaccharide is unique (Fig. 1) and, according to its function in intercellular aggregation, it was referred to as polysaccharide intercellular adhesin (PIA). In retrospect, it appears that the above-mentioned SAA is most likely PIA.

image

Figure 1. Structure of polysaccharide intercellular adhesin (PIA). The polysaccharide is a linear homoglycan composed of β-1,6-linked N- acetylglucosamine residues; ≈ 20% of the residues are deacetylated and are thus positively charged. Adapted from Mack et al. (1996a).

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Recently, a PIA-related substance has been de- scribed from Staphylococcus aureus MN8N. The N-acetylglucosamine residues of PNSG are completely succinylated, which led to its designation as poly N-succinyl β-1,6-glucosamine (PNSG) (McKenney et al., 1999). It is now clear that the succinyl groups were an artifact (G. Pier, personal communication). Indeed, the existence of PNSG would contradict the findings that antibodies against PNSG and against PIA are completely cross-reactive and that only N-acetylglucosamine oligomers and no succinylated residues are found after in vitro synthesis with extracts of Staphylococcus carnosus clones expressing various ica genes (Gerke et al., 1998).

Biological activity of PIA

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

In a collection of 179 S. epidermidis isolates, 51% produce PIA, and most of these strains form a biofilm (Mack et al., 1996b). As the core polymer of PIA has a β-1,6-linked N-acetylglucosamine structure, we tested whether biofilm-positive strains could be distinguished from biofilm-negative strains using a phosphatase-labelled wheat germ agglutinin (WGA) as a specific probe for the (GlcNAcβ-1,4)n. This enzyme-linked lectinsorbent assay (ELLA) does not prove the presence of PIA, but is useful as an initial screening assay (Thomas et al., 1997). Other detection methods, and genetic as well as biochemical techniques used to study biofilm formation in staphylococci, have been reviewed recently (Leriche et al., 2000; Cramton et al., 2001a). PIA-positive and PIA-negative strains can also be differentiated on Congo red agar (Heilmann and Götz, 1998); colonies of PIA-positive strains are black, whereas colonies of PIA-negative strains are red.

Haemagglutination of erythrocytes, a common prop-erty of S. epidermidis strains, may play a role in the pathogenesis of biomaterial-associated infections. PIA is somehow involved in haemagglutination; PIA-negative mutants (interruption or deletion of the ica genes) have a haemagglutination-negative phenotype (Fey et al., 1999; Mack et al., 1999). However, PIA alone does not mediate haemagglutination.

PIA also acts as an intercellular adhesin, and there are hints that PIA acts as an adhesin on glass and prob-ably other hydrophilic surfaces (see below). The role of PIA in phagocytosis and host defence is currently being investigated.

PIA biosynthesis (ica genes)

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

The genes responsible for these various phenotypes were identified by isolating and analysing biofilm-negative transposon–insertion mutants of S. epidermidis O-47. Two classes of mutants were isolated. One is defective in attachment to polystyrene, and the other is defective in slime production and is unable to accumulate in multi-layered cell clusters (Heilmann et al., 1996a). For the first time, the two postulated steps in biofilm formation were genetically verified. The accumulation-defective mutants are unable to form a biofilm on both polystyrene and glass surfaces, display no intercellular aggregation and do not produce PIA. The transposon had integrated at various sites in an operon, which we named ica for intercellular adhesion (Heilmann et al., 1996b). The operon is composed of the icaR (regulatory) gene and icaADBC (biosynthesis) genes. With PIA-specific antibodies and immunofluorescence studies, PIA was localized mainly on the cell surface. The ica genes cloned into the heterologous cloning host S. carnosus, which is apathogenic, lacks ica-related genes and is therefore unable to form a biofilm, produced interesting results: massive cell aggregation, formation of a biofilm on a glass surface and PIA production. Surprisingly, the S. carnosus clones hardly form a biofilm on polystyrene plates, which indicates that adherence of S. carnosus to polystyrene is weak, whereas adherence to glass is very pronounced. Although proof is lacking, we think that PIA itself mediates adherence to hydrophilic surfaces.

There are several pieces of evidence that icaR is a repressor. An internal deletion in icaR in the chromo- some of S. epidermidis O-47 augments PIA expression. By comparing S. carnosus PIA production in the presence or absence of icaR, again PIA production is lower in the presence of icaR. icaA promoter-directed reporter gene expression revealed that heterologously expressed icaR represses reporter gene expression (C. Gerke and F. Götz, unpublished). The exact IcaR-specific operator site has not yet been determined.

Functional analysis of the IcaABCD proteins

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

Our knowledge of the function of the IcaABCD proteins is only lowly. All encoding genes have been cloned individually or in combination in S. carnosus, and the in vitro biosynthesis of PIA has been analysed using the membrane fraction or cellular or extracellular extracts (Gerke et al., 1998). IcaA, C and D are located in the membrane fraction; IcaB is mainly present in the culture supernatant. IcaA contains four transmembrane helices and has N-acetylglucosaminyl-transferase activity with UDP-N-acetylglucosamine as substrate. Certain domains of the amino acid sequence show similarity to the chitinase (NodC) of rhizobia and the hyaluronan synthase (HasA) of Streptococcus pyogenes. IcaA alone has only low transferase activity; when icaA is co-expressed with icaD, the transferase activity increases ≈ 20-fold. IcaD might be a chaperone that directs the correct folding and membrane insertion of IcaA and, in addition, might act as a link between IcaA and IcaC. To date, no IcaD-like protein has been identified among the homologous β-glycosyl transferases; thus, IcaAD represents a novel enzyme combination (Gerke et al., 1998). N-Acetylglucosamine oligomers produced by IcaAD reach a maximal length of only 20 residues; only when the icaAD genes are co-expressed with icaC are longer oligomer chains synthesized that react with PIA-specific antiserum, which does not, however, react with short oligomeric N-acetylglucosamines (Fig. 2).

image

Figure 2. Model of PIA biosynthesis. IcaA and IcaD synthesize oligomers derived from UDP-N-acetylglucosamine; synthesis comes to a halt when a length of 10–20 residues is reached. In the presence of IcaC, longer oligomers are formed that react with PIA-specific antiserum. Because of the sequence similarity with deacetylases, IcaB might catalyse the deacetylation reaction. CM, cytoplasmic membrane. Adapted from Heilmann et al. (1996a), Gerke et al. (1998) and O. Schweitzer (unpublished results).

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An undecaprenyl phosphate lipid carrier that is often involved in the biosynthesis of bacterial exopolysaccharides does not appear to participate in the activity of IcaAD – lipid-linked intermediates have never been detected in vitro or in vivo, and the antibiotics tunicamycin and bacitracin have no effect on the synthesis of IcaAD-dependent oligomers or PIA in vitro and in vivo.

In in vitro synthesis studies, no deacetylated residues have been detected by mass spectroscopic analysis. We therefore speculate that the primary product is a completely acetylated and therefore uncharged N- acetylglucosamine polymer. Deacetylation is apparently a second step that leads to the positively charged N- glucosamine residues. Although the PIA-specific deacetylase has not yet been identified, a candidate is the 33 kDa IcaB, which is secreted into the culture broth. IcaB shows sequence similarity to the Rhizobium NodB protein, which is involved in nodulation signal synthesis and is a chitooligosaccharide deacetylase (John et al., 1993).

The 42 kDa IcaC with its transmembrane helices is very likely an integral membrane protein. Inactivation of icaA or icaC in the ica gene-expressing plasmid pCN27 leads to complete loss of the intercellular adhesion phenotype of S. carnosus, which suggests that these two genes are necessary for PIA expression (Heilmann et al., 1996b). Internal frameshift and deletion mutants in either icaB or icaD led to a strong decrease in PIA expression (O. Schweitzer and F. Götz, unpublished). The function of IcaC is unknown, but we speculate that it might be in-volved in the translocation of the polysaccharide through the cytoplasmic membrane.

Adherence to polymers and extracelluar matrix

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

A transposon–insertion mutant of S. epidermidis strain O-47 has been isolated that is biofilm negative even though the ica operon is intact and PIA is produced (Heilmann et al., 1996a). Although the mutant is unable to form a biofilm on polystyrene, biofilm formation on glass is entirely unaffected or even more pronounced, which led us to speculate and then demonstrate in binding assays that this mutant is defective in primary adhesion to polystyrene (Heilmann et al., 1997). The mutant is also significantly less hydrophobic than the wild type, but the most remarkable characteristic is the formation of huge cell clusters that resist even detergent treatment, which points to covalent interlinking of the cells. Both the altered surface hydrophobicity and the massive cell clumping suggest that cell surface compounds are changed. Indeed, in comparison with the wild type, the mutant lacks five cell surface-associated proteins with masses of 120, 60, 52, 45 and 38 kDa. This pleiotropic effect results from an 8 kb deletion, including a 4005 bp open reading frame (ORF) encoding the S. epidermidis major autolysin AtlE, which accompanied transposon insertion (Heilmann et al., 1997). AtlE is a homologue of S. aureus Atl, which has been identified by Oshida et al. (1995).

AtlE (1335 amino acids) is a multidomain protein composed of an N-terminal signal peptide, a propeptide (PP), an amidase domain (AM; 60 kDa), three highly cationic repeats (R1, R2, R3) and the glucosaminidase domain (GL; 52 kDa). Apart from the signal peptide processing, AtlE is also processed by an extracellular protease at two (or more) further positions: Val-303 and Thr-846. Differently sized degradation products are generated through partial processing, but all bind to the cell surface. Complementation studies have shown that production of only the 60 kDa AM is sufficient to restore the biofilm-positive phenotype and normal cell separation completely, which indicates that the amidase really acts as a major autolysin in S. epidermidis. The three cationic repeats might be involved in the attachment of the Atl proteins to the cell surface. Certain synthetic oligopeptides with sequences derived from the S. aureus Atl repeat-1 affect autolysis (Takano et al., 2000); therefore, the AtlE repeats might exert a similar function. AtlE binds strongly to vitronectin (Heilmann et al., 1997), a 70 kDa protein found in the extracellular matrix and in serum. Vitronectin is one of the few proteins that regulates both the complement and the coagulation systems; whether AtlE functions similarly is not known.

The staphylococcal major autolysins are multifunctional. Although atl and atlE mutants can survive, cell separation after division is severely affected, leading to covalently interlinked cell clusters. Atl and AtlE have binding motifs for the cell wall and vitronectin. We do not know whether the decrease in adherence of the atlE mutant to polystyrene results from the lack of hydrophobic interaction of the 60 kDa AM with the hydrophobic polymer or from formation of cell clusters that might be less resistant to shearing during the washing procedure.

Influence of the teichoic acid structure on biofilm formation

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

In a search for S. aureus mutants hypersensitive to positively charged antimicrobial peptides, mutants with an altered teichoic acid structure have been isolated (Peschel et al., 1999). The mutations are in the dltA operon, and the teichoic acids lack D-alanine. It has been postulated that the increased negative charge of the cell surface of the mutants leads to an increased scavenging of positively charged antimicrobial peptides, thus leading to the observed hypersensitivity. The dltA mutants are also more sensitive to vancomycin and antimicrobial host defence peptides, such as defensins, protegrins and platelet microbicidal proteins produced by mammalian skin, epithelia, phagocytes and platelets in response to staphylococcal infections (Peschel et al., 2000; Peschel and Vincent Collins, 2001).

Surprisingly, the dltA mutant is also biofilm negative even though PIA production appears to be unchanged. The mutant is severely affected in adherence to polystyrene or glass surfaces (Gross et al., 2001), the first step of biofilm formation. In this respect, the dltA mutant resembles the atlE mutant described above, with one exception: the atlE mutant is still able to form a biofilm on a glass surface. Biofilm formation of the dltA mutant can be completely restored by the addition of Mg2+ ions, which is in accordance with an earlier observation that low concentrations of Mg2+ (16 μM) significantly enhance the adhesion of all tested slime-positive S. epidermidis strains to plastic, whereas Ca2+ exerts little effect (Dunne and Burd, 1992). These findings corroborate the biological importance of the charge balance of the Gram-positive cell surface.

Other proteins that contribute to biofilm formation

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

Proteins other than the major autolysin AtlE contr- ibute to biofilm formation in staphylococci, such as the accumulation-associated protein (AAP) and the biofilm-associated protein (Bap). Mitomycin mutagenesis of S. epidermidis RP62A led to the isolation of a biofilm- negative mutant, M7, which lacks a 140 kDa exoprotein (Schumacher-Perdreau et al., 1994). The 140 kDa antigen is predominantly found under sessile growth conditions, and not all biofilm-positive S. epidermidis strains produce this antigen. However, the positive strains produce significantly larger amounts of biofilm than those that lack the 140 kDa antigen. Therefore, the 140 kDa antigen is referred to as accumulation-associated protein (AAP) (Hussain et al., 1997).

An S. aureus transposon–insertion mutant has been isolated with a significant decrease in attachment to inert surfaces, intercellular adhesion and biofilm formation (Cucarella et al., 2001). The transposon is inserted in the bap gene, which encodes a novel cell wall-associated, 2276-amino-acid protein. The bap gene has been found in 5% of the S. aureus bovine mastitis isolates, but it was absent in the 75 clinical human S. aureus isolates analysed. The core region of Bap consists of 52% of the protein, with 13 successive nearly identical repeats, each containing 86 amino acids. All staphylococcal isolates harbouring bap are highly adherent and strong biofilm producers. In a mouse infection model, Bap is involved in pathogenesis, causing a persistent infection (Cucarella et al., 2001).

We have to be aware that biofilm formation does not only occur with foreign body infections. If one closely examines native tissues removed from patients with recurrent S. aureus infections (e.g. heart tissue, cartilage, chronic wounds), the cells are frequently organized in confluent colonies with a biofilm-like appearance. Therefore, not only should adherence to plastic polymers be considered, but also any bacterial factor that mediates adherence to components of the extracellular matrix of the host. S. aureus is especially capable of adhering to a large variety of matrix components to initiate coloniza-tion. This adherence is frequently mediated by protein adhesins of the MSCRAMM (microbial surface com-ponents recognizing adhesive matrix molecules) family, which in many cases are covalently anchored to the cell wall peptidoglycan (Foster and Höök, 1998). The fibronectin-binding protein A and B (FnBPA, FnBPB), the collagen-binding protein Cna and the fibrinogen-binding proteins, clumping factor A and B (ClfA and ClfB), belong to this family. ClfA and ClfB are prototypes of a recently identified multigene family of putative surface proteins characterized by a common domain composed of a unique Ser–Asp dipeptide repeat referred to as Sdr (SD repeat protein). There are other Sdr proteins (SdrC, SdrD and SdrE) whose function is unclear. Interestingly, it has been demonstrated that CflA also mediates adherence to polyethylene or polyvinylchloride tubing, whereas FnBP has no effect (Vaudaux et al., 1995). This example shows that ClfA plays a dual role in wound and foreign body infection.

In general, S. epidermidis does not have that many adherence components for the extracellular matrix. It was therefore surprising when a CflA-related fibrinogen-binding protein, Fbe (SdrG), was identified (Nilsson et al., 1998). Its contribution in biofilm has not yet been analysed.

Veenstra et al. (1996) identified a 280 kDa surface protein, SSP1, and its degradation product, SSP2. SSP1 is involved in the attachment of S. epidermidis 354 to polystyrene. Ultrastructural analysis suggests that this adhesin protrudes from the cell surface as a fimbria-like polymer. Unfortunately, the corresponding gene has not yet been identified, which would allow further analysis.

Regulation of slime production and biofilm formation

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

Environmental (stress) factors

In earlier literature, the term ‘slime’ was used to describe the mucous substance of colonies of staphylococcal strains, the slimy film coating on culture tube walls and the extracellular matrix encasing the catheter surface (Christensen et al., 1982). In retrospect, in most cases, the ‘slime’ was very likely PIA; unfortunately, the chemical composition of ‘slime’ was not or was incorrectly assessed. Only with the well-studied S. epidermidis strains RP62A and O-47 are we certain that PIA and ‘slime’ are the same. A further proof is that the expression of the ica genes in S. carnosus leads to PIA production and to the generation of colonies with a slimy mucous consistency (Heilmann and Götz, 1998).

The early observations on ‘slime’ regulation already indicated that slime (PIA) production was influenced by many parameters. In certain strains, such as S. epidermidis RP62A, slime production frequently varies (Christensen et al., 1987; Baddour et al., 1990), and Christensen et al. (1987) spoke of ‘phenotypic variation’. In recent years, many factors have been identified that influence PIA (‘slime’) or biofilm production (summarized in Table 1).

Table 1. Factors that influence biofilm formation.
FactorReference
Factors that influence PIA expression
Glucose Mack et al. (1992)
Glucosamine, N-acetylglucosamine Gerke et al. (1998)
Oleic acid Campbell et al. (1983)
Quinupristin–dalfopristin, tetracyclineRachid et al. (2000a)
Urea (in S. saprophyticus) Hjelm and Lundell-Etherden (1991)
AnaerobiosisCramton et al. (2001b)
Fe limitation Deighton and Borland (1993); Elci et al., (1995); Evans et al. (1994)
High osmolarity, high temperatureRachid et al. (2000b)
Ethanol (S. epidermidis)Knobloch et al. (2001)
Global regulators
SigB (S. aureus)Rachid et al. (2000b)
RsbU (S. epidermidis)Knobloch et al. (2001)
Agr (S. aureus)Vuong et al. (2000)
Sar (S. aureus)Pratten et al. (2001)

Variants of highly slime-producing S. epidermidis strains that produce markedly less slime (frequency 10−2–10−3) or no slime (frequency >10−4) can easily be isolated. Oleic acid has an inducing effect on S. aureus, which could result from an ionic interaction of the positively charged PIA with the negatively charged oleic acid. Interestingly, the positive effect of oleic acid is more pronounced under oxygen-limited conditions (Campbell et al., 1986).

The effect of environmental factors on ica transcription in S. epidermidis has been studied directly by fusing the ica promoter on the chromosome with the Escherichia coliβ-galactosidase gene lacZ. Subinhibitory concentrations of tetracycline and quinupristin–dalfopristin, as well as high temperature and osmolarity, increase the ica promoter activity, whereas penicillin, oxacillin, chloramphenicol, clindamycin, gentamicin, ofloxacin, vancomycin and teicoplanin have no effect (Rachid et al., 2000a). Subinhibitory concentrations of erythromycin induce ica ex-pression weakly (2.5-fold). These results suggest that ica expression could be under the control of the alternative sigma factor SigB. Indeed, replacement of sigB on the chromosome with the erythromycin cassette ermB leads to a complete loss of ica transcription and biofilm formation under non-osmotic and osmotic stress conditions. sigB complementation completely restores the wild-type phenotype (Rachid et al., 2000b). PIA expression also appears to be controlled by the sigB operon in S. epidermidis. Insertion of a transposon in the rsbU gene (the first gene of the sigB operon) of S. epidermidis leads to a biofilm-negative phenotype and almost undetectable PIA production, whereas primary attachment is uninfluenced (Knobloch et al., 2001). Interestingly, ethanol stress enables the mutants to form a biofilm, whereas salt stress does not.

Many virulence factors in S. aureus are controlled by the accessory gene regulator (Agr) and staphylococcal accessory regulator (Sar). This regulation may be affected by the environment in which the organisms are grown. A sarA mutant strain adheres to glass better than the agrA mutant or the wild-type strain. All three strains adhere better to fibronectin-coated glass than to uncoated glass. Thus, Agr and Sar have pleiotropic effects on the surface expression of molecules responsible for binding to different substrata (Pratten et al., 2001).

There also appears to be a correlation between the ability of S. aureus to adhere to polystyrene and the agr quorum-sensing system phenotype, as 78% of agr-negative, but only 6% of agr-positive strains tested form a biofilm. This has been confirmed with defined agr mutants and by inhibition of the agr system with quorum-sensing blockers. The observed effect is not the result of differential expression of the autolysin Atl or of the exopolysaccharide, PIA, but seems to be caused at least in part by the surfactant properties of δ-toxin (Vuong et al., 2000).

Anaerobiosis induces ica expression

Until recently, little was known about the ability of S. aureus to form a biofilm. In routine microtitre plate assays, many S. aureus isolates appear to be biofilm and PIA negative. We only became perplexed when it was discovered that the ica operon is present in nearly all S. aureus strains tested (Cramton et al., 1999). Therefore, the ica operon must be strictly regulated. Indeed, anoxic conditions lead to increased transcription of the ica operon and PIA production in S. aureus and S. epidermidis, although the regulation is less stringent in S. epidermidis (Cramton et al., 2001b). That anaerobiosis is an important stimulus for ica expression explains the biofilm-negative phenotype observed in earlier assays, which were carried out under oxic conditions.

Many clinical S. aureus isolates possess capsule polymers of predominantly polysaccharide of types 5 and 8 (CP5, CP8; Hochkeppel et al., 1987). It is therefore of interest whether PIA (slime) and capsule polymers are produced at the same time and under the same conditions. As it was thought that these microcapsules play a role in colonization of lung tissue of cystic fibrosis (CF) patients, it was surprising that CP5 and CP8 were only poorly expressed in the lung tissue and nasal polyps of two CF patients and in rats, but were well expressed when the pouch isolates were cultivated in the presence of oxygen (Herbert et al., 1997; McKenney et al., 1999). CO2 (or rather anoxic conditions) is an environmental signal that downregulates the transcription of cp5; how-ever, this regulation is strain dependent (Herbert et al., 2001). In the CP5-positive S. aureus strain Newman, adherence to endothelial cells is reduced, whereas in cp5 mutants, adherence is increased (Pohlmann-Dietze et al., 2000). As CP5 expression is repressed under the anoxic conditions within the lung mucus, PIA might play a role in lung infection. McKenney et al. (1999) have shown that PIA (wrongly described as PNSG) is expressed in lungs infected with S. aureus, whereas CP8 is not expressed.

PIA polysaccharide and the capsule polysaccharides CP5 and CP8 are inversely controlled with respect to oxic and anoxic stimuli. As PIA is expressed in the lung environment, it is believed that PIA plays a crucial role in lung infection and any other kind of infection in which oxygen is, or becomes, limited. It would be interesting to determine how wild-type S. aureus and an ica mutant differ in virulence in a CF animal model.

IS256 integrates into the ica operon

An intact ica operon is more prevalent in clinical S. epidermidis isolates (septicaemic diseases, shunt- associated meningitis) than in skin isolates of non- hospitalized persons (Ziebuhr et al., 1997). Defined isolates of biofilm-negative variants occur at a frequency of ≈ 10−5 and, in 30% of these variants, the insertion sequence element IS256 is integrated at specific hot-spots within icaA or icaC (Ziebuhr et al., 1999; 2000a). Furthermore, the transposition of IS256 into the ica operon is reversible. After repeated passages, the biofilm-forming phenotype can be restored. In these revertants, IS256 is precisely excised, including the initially duplicated 8 bp target sites. These results elucidate, for the first time, a molecular mechanism for turning PIA expression on and off (Ziebuhr et al., 2000b). The recently sequenced genomes of S. aureus N315 and Mu50 contain, in addition to prophages and various transposons, 10 copies of IS1181 and two of IS431, distributed over the genome (Kuroda et al., 2001). We have prob-ably underestimated the contribution of IS elements and transposons to genetic flexibility and environmental adaptation in staphylococci.

Decreased virulence of biofilm-negative mutants in animal models

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

With the creation of defined isogenic staphylococcal mutants, it has been possible to compare wild type and mutants in vivo. Various animal models are currently used to investigate the significance of factors involved in staphylococcal biofilm formation. Some of the models have been reviewed recently (Yasuda et al., 1999; Rupp and Fey, 2001). Rupp et al. (1999a,b) have tested the relevance of PIA production in two different animal models – the rat central venous catheter (CVC)-associated infection model and a mouse model with a subcutaneously implanted intravenous catheter. The PIA-positive wild-type S. epidermidis 1457 causes CVC-associated infection (71% versus 14%) and subcutaneous abscesses significantly more frequently than the mutant strain. Furthermore, the wild-type strain adheres to the implanted catheter much better and is significantly less likely to be eradicated from the inoculation site by host defence than the mutant. atlE and ica mutants of S. epidermidis O-47 are significantly less virulent in a CVC infection model than the wild type (Rupp et al., 2001). Also, metastatic disease is less common in rats inoculated with atlE or ica mutants. These results confirm the importance of AtlE and PIA in the pathogenesis of S. epidermidis experimental CVC infection. In a rabbit endocarditis model, PS/A- negative strains develop significantly less endocarditis (Shiro et al., 1994).

In a comparison of S. epidermidis RP62A and its mitomycin-induced M7 mutant in a rabbit aortic valve endocarditis model, no differences were observed, indicating that AAP might not be an important virulence determinant in vivo (Perdreau-Remington et al., 1998). On the other hand, mutant M7 can be eradicated more easily in vitro and in animal models by amikacin, levofloxacin, rifampin or teicoplanin (Schwank et al., 1998).

Concepts for the prevention of polymer-associated infections

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

As soon as it was clear that biofilm-forming CNS play a major role in nosocomial bacteraemia and infection of prosthetic medical devices, many research groups investigated the potential of antibiotics in preventing biofilm formation or in curing staphylococcal foreign body infection (Götz and Peters, 2000). In general, staphylococcal cells embedded in a biofilm or in microcolonies are much more resistant to antibiotics than planktonic cells. Many patients with a chronic polymer-associated staphylococcal infection have been treated with various antibiotics, mostly without much success, and the replacement of the implant is usually unavoidable. In a new chronic osteomyelitis rat infection model, the influence of various antibiotics on biofilms of S. aureus has been investigated. Stainless steel implants precolonized for 12 h with a highly adherent S. aureus isolate were introduced into the rat tibiae and treated for 21 days with cefuroxime, vancomycin, tobramycin or ciprofloxacin. A reduction in bone colonization was achieved only with vancomycin or cefuroxime; however, cefuroxime was only able to sterilize one out of eight implants (Monzon et al., 2001). This recent study, like the many previous studies, emphasizes that, once a biofilm is formed, treatment with currently available antibiotics is difficult. In view of the failure of so many antibiotics to treat chronic staphylococcal infections, there is a great need for antimicrobials effective in biofilms. The question frequently raised is whether PIA (slime) is a diffusion barrier for antibiotics, which would explain the increased resistance of biofilm-imbedded cells. To answer this question, antibiotic (vancomycin and rifampin) penetration experiments have been carried out in a two-chamber system; the chambers were separated by a dialysis membrane covered on one side with an S. epidermidis biofilm (Dunne et al., 1993). There was no indication that slime acts as a diffusion barrier. This finding is not quite unexpected, as even the multilayered staphylococcal cell wall provides no real barrier for antibiotics. It has also been discussed whether the different metabolic activities of biofilm and planktonic cells play a role in re-sistance; however, no real clues have been discovered. Thus, the reason for the increased antibiotic resistance of biofilm-imbedded staphylococci remains unknown (Mah and O’Toole, 2001).

Several immunization trials with PIA carried out by McKenney et al. (1999) indicate that PIA might be a useful antigen for vaccination to protect against S. aureus infection. Immunization against PIA (incorrectly regarded as PNSG) derived from S. aureus protected mice against kidney infections and death from strains that produce little PIA in vitro. Whether vaccination against S. aureus and S. epidermidis in the long run would be successful is questionable, especially as S. aureus produces numerous extracellular and surface-bound virulence factors, and S. aureus vaccination trials have a long tradition of failure – to date, no staphylococcal vaccine has found its way into practical application.

One of the prerequisites for chronic polymer-associated infections by staphylococci is adherence to the polymer surface; therefore, great efforts have been made to find materials that prevent adherence. In practice, however, there is no synthetic polymer or biocompatible metal onto which S. epidermidis does not bind under clinical conditions (Götz and Peters, 2000).

Jansen et al. (1989) started very early to investigate the influence of modification of polymeric materials on staphylococcal adherence and biofilm formation. Various anti-biotics were incorporated or coupled to polymers, thus generating anti-infective catheters. Some of the materials exert a positive effect in the prevention of catheter-related infections. For example, a silicone ventricular catheter coated with a combination of rifampin and trimethoprim reduces colonization with S. aureus in vitro (Kohnen et al., 1998; Kockro et al., 2000).

The positive effect of silver-impregnated polymers, e.g. in the ‘silver catheter’, in reducing staphylococcal colonization or infection has been described (McLean et al., 1993; Dasgupta, 1994; Boswald et al., 1995; Jansen and Kohnen, 1995; Sampath et al., 1995; Gatter et al., 1998; Illingworth et al., 2000). Other materials tested were plasma-treated hydrogel-coated ionized fluoroplastics and heparinized hydrophilic polymers (Nagaoka and Kawakami, 1995; Appelgren et al., 1996; John et al., 1996; Biedlingmaier et al., 1998). Positive results have also been achieved with gelatine; for example, gelatine-impregnated polyester grafts show good in vivo re- sistance to coagulase-negative staphylococcal biofilm infection (Farooq et al., 1999).

Many in vitro results with new materials look very promising at the beginning, but the in vivo situation is frequently disillusioning. The main problem is that, as soon as the material is implanted, it comes into contact with blood, and the surface of the material is sooner or later covered with blood compounds to which staphylococci can adhere. S. aureus in particular has a broad arsenal of cell wall-associated compounds (murein, teichoic acids, various polysaccharides, binding proteins for tissue and plasma factors) that can interact with the one or other blood compound.

One must also be sceptical with respect to a long-range in vivo benefit of silver or other antimicrobial compounds in implant material. In a pilot in vitro study, S. epidermidis adhered even better to the surfaces of the silver-coated sewing cuff than to the uncoated cuff. These pilot in vitro results cast doubt on the anti-infective efficacy of silver-coated materials (Bechert et al., 1999; Cook et al., 2000). Furthermore, the use of silver ions or antibiotics in routine catheters will certainly lead to the selection of resistant strains. The search for alternative clinically applicable materials is not yet finished. Ideally, the material should allow binding of defence cells, but must resist the adherence of blood compounds and bacteria.

Conclusion and prospects

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

Biofilm formation in staphylococci is multifactorial, and the ability to form a biofilm makes the strains much better able to survive in the normally hostile environment of tissue and blood. Biofilm formation appears to be a bacterial survival strategy that is turned on when, for example, oxygen and Fe ions become limited, when sublethal concentrations of certain antibiotics are present or other stress situations emerge. The modulation of biofilm formation by various environmental conditions appears to be an ad-vantage for successful infection. PIA (slime) production certainly plays a major role in generating a biofilm, and the corresponding ica genes are predominantly present in clinical isolates. On the other hand, PIA only leads to intercellular adhesion, which might also be mediated by proteins or non-proteinaceous cell wall structures. In this connection, it would be interesting to know more about the function of the biofilm cofactors, such as the accumulation-associated protein (AAP) and the biofilm-associated protein (Bap), or the D-alanine esterification of teichoic acid (DltA). Colonization of tissues or implants is frequently a first step in infection, and biofilm formation plays a crucial role. However, for the invasion of endothelial and epithelial cells, sepsis and spreading, it might be advantageous for some staphylococcal cells to be able to escape the biofilm network, to become biofilm negative and to move on to other tissues. On their way, they might regain a biofilm-positive phenotype and settle down on heart valves, endocardium, periosteum, etc. One can imagine that the observed genetic variability in biofilm formation, which ranges from super- to non-biofilm formers, underlies a well-directed survival and spreading programme that we are only gradually learning to understand. This strategy makes it very difficult to master chronic and polymer-associated infections. If once a staphylococcal biofilm is formed the cells are nearly invulnerable, they are then shielded from the immune system and resist antibiotic treatment. Therefore, it is very important to understand the various aspects of biofilm formation. Only then shall we be able to develop more specific ways of overcoming staphylococcal resistance in chronic infections.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References

Our research was supported by the German Bundes- ministerium für Bildung, Wissenschaft, Forschung und Technologie (DLR: 01KI9751/1), the Deutsche Forschungsgemeinschaft Forschergruppe (FOR449/1-2) and the graduate college ‘Infection biology’ (IIIGK-GRK-685/101). I thank Karen A. Brune for critical reading of the manuscript.

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  1. Top of page
  2. Summary
  3. Introduction
  4. Biological activity of PIA
  5. PIA biosynthesis (ica genes)
  6. Functional analysis of the IcaABCD proteins
  7. Adherence to polymers and extracelluar matrix
  8. Influence of the teichoic acid structure on biofilm formation
  9. Other proteins that contribute to biofilm formation
  10. Regulation of slime production and biofilm formation
  11. Decreased virulence of biofilm-negative mutants in animal models
  12. Concepts for the prevention of polymer-associated infections
  13. Conclusion and prospects
  14. Acknowledgements
  15. References
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