SEARCH

SEARCH BY CITATION

Keywords:

  • Antibiotic resistance;
  • carriage;
  • Gram-positive;
  • multidrug resistance;
  • outbreaks;
  • review;
  • virulence

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Routes of MRSA Dissemination
  5. MDR Coagulase-Negative Staphylococci
  6. GRE
  7. Infection Control Programmes Profit from New Bacteriological Techniques
  8. Resistance to Old and New Antimicrobial Agents
  9. Acknowledgements
  10. Transparency Declaration
  11. References

The spread of methicillin-resistant Staphylococcus aureus is continuous. The emergence of community-acquired methicillin-resistant S. aureus (CA-MRSA) was rapidly followed by its introduction into and dissemination in hospitals in countries where CA-MRSA prevalence is high. Vancomycin-resistant enterococci have recently been responsible for outbreaks in European hospitals in relation to dissemination of hospital-adapted isolates. Although new antimicrobials have been recently introduced into therapy to fight multidrug-resistant Gram-positive cocci, resistance to these compounds has already emerged in rare strains. This review presents recent data concerning the advance of our knowledge related to these problems.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Routes of MRSA Dissemination
  5. MDR Coagulase-Negative Staphylococci
  6. GRE
  7. Infection Control Programmes Profit from New Bacteriological Techniques
  8. Resistance to Old and New Antimicrobial Agents
  9. Acknowledgements
  10. Transparency Declaration
  11. References

The major resistance issues concerning Gram-positive organisms are those related to methicillin-resistant Staphylococcus aureus (MRSA) and glycopeptide-resistant enterococci (GRE). The epidemicity and the means of dissemination of these microorganisms are incompletely understood, although progress has been made in deciphering the complex interactions among host, pathogen, and environment. Progress has also been made with new molecular methods that show great promise in strengthening our ability to detect colonization or infection with these bacteria. Concerns about dissemination of multidrug-resistant (MDR) bacteria have prompted the licensing of a few new agents that have, for the most part, narrow spectra of activity, and are focused on Gram-positive pathogens. However, resistance to some of these new agents emerged in rare bacteria as soon as they were introduced into therapy.

The Routes of MRSA Dissemination

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Routes of MRSA Dissemination
  5. MDR Coagulase-Negative Staphylococci
  6. GRE
  7. Infection Control Programmes Profit from New Bacteriological Techniques
  8. Resistance to Old and New Antimicrobial Agents
  9. Acknowledgements
  10. Transparency Declaration
  11. References

From MRSA nasopharyngeal carriage to infection

S. aureus has the capacity to both colonize human hosts without causing too much harm and act as a major pathogen. The role of nasopharyngeal carriage of S. aureus in the spread of microorganisms, including MDR strains, and as a risk factor for infection in the host is well documented [1].

Several genomes of S. aureus have now been completely sequenced. Sequence analysis has shown that all S. aureus strains contain major virulence factors and that they have the capacity to become invasive [2]. Staphylococcal virulence is probably better explained by modulation of the expression of virulence factors by accessory genes. However, and most importantly, individual variations in intermittent or persistent carrier status cannot be understood without identification of host factors that contribute to colonization. The complete picture of the contribution of host genetic polymorphism to nasal carriage and the occurrence of infections is far from being completely elucidated. Recent studies have shown associations between polymorphisms in interleukin-4, complement factor H and C-reactive protein, on the one hand, and S. aureus carrier status and the occurrence of boils on the other [3]. The observation that the host genotype was associated with carriage of certain staphylococcal types confirms that nasopharyngeal carriage results from complex interactions among host, pathogen, and environment, including antibiotic selective pressure.

Dissemination of MRSA: tracing the bacteria and identifying new reservoirs

The spread of MDR bacteria results from an interplay of transfer of resistance genes between strains, bacterial species or genera and of dissemination of bacterial clones in various environments. Gene transfer occurs through horizontal transfer of plasmids, transposition (e.g. vanA borne by transposon Tn1546 in enterococci), dissemination of resistance cassettes (e.g. SCCmec in MRSA), or transformation (e.g. pbp genes of Streptococcus pneumoniae), depending on the microorganism and on the genetic support of the resistance determinant.

Tools are available for tracing the sources of dissemination and deciphering the genomic evolution of MDR organisms that allow adaptation to diverse hosts and environments. Various genotyping techniques based on PCR (rep-PCR, amplified fragment length polymorphism), pulsed-field gel electrophoresis, sequencing (multilocus variant analysis, single-locus sequence typing, multilocus sequence typing (MLST)) and DNA microarrays may be used.

The choice of the technique will depend on the objectives of the study, which can be limited to local outbreaks or can extend to national or global surveillance; this has recently been discussed by van Belkum et al. [4].

In staphylococci, the mecA gene encoding penicillin-binding protein 2a, responsible for methicillin resistance, is borne by 21–67-kb mobile genetic elements, termed the staphylococcal cassette chromosome mec (SCCmec) [5]. The SCCmec element integrates into the S. aureus chromosome at a unique site (attBSCC). SCCmec elements are characterized by the presence of flanking terminal direct and, in most cases, inverted repeats, two essential genetic components (the mec gene complex and the ccr gene complex), and three junkyard (or jigsaw) regions (J). To date, six classes (A, B, C1, C2, D, and E) of mec gene complex and five types (1, 2, 3, 4, and 5) of ccr gene complex have been described. Different combinations of ccr and mec complexes allowed the definition of six types in S. aureus, with subypes defined as variations in the J regions [6]. SCCmec typing is used in combination with other typing techniques, such as MLST, for tracing MRSA clones. Using different typing techniques, in particular MLST, the spread of hospital-acquired (HA)-MRSA and community-acquired (CA)-MRSA clones has been traced. The role of travel and of population migrations in the spread of MDR organisms is now well established. For instance, in Belgium, 25% of CA-MRSA infections were acquired after travel abroad [7]. The origin of new epidemic HA-MRSA clones is still under discussion. They may result from international cross-border spread or de  novo emergence of clones by SCCmec type IV insertion into recipient methicillin-susceptible S. aureus lineages.

New reservoirs of MRSA have been identified using MLST techniques. In The Netherlands, unexpected cases of MRSA carriage and infection were linked to pig farming [8]. Isolates belonged to sequence type ST398. This particular type was found in staphylococci from pigs in France [9], and was highly prevalent in MRSA isolated in 39% of pigs in slaughterhouses in The Netherlands [10].

Finally, these powerful typing techniques have shed light on the circulation, the reservoirs and the origin and evolution of MDR organisms. However, predictions about future epidemic MDR clones and evolution remain uncertain.

MRSA strains have abolished borders between the community and hospitals

As already mentioned, MRSA isolates have spread internationally. However, they have traditionally been associated with infections in hospitals, to which they were mostly confined. Recently, new MRSA strains have emerged and rapidly spread in the community. They cause infections that are acquired by persons who have not been recently hospitalized or undergone a medical procedure (related to, for example, surgery, catheters, percutaneous medical devices, or dialysis). These so-called CA-MRSA strains are responsible for infections with a particular clinical presentation, and are associated with skin and soft tissue infections, such as pimples and boils, that occur in previously healthy and young persons.

Although there is no consensus definition, CA-MRSA isolates from throughout the world have several common characteristics. The most important are the production of Panton–Valentine leukocidin (PVL), which is infrequent in other S. aureus strains, and the presence of short SCCmec elements (of type IV or V). CA-MRSA isolates initially lacked multiple resistance to antibiotics. However, they can be classified as MDR organisms, because they are resistant to the β-lactam class of antibiotics, which includes major antistaphylococcal agents, and to some other antimicrobials, such as tetracyclines and fusidic acid, when isolated in Europe. In addition, new variants of CA-MRSA clones resistant to clindamycin and quinolones have recently emerged [11].

In the USA, CA-MRSA became more prevalent than methicillin-susceptible S. aureus in community-acquired S. aureus infections. In a recent study including 422 emergency department patients, 59% of S. aureus isolates from skin and soft tissue infections requiring drainage were resistant to methicillin, with variations from 20% to 72%, depending upon the state [12]. Most infections in the USA are due to an MRSA clone characterized by sequence type ST8 and called USA300.

Two recent major events in the evolution of the USA300 clone were observed. First, it started to infiltrate hospitals and to replace the traditional HA-MRSA strains. From 2000 to 2006, the proportion of MRSA strains isolated from hospital-onset bloodstream infections and displaying a community phenotype of resistance increased from 24% to 49% in a US hospital [13]. The total number of bloodstream infections remained stable, suggesting that the CA-MRSA strains replaced the HA-MRSA strains without causing additional infections. However, spread of CA-MRSA infections in hospitals is worrying, as it would occur among a more debilitated, older patient population and would provoke more severe infections. In addition, the skin tropism may add to the capacity of CA-MRSA to disseminate, which may eventually lead to an increased global burden of infections.

Subsequently, variants of USA300 MRSA that were resistant to clindamycin, ciprofloxacin and mupirocin became common among men who have sex with men [14]. Possibly, MDR MRSA infections will have to be added to the list of sexually transmitted diseases.

CA-MRSA strains have also been described on other continents. Clones were initially reported as being continent-specific, with the ST80 type spreading in Europe. However, recent data show intercontinental exchanges of CA-MRSA clones and emergence of new PVL-positive clones, resulting in a more complex situation.

The prevalence of CA-MRSA is not uniform in Europe. It ranges from low in France (3.6% of MRSA) and in England and Wales [15,16] to high in Greece, with 75% of MRSA strains in the community containing PVL genes [17]. The prevalence is also high at the southern boundary of Europe, with 72% of MRSA strains containing PVL genes in Algeria [11]. The high prevalence of CA-MRSA in certain countries was found to be associated with an increased prevalence of CA-MRSA in hospitals [17] and, in Algeria, with diversification of the resistance profiles [11].

Despite the recent developments, the European ST80 clone seems to have less potential for dissemination than the USA300 clone. The epidemiological success of the USA300 clone (which may be considered as a ‘superbug’) has tentatively been explained by several particular characteristics. USA300 produces novel cytolytic peptides called phenol-soluble modulins that have the capacity to recruit, activate and subsequently damage human neutrophils [18]. Phenol-soluble modulin-alpha is rarely detected in HA-MRSA strains. Recently, the arginine catabolic mobile element was found in association with the staphylococcal chromosomal cassette mec in USA300 lineages, but it was absent in most of the other CA-MRSA isolates, including ST80 [19]. This element encodes an arginine deiminase pathway and a putative oligopeptide permease operon (Opp-3). Arginine deiminase could enhance arginine catabolism and the survival of USA300 in anaerobic environments. Opp-3 may contribute to the overall fitness of USA300 by improving growth, adhesion to host cells, and expression of virulence determinants. The fitness of CA-MRSA may also be influenced by the polymorphism of the S. aureus PVL [20].

The possibility should be considered that the USA300 clone that has recently arrived in Europe may replace ST80, leading to a situation similar to that seen in the USA.

MDR Coagulase-Negative Staphylococci

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Routes of MRSA Dissemination
  5. MDR Coagulase-Negative Staphylococci
  6. GRE
  7. Infection Control Programmes Profit from New Bacteriological Techniques
  8. Resistance to Old and New Antimicrobial Agents
  9. Acknowledgements
  10. Transparency Declaration
  11. References

Staphylococcus epidermidis and other coagulase-negative staphylococci are major constituents of the permanent normal human flora. Coagulase-negative staphylococci are considered to be poorly pathogenic for normal hosts, apart from some species, e.g. Staphylococcus saprophyticus and Staphylococcus lugdunensis, which are responsible for common urinary tract infections and skin and soft tissue infections, respectively. Isolates belonging to these latter species are usually susceptible to most antistaphylococcal agents. By contrast, S. epidermidis and S. haemolyticus often combine resistances to major antistaphylococcal agents, and are important causes of material-associated infections.

The ‘virulence’ potential of S. epidermidis relies on the presence of several surface and surface-associated proteins, and on the capacity of this microorganism to produce biofilm. Several surface proteins, the so-called MSCRAMMS, characterized by the presence of LPXTG motifs, have been identified in S. epidermidis, including fibrinogen-binding (Fbe) and collagen-binding (SdrF) proteins, accumulation-associated protein (Aap), and Bap homologue protein (Bhp). In combination with surface-associated proteins, such as fibronectin-binding protein (Embp), collagen-binding protein (GehD), and adhesins (Aae and AtlE), and the polysaccharide intercellular adhesin (PIA), these proteins interact at the different steps of adhesion to surfaces and of the development of biofilm. AtlE and PIA have been found to be involved in the early attachment to polymer surfaces, and PIA and Aap play a role in the proliferation and accumulation of bacterial cells that form part of the biofilm.

In a European study in intensive-care units, coagulase-negative staphylococci were found to be the first cause of bloodstram infections (19.4%) and, overall, accounted for 10.5% of infections (fourth most frequent cause of infection) [21].

The role of coagulase-negative staphylococci in catheter-related infections, prosthetic joint infections and pacemaker endocarditis is well established. As an example, the rate of intravascular catheter infection has been estimated to be 3–5% in the UK [22]. In addition, these bacteria cause 8% of cases of native valve endocarditis not associated with drug use, with a 25% mortality rate, which is similar to that due to S. aureus [23].

Multidrug resistance is common in S. epidermidis and even more so in Staphylococcus haemolyticus. Less than 30% of S. epidermidis isolates circulating in the hospital environment are susceptible to oxacillin, and only about 40%, 45–60% and 45% are susceptible to gentamicin, clindamycin and ciprofloxacin, respectively [21,24]. Resistance to glycopeptides, usually associated with resistance to methicillin, has also been reported in S. epidermidis and S. haemolyticus. However, the influence of technical factors such as inoculum size, medium and incubation time on susceptibility tests, and different standards used by the CLSI, EUCAST and other committees for testing, make the prevalence of glycopeptide resistance difficult to evaluate. With regard to teicoplanin, local variations may explain the wide range in rates of resistance, reported to vary from 1.5% [21] to 55% [25]. Vancomycin resistance seems to be rare in coagulase-negative staphylococci.

In addition to their role as potential pathogens under certain circumstances, MDR coagulase-negative staphylococci have been suspected to be providers to S. aureus of mecA and of other resistance genes. The presence of a mecA-like gene in all isolates of Staphylococcus sciuri might be taken to incriminate this species as a major source of mecA [26]. However, S. sciuri is rarely isolated from humans, and the mecA-like gene displays only 80% identity with mecA. This argues against a recent transfer of the gene to human staphylococci, but suggests that the S. sciuri gene is the ancestor of the mecA gene currently spreading among clinical isolates of S. aureus.

Several observations support the hypothesis of transfer of methicillin resistance between coagulase-negative staphylococci and S. aureus. First, a recent study confirmed the high prevalence of methicillin-resistant coagulase-negative staphylococci in nasal carriers, between 11% and 31% in four countries (on three continents) [27]. Andremont et al. (personal communication) found, in a recent unpublished study conducted in France, that 3.5% of study patients had nasal co-colonization with methicillin-susceptible S. aureus and methicillin-resistant coagulase-negative staphylococci, showing that opportunities for mecA gene transfer are not rare. Molecular studies provided evidence of interspecies transfer of mec cassetttes. The type IV SCCmec, which is widespread in MRSA, is the most frequent in S. epidermidis [28]. The nucleotide sequences of the cassettes in both species were identical [29]. Notably, S. epidermidis showed a higher diversity of mec cassettes than S. aureus, and new types were found in recent studies [27,30]. Therefore, S. epidermidis strains form an abundant reservoir of various SCCmec types that could subsequently be transferred to S. aureus and other staphylococcal species. Future emergence of new cassette types in S. aureus can be confidently predicted.

GRE

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Routes of MRSA Dissemination
  5. MDR Coagulase-Negative Staphylococci
  6. GRE
  7. Infection Control Programmes Profit from New Bacteriological Techniques
  8. Resistance to Old and New Antimicrobial Agents
  9. Acknowledgements
  10. Transparency Declaration
  11. References

Since the 1980s, enterococci have been identified as an important cause of nosocomial infections, generally ranking as the third or fourth most prevalent genus among nosocomial pathogens.

Eighty-five to 90% of enterococcal infections are due to Enterococcus faecalis and c. 10% to Enterococcus faecium. In the latter species, acquisition of resistance is a major cause of concern. Most prominently, resistance to penicillin–ampicillin, aminoglycosides (high-level resistance), and glycopeptides is being reported in an increasing number of isolates, which limits the therapeutic possibilities.

Glycopeptides act by blocking cell wall formation, and resistance is due to the synthesis of modified peptidoglycan precursors, terminating in d-alanyl-d-lactate or d-alanine-d-serine. Six types of acquired glycopeptide resistance (types VanA, B, D, E, G, and L) have been described. The vanA and vanB resistance genotypes are by far the most prevalent in Europe, and are mostly found in E. faecium. GRE are now highly prevalent in the USA and some Asian countries. In Europe, the situation is different, with prevalence ranging from <1% to >40%.

A major concern is the transfer of plasmid-mediated VanA resistance to MRSA. Seven isolates have been reported in the USA since 2002 (which were not cross-transmitted to other patients) [31], one isolate in Iran [32], and another isolate in India [33]. The risk of emergence of MRSA resistant to vancomycin is highest in countries with a high prevalence of both MRSA and GRE. This unwelcome perspective has reinforced the willingness of health authorities in several countries to make efforts to prevent the further spread of GRE [34].

In Europe, the major reservoir of GRE was initially the healthy population in the community, 2–10% of whom were carriers. The source was most likely the GRE reservoir in animals, where it most likely arose due to the widespread use of avoparcin (a glycopeptide similar to teicoplanin) as a growth promoter during the 1980s.

Recently, the epidemiology has changed with the report of hospital outbreaks, and it now resembles that described in the USA since the 1990s. A recent review by Werner et al. [34] described the emergence and spread of GRE in Europe.

Defined clonal groups of E. faecium have been identified using DNA sequence-based typing (MLST) and phylogenetic analyses [35]. The clones that have been grouped in a superclonal complex called CC17 are mostly ampicillin-resistant and high-level ciprofloxacin-resistant, and show an enhanced capacity to disseminate in the nosocomial setting. These clones have been circulating in hospitals for 20 years, but acquired transferable glycopeptide resistance only recently. They possess additional genomic material, which includes a pathogenicity island bearing a putative virulence gene for an enterococcal surface protein, esp, and genes encoding different cell wall-anchored surface proteins, a putative hyaluronidase gene, hylEfm, and a gene encoding a collagen-binding protein, acm [36]. Also, DNA microarray studies showed that E. faecium CC17 contains a much larger accessory genome than non-hospital isolates [37].

Although there are now numerous data from population analysis, the reasons for the rise of GRE in Europe since 2004 have remained unknown. The bases of the epidemicity of E. faecium CC17 are incompletely elucidated. However, it has been learned from hospitals facing GRE outbreaks that spread can be successfully stopped by immediate infection control measures, including extensive and rapid screening of GRE carriers [38].

Infection Control Programmes Profit from New Bacteriological Techniques

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Routes of MRSA Dissemination
  5. MDR Coagulase-Negative Staphylococci
  6. GRE
  7. Infection Control Programmes Profit from New Bacteriological Techniques
  8. Resistance to Old and New Antimicrobial Agents
  9. Acknowledgements
  10. Transparency Declaration
  11. References

Culture-based and nucleic acid-based detection methods for major MDR Gram-positive organisms in carriers or infected patients have greatly improved during the last 10 years. The techniques apply to only a few, but important, organism–antimicrobial agent combinations, allowing identification in a single step of the bacterial species and of major antibiotic resistance characteristics. MRSA and GRE, which are often multiply resistant to other antimicrobials, are relevant targets for these techniques [39].

Whereas conventional methods required 2–4 days for the detection of MRSA and GRE, new chromogenic media have the potential to produce results within 24 h. Most chromogenic media show high specificities after 24 h of incubation, with no necessity for confirmatory tests; however, sensitivities vary widely, from 50% to 99%, depending on multiple factors. Pre-enrichment results in a substantial increase in sensitivity, but may increase the time required to obtain results. The best balance between rapid results and increased sensitivities, in terms of impact on infection control, has still to be found.

Several nucleic acid-based methods have been developed for the detection of MRSA. Those that are based on hybridization can be performed only with bacterial cultures. A major advantage of these techniques is the rapid identification of MRSA in positive blood cultures, where 96–100% agreement with phenotypic techniques is obtained [40]. Amplification-based techniques may be carried out with isolates, with blood cultures and, most interestingly for prevention of cross-transmission of MDR bacteria, with screening samples. Commercial tests are now available with sensitivities and negative predictive values that are equivalent to or slightly better than those obtained with culture. Therefore, these tests can be used to justify the removal of patients from isolation. However, positive results still require confirmation.

Chromogenic media and molecular techniques are also available for the detection of vancomycin-resistant enterococci. PCR-based techniques are used for species confirmation and characterization of vancomycin resistance genes of isolates from cultures, e.g. stool cultures and enrichment broths, and also directly from rectal swabs.

Again, the negative and positive predictive values are useful in screening programmes, and may help to control the rapid diffusion of GRE from patient to patient.

However, molecular methods allow detection only of known mechanisms of resistance and, so far, do not apply to cases where resistance is due to deregulation of gene expression. For instance, no molecular method is available for the detection and identification of glycopeptide-intermediate S. aureus (GISA), where resistance is not due to specific genes but to the alteration of expression of multiple genes involved in cell wall synthesis.

If it is clear that the excellent negative predictive values of most rapid molecular techniques may save a large number of unnecessary isolation days in screening programmes. However, the high cost of these techniques prevents their widespread use. In addition, there is no single and stand-alone rapid assay that can recognize a large variety of MDR organisms in clinical samples. More studies are needed to demonstrate the medical and economic benefits of control strategies using the molecular tools. Probably, these techniques are particularly useful for the management of outbreaks.

Resistance to Old and New Antimicrobial Agents

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Routes of MRSA Dissemination
  5. MDR Coagulase-Negative Staphylococci
  6. GRE
  7. Infection Control Programmes Profit from New Bacteriological Techniques
  8. Resistance to Old and New Antimicrobial Agents
  9. Acknowledgements
  10. Transparency Declaration
  11. References

Staphylococci with diminished susceptibility to glycopeptides

Vancomycin is still the alternative to oxacillin in cases of resistance (or intolerance). The definitions and clinical consequences of diminished susceptibility to vancomycin remain controversial. Different standards used by national committees for antibiotic susceptibility testing and lack of a genetic reference technique make difficult the identification of GISA strains and even more of heterointermediate GISA (hGISA) strains, which appear to be susceptible to vancomycin but consist of subpopulations of intermediate susceptibility to this glycopeptide [41]. Although the population analysis profile, and in particular the population analysis profile/AUC ratio, became the reference technique, it cannot be easily applied routinely in clinical microbiology laboratories [42].

GISA and hGISA share thickened cell walls, reduced peptidoglycan cross-linking, and reduced autolytic activity. They display higher MICs than fully vancomycin-susceptible S. aureus [43].

The prevalence of hGISA has been evaluated in several studies, and is higher among MRSA strains (c. 2%) than among methicillin-susceptible S. aureus strains (0.05%) [44].

The clinical impact of diminished susceptibility to vancomycin has been evaluated in several studies. However, in most cases studied, isolates were obtained during the late stages of vancomcyin therapy. In addition, vancomycin blood levels are rarely available. From the studies, it is clear that rare strains with diminished susceptibility to vancomycin exist and may represent a single GISA/hGISA phenotype. However, it cannot be concluded from most studies whether hGISA was the cause of clinical failure or whether it developed during therapy in relation either to suboptimal dosing of vancomycin or to the presence of foreign material. Limited data suggest that initial infection with GISA and hGISA may be associated with poor outcome.

Resistance to new antimicrobial agents

Therapeutic alternatives for the treatment of infections with GRE and MRSA with diminished susceptibility to glycopeptides are restricted to antibiotics introduced recently into clinical practice, such as linezolid, tigecycline, and daptomycin. However, these drugs are only approved for certain indications, and resistance has already been reported for some.

Tigecycline is a glycylcycline, derived from minocycline, that overcomes the two major mechanisms of resistance to tetracyclines, drug-specific efflux by pump acquisition and ribosomal protection. This bacteriostatic antibiotic has a wide spectrum of activity, and MIC90 values for enterococci and staphylococci, including MDR isolates, are 0.12 or 0.25 mg/L [45]. Resistance in clinical isolates of staphylococci has not been reported. However, mutants of MRSA N315 and Mu3 could be selected after 16 passages (MIC 4 mg/L) [46]. Transcription analysis revealed a 100-fold increased expression of a gene cluster, mepRAB, in mutants. The mepRAB gene cluster encodes a MarR-like transcriptional regulator (mepR), a novel MATE family efflux pump (mepA), and a hypothetical protein of unknown function (mepB). Mutations in mepR presumably inactivated the MepR repressor.

Daptomycin is a bactericidal glycolipopeptide that has a membrane-based mode of action involving calcium-dependent membrane depolarization of and potassium release from S. aureus. A model for the bactericidal action of daptomycin involving oligomerization of daptomycin and disruption of the functional integrity of the cytoplasmic membrane has been proposed [47].

Daptomycin-resistant S. aureus and Enterococcus spp. have been found during prolonged treatment with daptomycin [48–50]. An association between glycopeptide-intermediate S. aureus strains and diminished susceptibility to daptomycin has been reported [51].

Although precise mechanisms have yet to be determined, reduction of diffusion of daptomycin because of thickening of cell walls induced by vancomycin has been proposed. Whether this impacts on the efficacy of daptomycin negatively when this antibiotic is used in cases of failure of vancomycin therapy is unknown.

Linezolid is the first available oxazolidinone. This bacteriostatic antibiotic has a spectrum of activity limited to Gram-positive organisms. Infrequent resistance caused by mutation of the target site, the 23S rRNA, has been reported in enterococci and staphylococci. Essentially, substitution of guanine at position 2576 was observed in clinical isolates. Most bacteria have several rRNA operon copies, the number of which depends on the species (four to six in major Gram-positive species). The level of resistance is proportional to the number of mutated RNA operon copies.

As consumption of linezolid increased in the USA for the treatment of GRE infections, it is not surprising that outbreaks of E. faecalis and E. faecium resistant to linezolid were reported [52].

The recent emergence in staphylococci of transferable resistance to linezolid may be a major cause of concern. Resistance is due to the cfr gene, which encodes an rRNA methylase that transfers an additional methyl group to A2503 in 23S rRNA. As several antibiotics share this same nucleotide as a binding site, Cfr impairs ribosomal binding of five different classes of antibiotics: phenicols, lincosamides, oxazolidinones, pleuromutilins, and streptogramins  A. This resistance was first reported in Staphylococcus spp. of animal origin in Europe [53], and has recently been reported in S. aureus and S. epidermidis from humans in Colombia and the USA [54,55]. Comparison between the DNA sequences surrounding cfr in animal S. aureus (pSCF53) and human S. aureus (004-737X) as well as in S. epidermidis (426-3147L) isolates showed similarities that suggested genetic transfer between strains from animals and humans [55].

In conclusion, bacteria have already selected some specific ways to resist new antibiotics, i.e. efflux for tigecycline, cell wall structure modification for daptomycin, and ribosomal target modification for linezolid. So far, resistant isolates are rare. However, a pessimistic point of view would be to say that, because resistant isolates already exist, the lifespan of the new antibiotics is already threatened. A more optimistic point of view would be to say that the availability of several antibiotic alternatives against MDR bacteria may delay the spread of resistance. In addition, strict antibiotic usage protocols and hygiene precautions may delay the spread of resistant microorganisms.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Routes of MRSA Dissemination
  5. MDR Coagulase-Negative Staphylococci
  6. GRE
  7. Infection Control Programmes Profit from New Bacteriological Techniques
  8. Resistance to Old and New Antimicrobial Agents
  9. Acknowledgements
  10. Transparency Declaration
  11. References

This review is based on the presentations by A. Andremont, P. Courvalin, O. Dumitrescu, R. Howe, M. Ieven, R. Leclercq, G. Peters, A. van Belkum, F. Vandenesch and W. Witte during the ESCMID conference entitled Fighting infections due to MDR Gram positives’(Venice, May 2008).

Transparency Declaration

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Routes of MRSA Dissemination
  5. MDR Coagulase-Negative Staphylococci
  6. GRE
  7. Infection Control Programmes Profit from New Bacteriological Techniques
  8. Resistance to Old and New Antimicrobial Agents
  9. Acknowledgements
  10. Transparency Declaration
  11. References

For three years, Roland Leclercq has received funds for speaking from Wyeth and Sanofi-Aventis. No conflicting or dual interest, financial or of any other nature, may affect professional judgment in relation to the submitted article.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Routes of MRSA Dissemination
  5. MDR Coagulase-Negative Staphylococci
  6. GRE
  7. Infection Control Programmes Profit from New Bacteriological Techniques
  8. Resistance to Old and New Antimicrobial Agents
  9. Acknowledgements
  10. Transparency Declaration
  11. References
  • 1
    Von Eiff C, Becker K, Machka K, Stammer H, Peters G. Nasal carriage as a source of Staphylococcus aureus bacteremia. Study group. N Engl J Med 2001; 344: 1116.
  • 2
    Lindsay JA, Moore CE, Day NP et al. Microarrays reveal that each of the 10 dominant lineages of Staphylococcus aureus has a unique combination of surface-associated and regulatory genes. J Bacteriol 2006; 188: 669676.
  • 3
    Van Belkum A, Emonts M, Wertheim H et al. The role of human innate immune factors in nasal colonization by Staphylococcus aureus. Microbes Infect 2007; 9: 14711477.
  • 4
    Van Belkum A, Tassios PT, Dijkshoorn L et al. Guidelines for the validation and application of typing methods for use in bacterial epidemiology. Clin Microbiol Infect 2007; 13 (suppl 3): 146.
  • 5
    Hanssen AM, Ericson Sollid JU. SCCmec in staphylococci: genes on the move. FEMS Immunol Med Microbiol 2006; 46: 820.
  • 6
    Kondo Y, Ito T, Ma XX et al. Combination of multiplex PCRs for staphylococcal cassette chromosome mec type assignment: rapid identification system for mec, ccr, and major differences in junkyard regions. Antimicrob Agents Chemother 2007; 51: 264274.
  • 7
    Denis O, Deplano A, De Beenhouwer H et al. Polyclonal emergence and importation of community-acquired methicillin-resistant Staphylococcus aureus strains harbouring Panton–Valentine leucocidin genes in Belgium. J Antimicrob Chemother 2005; 56: 11031106.
  • 8
    Voss A, Loeffen F, Bakker J, Klaassen C, Wulf M. Methicillin-resistant Staphylococcus aureus in pig farming. Emerg Infect Dis 2005; 11: 19651966.
  • 9
    Armand-Lefevre L, Ruimy R, Andremont A. Clonal comparison of Staphylococcus aureus isolates from healthy pig farmers, human controls, and pigs. Emerg Infect Dis 2005; 11: 711714.
  • 10
    Van Duijkeren E, Ikawaty R, Broekhuizen-Stins MJ et al. Transmission of methicillin-resistant Staphylococcus aureus strains between different kinds of pig farms. Vet Microbiol 2008; 126: 383389.
  • 11
    Ramdani-Bouguessa N, Bes M, Meugnier H et al. Detection of methicillin-resistant Staphylococcus aureus strains resistant to multiple antibiotics and carrying the Panton–Valentine leukocidin genes in an Algiers hospital. Antimicrob Agents Chemother 2006; 50: 10831085.
  • 12
    Moran GJ, Krishnadasan A, Gorwitz RJ et al. Methicillin-resistant S. aureus infections among patients in the emergency department. N Engl J Med 2006; 355: 666674.
  • 13
    Popovich KJ, Weinstein RA, Hota B. Are community-associated methicillin-resistant Staphylococcus aureus (MRSA) strains replacing traditional nosocomial MRSA strains? Clin Infect Dis 2008; 46: 787794.
  • 14
    Diep BA, Chambers HF, Graber CJ et al. Emergence of multidrug-resistant, community-associated, methicillin-resistant Staphylococcus aureus clone USA300 in men who have sex with men. Ann Intern Med 2008; 148: 249257.
  • 15
    Dauwalder O, Lina G, Durand G et al. Epidemiology of invasive methicillin-resistant Staphylococcus aureus clones collected in France in 2006 and 2007. J Clin Microbiol 2008; 46: 34543458.
  • 16
    Holmes A, Ganner M, McGuane S, Pitt TL, Cookson BD, Kearns AM. Staphylococcus aureus isolates carrying Panton–Valentine leucocidin genes in England and Wales: frequency, characterization, and association with clinical disease. J Clin Microbiol 2005; 43: 23842390.
  • 17
    Chini V, Petinaki E, Foka A, Paratiras S, Dimitracopoulos G, Spiliopoulou I. Spread of Staphylococcus aureus clinical isolates carrying Panton–Valentine leukocidin genes during a 3-year period in Greece. Clin Microbiol Infect 2006; 12: 2934.
  • 18
    Wang R, Braughton KR, Kretschmer D et al. Identification of novel cytolytic peptides as key virulence determinants for community-associated MRSA. Nat Med 2007; 13: 15101514.
  • 19
    Diep BA, Stone GG, Basuino L et al. The arginine catabolic mobile element and staphylococcal chromosomal cassette mec linkage: convergence of virulence and resistance in the USA300 clone of methicillin-resistant Staphylococcus aureus. J Infect Dis 2008; 197: 15231530.
  • 20
    Dumitrescu O, Tristan A, Meugnier H et al. Polymorphism of the Staphylococcus aureus Panton–Valentine leukocidin genes and its possible link with the fitness of community-associated methicillin-resistant S. aureus. J Infect Dis 2008; 198: 792794.
  • 21
    Fluit AC, Verhoef J, Schmitz FJ, European SENTRY Participants. Frequency of isolation and antimicrobial resistance of gram-negative and gram-positive bacteria from patients in intensive care units of 25 European university hospitals participating in the European arm of the SENTRY Antimicrobial Surveillance Program 1997–1998. Eur J Clin Microbiol Infect Dis 2001; 20: 617625.
  • 22
    Berrington A, Gould FK. Use of antibiotic locks to treat colonized central venous catheters. J Antimicrob Chemother 2001; 48: 597603.
  • 23
    Chu VH, Woods CW, Miro JM et al. Emergence of coagulase-negative staphylococci as a cause of native valve endocarditis. Clin Infect Dis 2008; 46: 232242.
  • 24
    Von Eiff C, Reinert RR, Kresken M, Brauers J, Hafner D, Peters G. Nationwide German multicenter study on prevalence of antibiotic resistance in staphylococcal bloodstream isolates and comparative in vitro activities of quinupristin–dalfopristin. J Clin Microbiol 2000; 38: 28192823.
  • 25
    Trueba F, Garrabe E, Hadef R et al. High prevalence of teicoplanin resistance among Staphylococcus epidermidis strains in a 5-year retrospective study. J Clin Microbiol 2006; 44: 19221923.
  • 26
    Wu S, Piscitelli C, De Lencastre H, Tomasz A. Tracking the evolutionary origin of the methicillin resistance gene: cloning and sequencing of a homologue of mecA from a methicillin susceptible strain of Staphylococcus sciuri. Microb Drug Resist 1996; 2: 435441.
  • 27
    Ruppé E, Barbier F, Mesli Y et al. Diversity of SCCmec structures in methicillin-resistant Staphylococcus epidermidis and Staphylococcus haemolyticus among outpatients from four countries. Antimicrob Agents Chemother 2009; 53: 442449.
  • 28
    Miragaia M, Carriço JA, Thomas JC, Couto I, Enright MC, De Lencastre H. Comparison of molecular typing methods for characterization of Staphylococcus epidermidis: proposal for clone definition. J Clin Microbiol 2008; 46: 118129.
  • 29
    Wisplinghoff H, Rosato AE, Enright MC, Noto M, Craig W, Archer GL. Related clones containing SCCmec type IV predominate among clinically significant Staphylococcus epidermidis isolates. Antimicrob Agents Chemother 2003; 47: 35743579.
  • 30
    Miragaia M, Thomas JC, Couto I, Enright MC, De Lencastre H. Inferring a population structure for Staphylococcus epidermidis from multilocus sequence typing data. J Bacteriol 2007; 189: 25402552.
  • 31
    Sievert DM, Rudrik JT, Patel JB, McDonald LC, Wilkins MJ, Hageman JC. Vancomycin-resistant Staphylococcus aureus in the United States, 2002–2006. Clin Infect Dis 2008; 46: 668674.
  • 32
    Aligholi M, Emaneini M, Jabalameli F, Shahsavan S, Dabiri H, Sedaght H. Emergence of high-level vancomycin-resistant Staphylococcus aureus in the Imam Khomeini Hospital in Tehran. Med Princ Pract 2008; 17: 432434.
  • 33
    Saha B, Singh AK, Ghosh A, Bal M. Identification and characterization of a vancomycin-resistant Staphylococcus aureus isolated from Kolkata (South Asia). J Med Microbiol 2008; 57: 7279.
  • 34
    Werner G, Coque TM, Hammerum AM et al. Emergence and spread of vancomycin resistance among enterococci in Europe. Euro Surveill 2008; 13: pii19046.
  • 35
    Willems RJ, Top J, Van Santen M et al. Global spread of vancomycin-resistant Enterococcus faecium from distinct nosocomial genetic complex. Emerg Infect Dis 2005; 11: 821828.
  • 36
    Willems RJ, Bonten MJ. Glycopeptide-resistant enterococci: deciphering virulence, resistance and epidemicity. Curr Opin Infect Dis 2007; 20: 384390.
  • 37
    Leavis H, Top J, Shankar N et al. A novel putative enterococcal pathogenicity island linked to the esp virulence gene of Enterococcus faecium and associated with epidemicity. J Bacteriol 2004; 186: 672682.
  • 38
    Borgmann S, Schulte B, Wolz C et al. Discrimination between epidemic and non-epidemic glycopeptide-resistant E. faecium in a post-outbreak situation. J Hosp Infect 2007; 67: 4955.
  • 39
    Malhotra-Kumar S, Haccuria K, Michiels M et al. Current trends in rapid diagnostics for methicillin-resistant Staphylococcus aureus and glycopeptide-resistant enterococcus species. J Clin Microbiol 2008; 46: 15771587.
  • 40
    Stamper PD, Cai M, Howard T, Speser S, Carroll KC. Clinical validation of the molecular BD GeneOhm StaphSR assay for direct detection of Staphylococcus aureus and methicillin-resistant Staphylococcus aureus in positive blood cultures. J Clin Microbiol 2007; 45: 21912196.
  • 41
    Hiramatsu K. Vancomycin-resistant Staphylococcus aureus: a new model of antibiotic resistance. Lancet Infect Dis 2001; 1: 147155.
  • 42
    Wootton M, Howe RA, Hillman R, Walsh TR, Bennett PM, MacGowan AP. A modified population analysis profile (PAP) method to detect hetero-resistance to vancomycin in Staphylococcus aureus in a UK hospital. J Antimicrob Chemother 2001; 47: 399403.
  • 43
    Wootton M, Walsh TR, MacGowan AP. Evidence for reduction in breakpoints used to determine vancomycin susceptibility in Staphylococcus aureus. Antimicrob Agents Chemother 2005; 49: 39823983.
  • 44
    Liu C, Chambers HF. Staphylococcus aureus with heterogeneous resistance to vancomycin: epidemiology, clinical significance, and critical assessment of diagnostic methods. Antimicrob Agents Chemother 2003; 47: 30403045.
  • 45
    Waites KB, Duffy LB, Dowzicky MJ. Antimicrobial susceptibility among pathogens collected from hospitalized patients in the United States and in vitro activity of tigecycline, a new glycylcycline antimicrobial. Antimicrob Agents Chemother 2006; 50: 34793484.
  • 46
    McAleese F, Petersen P, Ruzin A et al. A novel MATE family efflux pump contributes to the reduced susceptibility of laboratory-derived Staphylococcus aureus mutants to tigecycline. Antimicrob Agents Chemother 2005; 49: 18651871.
  • 47
    Muthaiyan A, Silverman JA, Jayaswal RK, Wilkinson BJ. Transcriptional profiling reveals that daptomycin induces the Staphylococcus aureus cell wall stress stimulator and genes responsive to membrane depolarization. Antimicrob Agents Chemother 2008; 52: 980990.
  • 48
    Fowler VG Jr, Boucher HW, Corey GR et al. Daptomycin versus standard therapy for bacteremia and endocarditis caused by Staphylococcus aureus. N Engl J Med 2006; 355: 653665.
  • 49
    Hayden MK, Rezai K, Hayes RA, Lolans K, Quinn JP, Weinstein RA. Development of daptomycin resistance in vivo in methicillin-resistant Staphylococcus aureus. J Clin Microbiol 2005; 43: 52855287.
  • 50
    Munoz-Price LS, Lolans K, Quinn JP. Emergence of resistance to daptomycin during treatment of vancomycin-resistant Enterococcus faecalis infection. Clin Infect Dis 2005; 41: 565566.
  • 51
    Cui LZ, Tominaga E, Neoh HM, Hiramatsu K. Correlation between reduced daptomycin susceptibility and vancomycin resistance in vancomycin-intermediate Staphylococcus aureus. Antimicrob Agents Chemother 2006; 50: 10791082.
  • 52
    Kainer MA, Devasia RA, Jones TF et al. Response to emerging infection leading to outbreak of linezolid-resistant enterococci. Emerg Infect Dis 2007; 13: 10241030.
  • 53
    Long KS, Poehlsgaard J, Kehrenberg C, Schwarz S, Vester B. The Cfr rRNA methyltransferase confers resistance to phenicols, lincosamides, oxazolidinones, pleuromutilins, and streptogramin  A antibiotics. Antimicrob Agents Chemother 2006; 50: 25002505.
  • 54
    Toh SM, Xiong L, Arias CA et al. Acquisition of a natural resistance gene renders a clinical strain of methicillin-resistant Staphylococcus aureus resistant to the synthetic antibiotic linezolid. Mol Microbiol 2007; 64: 15061514.
  • 55
    Mendes RE, Deshpande LM, Castanheira M, DiPersio J, Saubolle MA, Jones RN. First report of cfr-mediated resistance to linezolid in human staphylococcal clinical isolates recovered in the United States. Antimicrob Agents Chemother 2008; 52: 22442246.