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1. Summary, 46S

2. Introduction, 46S

3. Cell wall composition and structure, 47S

 3.1 Gram-positive bacteria, 47S

 3.2 Mycobacteria, 47S

4. Barrier function of the cell wall, 47S

 4.1 Penetration of Gram-positive bacterial cell walls, 47S

 4.1.1 Observations on the size exclusion limit, 47S

 4.1.2 The effects of charged groups in the cell wall, 49S

 4.1.3 Vancomycin resistance in Staphylococcus aureus, 49S

 4.2 Penetration of mycobacterial cell walls, 50S

 4.2.1 Rifampicin, 50S

 4.2.2 Isoniazid, 50S

 4.2.3 Ethambutol, 51S

 4.2.4 Pyrazinamide, 51S

 4.2.5 Fluoroquinolones, 51S

 4.2.6 β-Lactams, 51S

 4.2.7 Streptomycin, 52S

 5. Conclusions, 52S

 6. References, 52S

1. SUMMARY

  1. Top of page
  2. 1. SUMMARY
  3. 2. INTRODUCTION
  4. 3. CELL WALL COMPOSITION AND STRUCTURE
  5. 4. BARRIER FUNCTION OF THE CELL WALL
  6. 5. CONCLUSIONS
  7. References

Gram-positive bacteria possess a permeable cell wall that usually does not restrict the penetration of antimicrobials. However, resistance due to restricted penetration can occur, as illustrated by vancomycin-intermediate resistant Staphylococcus aureus strains (VISA) which produce a markedly thickened cell wall. Alterations in these strains include increased amounts of nonamidated glutamine residues in the peptidoglycan and it is suggested that the resistance mechanism involves `affinity trapping' of vancomycin in the thickened cell wall. VISA strains have reduced doubling times, lower sensitivity to lysostaphin and reduced autolytic activity, which may reflect changes in the D-alanyl ester content of the wall and membrane teichoic acids. Mycobacterial cell walls have a high lipid content, which is assumed to act as a major barrier to the penetration of antimicrobial agents. Relatively hydrophobic antibiotics such as rifampicin and fluoroquinolones may be able to cross the cell wall by diffusion through the hydrophobic bilayer composed of long chain length mycolic acids and glycolipids. Hydrophilic antibiotics and nutrients cannot diffuse across this layer and are thought to use porin channels which have been reported in many species of mycobacteria. The occurrence of porins in a lipid bilayer supports the view that the mycobacterial wall has an outer membrane analogous to that of Gram-negative bacteria. However, mycobacterial porins are much less abundant than in the Gram-negative outer membrane and allow only low rates of uptake for small hydrophilic nutrients and antibiotics.

2. INTRODUCTION

  1. Top of page
  2. 1. SUMMARY
  3. 2. INTRODUCTION
  4. 3. CELL WALL COMPOSITION AND STRUCTURE
  5. 4. BARRIER FUNCTION OF THE CELL WALL
  6. 5. CONCLUSIONS
  7. References

The cell wall plays a vital role in bacterial growth and survival in hostile environments. Its functions include: support of the delicate cytoplasmic membrane against the high internal osmotic pressure; control of cell shape; mediation of adhesion to surfaces and other cells; involvement in the export of cellular products. With the exception of L-forms and mycoplasma, which lack rigid cell walls, these functions apply to all forms of bacteria. However, the role of the cell wall as a barrier to the penetration of toxic molecules differs considerably between Gram-positive bacteria, Gram-negative bacteria and mycobacteria. The outer membrane of Gram-negative bacteria poses a significant barrier to the penetration of small hydrophilic molecules, restricting their rate of penetration and excluding larger molecules. The detailed information available on the composition and architecture of Gram-negative outer membranes, and the role of porins as aqueous channel-forming proteins has provided a sound basis to support this view. This exclusion barrier is one of the factors which accounts for the greater resistance of Gram-negative bacteria to antimicrobials compared with Gram-positive bacteria, which possess a permeable cell wall. Only recently has resistance due to restricted penetration through the Gram-positive cell wall become a significant problem, with the emergence of vancomycin-resistant Staphylococcus aureus due to a thickened cell wall. Mycobacterial cell walls have a high lipid content, which is assumed to act as a major barrier to the penetration of antimicrobial agents. The emergence of multiple antibiotic resistance in Mycobacterium tuberculosis emphasizes the need to develop new agents, which are not affected by this uptake barrier.

As more information is gained on the structure of mycobacterial cell walls and the mechanisms of action of antimycobacterial agents, a clearer view of its role as a penetration barrier is beginning to emerge.

3. CELL WALL COMPOSITION AND STRUCTURE

  1. Top of page
  2. 1. SUMMARY
  3. 2. INTRODUCTION
  4. 3. CELL WALL COMPOSITION AND STRUCTURE
  5. 4. BARRIER FUNCTION OF THE CELL WALL
  6. 5. CONCLUSIONS
  7. References

3.1 Gram-positive bacteria

1Figure 1a gives a schematic view of the arrangement of components in the cell walls of Gram-positive bacteria. Gram-positive walls have an open, hydrophilic structure, which retains the cell shape when isolated and purified. The major component is peptidoglycan, which accounts for 50% of the weight of the wall (Koch 2000). Linear anionic polymers, termed teichoic or teichuronic acids are covalently linked to the peptidoglycan, giving the wall a net negative charge. Teichoic acids are linear polymers of repeating units of ribitol or glycerol units linked by phosphodiesters. Teichuronic acids do not contain phosphate, instead they are made up from linear chains of sugar units bearing uronic acid residues. Another important form of teichoic acid found in the Gram-positive cell wall is lipoteichoic acid (LTA). LTA is a glycerolphosphate teichoic acid chain linked covalently to a glycolipid (typically a glycosyl diglyceride) located on the outer face of the cytoplasmic membrane. The glycerophosphate chain extends through the cell wall and is exposed on the cell surface. A number of functionally important proteins are also found both covalently and noncovalently linked to peptidoglycan. These mediate interactions between cells and their environment. Many pathogens interact specifically with host cells and tissues in infections by producing surface-exposed proteins which bind to host proteins. Some Gram-positive bacteria also produce capsular polysaccharides, which are loosely associated with the cell wall. Capsules form an additional barrier around the cells, protecting against engulfment by predatory cells in natural environments and by host phagocytic cells in infection.

image

Figure 1.  Schematic representations of the arrangement of components in the cell walls of Gram-positive bacteria (a) and mycobacteria (b). CM, cytoplasmic membrane; PG, peptidoglycan; TA, teichoic acid; TU, teichuronic acid; AG, arabinogalactan; MY, mycolic acid; GL glycolipids (various species indicated by different symbols). Both walls contain proteins, indicated by shaded areas. The Gram-positive wall contains lipoteichoic acid, linked to the cytoplasmic membrane by a glycolipid (□). The mycobacterial wall contains lipoarabinomannan, possibly linked to the membrane by a phosphatidylinositol lipid (□)

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3.2 Mycobacteria

The mycobacterial cell wall, shown schematically in Fig. 1(b), has a more complex structure than Gram-positive bacterial cell walls (Brennan and Nikaido 1995; Daffe and Draper 1998; Draper 1998). It is rich in high molecular weight lipids (Christensen et al. 1999) which form a protective barrier analogous to the outer membrane of Gram-negative bacteria (Jarlier and Nikaido 1994). The inner region of the wall contains peptidoglycan linked to a second polysaccharide polymer, arabinogalactan. These components form the basic structural skeleton of the cell wall. Their precise arrangement in the wall is not known, although a recent model suggests that the glycan chains of peptidoglycan are organized perpendicularly to the cell wall (Dmitriev et al. 2000). Mycolic acids are long chain length branched fatty acids, typically containing 70–90 carbon atoms, which are characteristic of mycobacteria and account for up to 60% of the whole cell dry weight. They are covalently linked to the arabinogalactan polymer, forming the inner region of the thick waxy coat surrounding the peptidoglycan/arabinogalactan skeleton. The outer surface of this coat contains a number of other complex species of lipids and waxes, including glycopeptidolipids, trehalose-containing lipooligosaccharides, sulpholipids, phthiocerol dimycocerosate and phenolic glycolipids. Spanning the outer waxy layer are porin proteins which have similar properties to those of the Gram-negative outer membrane, providing a route of access for low molecular weight hydrophilic nutrients. Lipoarabinomycolate (LAM) is thought to be anchored via phosphatidylinositol to the outer face of the cytoplasmic membrane and protrude through the wall to the outer cell surface in the same manner as LTA in Gram-positive bacteria. Although not well characterized, many mycobacteria are thought to produce a capsule external to the cell wall containing polysaccharide and protein (Draper 1998).

4. BARRIER FUNCTION OF THE CELL WALL

  1. Top of page
  2. 1. SUMMARY
  3. 2. INTRODUCTION
  4. 3. CELL WALL COMPOSITION AND STRUCTURE
  5. 4. BARRIER FUNCTION OF THE CELL WALL
  6. 5. CONCLUSIONS
  7. References

4.1 Penetration of Gram-positive bacterial cell walls

4.1.1 Observations on the size exclusion limit.

Measurements of the penetration of polysaccharides show that peptidoglycan and its associated anionic polymers provide an open network which is accessible to molecules of molecular weights in the range 30 000 to 57 000 Da (Scherrer and Gerhardt 1971). Most antimicrobials are therefore not excluded on the basis of their size alone. Biocides such as the phenols, alcohols, aldehydes, quaternary ammonium compounds and bisbiguanides are all small molecules, which penetrate the wall with ease. Gram-positive bacteria are generally sensitive to all these classes of antimicrobial agent (Russell 1999).

Resistance of Gram-positive bacteria to antibiotics is generally related to mechanisms involving destruction or inactivation, changes in the target site, or active efflux (Russell 1998). Exclusion by the cell wall is not usually responsible for resistance. Among the larger-sized antibiotics, glycopeptides (1240 Da), rifampicin (823 Da) and fusidic acid (516 Da) are notable for their activity against Gram-positive bacteria. Antimicrobial peptides such as nisin (3354 Da) and defensins (3000–3500 Da) are able to penetrate the wall to interact with the cytoplasmic membrane (Friedrich et al. 2000). There is, however, an upper limit to penetration of the wall, which is illustrated by proteins of the innate host defence system of the body. Lysozyme (14 400 Da) can reach the peptidoglycan in the cell wall and secreted phospholipase A2 (14 000 Da) can penetrate to the cell membrane to reach its phospholipid target, phosphatidyl glycerol (Foreman-Wykert et al. 1999; Buckland and Wilton 2000). Although complement C3 (180 000 Da) binds to teichoic acid on the surface of the wall, the membrane attack complex cannot penetrate to the cytoplasmic membrane. Consequently Gram-positive bacteria are opsonized by exposure to complement but are not killed.

Another indication that the exclusion limit of the wall is around 50 000 Da comes from studies using fluorescent in situ hybridization (FISH). Sensitive detection of whole cells of Lactobacillus lactis using horseradish peroxidase-labelled oligonucleotide probes (44 000 Da for HRP and 6000 for the 20-mer oligonucleotide) followed by the fluorescent tyramide molecule requires permeabilization of the wall by autolytic action or treatment with exogenous enzymes (Bidnenko et al. 1998). The peptidoglycan layer is therefore considered to present a barrier to penetration of the HRP-labelled probes. The permeability properties of Gram-positive walls may be dependent upon the nature of the peptidoglycan, particularly its degree of cross-linking and glycan chain length. Studies on the uptake kinetics of fluorescent nucleic acid binding dyes by different Gram-positive bacteria suggest that the wall of Bacillus cereus is more permeable than walls of Staph. aureus and Enterococcus faecalis to mithramycin and ethidium bromide (Walberg et al. 1999). Observations on the release of exocellular products from whole cells provide a complementary illustration of the large size exclusion properties of the wall. Enzymes such as the β-lactamase (20 000–30 000 Da), haemolysins and other exocellular toxins can all diffuse through the wall after crossing the cytoplasmic membrane.

4.1.2 The effects of charged groups in the cell wall.

The negative charge generated by the phosphate groups on teichoic acids and carboxylate groups on teichuronic acids and peptidoglycan might be expected to influence the penetration of charged antibiotics. However, Gram-positive bacteria are sensitive to cationic agents (aminoglycosides or quaternary ammonium compounds) and to anionic agents (fusidic acid, quinolones), provided no other resistance mechanism is involved. This suggests that penetration of the wall by charged molecules is neither hindered nor assisted by the negative charges in the wall polymers. Aminoglycosides bind to the negatively charged phosphate groups on teichoic acids, displacing magnesium ions. Mutants lacking teichoic acid do not bind aminoglycosides in this manner but do not show significantly altered sensitivity, indicating that this interaction is of little importance in the overall cellular uptake (Kusser et al. 1985).

4.1.3 Vancomycin resistance in Staphylococcus aureus.

Peptidoglycan is an indispensable component of bacterial cell walls. It is responsible for the shape and integrity of the bacterial cell and has no counterpart in mammalian cells. Agents which interfere with peptidoglycan synthesis (including β-lactams and glycopeptides) are highly active and selective antibacterial agents, provided they are able to penetrate the cell wall and reach their target sites. The targets for β-lactams and glycopeptides are dual-acting transpeptidase–transglycosylase (Tpase–TGase) enzymes located on the outer face of the cytoplasmic membrane (Goldman and Gange 2000). The TGase produces linear strands of peptidoglycan and is the target of the glycopeptides, the TPases cross-link the peptides in the peptidoglycan and are the targets of the β-lactams. Both types of antibiotic must cross the wall to reach their targets.

Methicillin-resistant Staph. aureus (MRSA) and vancomycin-resistant enterococci (VRE) strains owe their resistance to changes in the TPases and peptidoglycan precursors of the TGase, respectively. Target site changes involve expression of the alternative penicillin binding protein, MecA (PBP2′) in MRSA and an altered peptidoglycan precursor terminating in D-alanyl-D-lactate rather than D-alanyl-D-alanine in VRE. In both cases expression of a number of associated genes is required to provide modified peptidoglycan precursors for full resistance. Vancomycin (glycopeptide) resistance in Staph. aureus was expected to result from transfer of vancomycin resistance (van) genes from VRE to MRSA (Smith and Jarvis 1999). Although this transfer has been demonstrated experimentally (Noble et al. 1992), it is not responsible for the intermediate level of resistance reported to date in laboratory or clinical isolates. The vancomycin-intermediate Staph. aureus (VISA) phenotype seen in passage-selected vancomycin-resistant Staph. aureus strains involves production of thickened and diffuse cell walls following growth in subinhibitory levels of vancomycin (Pfeltz et al. 2000). Decreased doubling times, lower sensitivity to lysostaphin and reduced autolytic enzyme activity all indicate an altered cell wall structure. Vancomycin was shown to inhibit cell wall turnover and autolysis in another laboratory VISA strain (Sieradski and Tomasz 1997). The D-alanine substituents present on the wall and membrane teichoic acids influence vancomycin resistance, since cells lacking D-alanyl esters in their teichoic acids have increased sensitivity to glycopeptides and lysostaphin, but show decreased autolytic activity (Peschel et al. 2000).

Studies on Mu50, a clinical isolate of Staph. aureus with reduced sensitivity to vancomycin (termed glycopeptide-intermediate Staph. aureus, GISA) have shown a markedly thickened cell wall with increased amounts of nonamidated glutamine residues in the peptidoglycan (Cui et al. 2000). It has been suggested that the mechanism of resistance involves trapping of vancomycin in the thickened cell wall (`affinity-trapping'), denying access to the transglycosylase target on the outer face of the cytoplasmic membrane. Reduced cross-linking of the peptidoglycan and an increased affinity of binding vancomycin by the nonamidated peptidoglycan might also contribute to the resistance. Comparative studies on peptidoglycan structure in a range of different clinical GISA strains suggest that a spectrum of alterations in peptidoglycan occurs and that an increase in the glutamate content alone is not responsible for resistance in all cases (Boyle-Vavra et al. 2001).

4.2 Penetration of mycobacterial cell walls

The lipid-rich mycobacterial cell wall presents a significant barrier to the penetration of antimicrobials (Brennan and Nikaido 1995; Draper 1998; McDonnell and Russell 1999). Relatively hydrophobic antibiotics such as rifampicin and fluoroquinolones may be able to cross the cell wall by diffusion through the hydrophobic bilayer composed of long chain length mycolic acids and glycolipids. Hydrophilic antibiotics and nutrients cannot diffuse across this layer and are thought to use porin channels which have been reported in many species of mycobacteria. The occurrence of porins in a lipid bilayer supports the view that the mycobacterial wall has an outer membrane analogous to that of Gram-negative bacteria. However, mycobacterial porins are much less abundant than in the Gram-negative outer membrane (Trias et al. 1992) and allow only low rates of uptake for small hydrophilic nutrients and antibiotics.

4.2.1 Rifampicin.

Rifampicin is a broad spectrum antibiotic with potent activity against Myco. tuberculosis. It inhibits RNA synthesis by binding to the β-subunit of RNA polymerase, effectively blocking transcription. Resistance to rifampicin occurs readily and is usually caused by spontaneous mutations in one of three loci in the RNA polymerase gene (rpoB). The route by which rifampicin crosses the mycobacterial cell wall is not clear; it is too large to pass through the porin channels and presumably crosses the hydrophobic wall by diffusion. Piddock et al. (2000) measured the accumulation of rifampicin by Myco. tuberculosis, Myco. aurum and Myco. smegmatis, determining the amount of `intracellular' drug by subtraction of surface adsorbed drug. Uptake of rifampicin was greatly enhanced by growth of Myco. aurum in subinhibitory levels of ethambutol. The effect was less pronounced, but still significant with the other organisms, suggesting that the arabinogalactan content of the wall influences rifampicin uptake and possibly explaining the observed antimycobacterial synergy between rifampicin and ethambutol. In the same study, efflux pump inhibitors were employed to demonstrate a small but reproducible activity of efflux systems towards rifampicin in the strains (Piddock et al. 2000).

4.2.2 Isoniazid.

Isoniazid (Fig. 2) is a small, essentially hydrophilic molecule which should enter mycobacteria via the porin channels, however, studies on Myco. tuberculosis mutants with altered mycolate content showed unexpected features to its uptake. Inactivation of antigen 85C, one of three mycoloyltransferases produced by the organism, produced a 40% reduction in the transfer of mycolic acids to arabinogalactan in the cell wall (Jackson et al. 1999). This resulted in faster diffusion of the hydrophobic detergent, chenodeoxycholate and the hydrophilic nutrient, glycerol into the cells but had no effect on the rate of uptake of isoniazid. Selective antimycobacterial activity of isoniazid derives from its interference with the synthesis of mycolic acids. It is a prodrug which is converted after uptake by the catalase-peroxidase enzyme, KatG to the active species, 4-pyridylmethanol. Mycolic acid synthesis is blocked by the activated species by noncovalent binding to the target enzyme, InhA, an enoyl acyl carrier protein reductase (Lei et al. 2000). This inhibition results in accumulation of mycolate intermediates and cell lysis (Vilcheze et al. 2000). The related antimycobacterial agent, ethionamide, probably acts in a similar manner to isoniazid since mutation in the inhA gene results in resistance to both agents (Banerjee et al. 1994).

image

Figure 2.  The structures of some antimycobacterial agents used in the treatment of tuberculosis (pyrazinamide, isoniazid, ethionamide, ethambutol) and related compounds (pyrazinoic acid, nicotinamide, 4-pyridylmethanol)

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Isoniazid is very potent against Myco. tuberculosis but 100-fold less active against Myco. avium. Although the different sensitivities of these organisms have been attributed to different penetration characteristics through the walls, direct measurement of uptake of isoniazid shows no significant difference between the two species. The sensitivity of Myco. tuberculosis appears to result from its four-fold greater conversion rate of isoniazid compared with Myco. avium (Mdluli et al. 1998).

4.2.3 Ethambutol.

Like isoniazid, ethambutol (Fig. 2) is a small, hydrophilic agent and should therefore cross the mycobacterial wall via the porin channels. Ethambutol exerts its selective antimycobacterial activity by interfering with the synthesis and assembly of arabinogalactan (Takayama and Kilburn 1989), with secondary effects upon lipoarabinogalactan (Deng et al. 1995). The exact site of action remains to be determined but appears to be the arabinosyl transferases involved in assembly of the arabinan portion of the polysaccharide rather than enzymes involved in formation of precursors. Interference with arabinan assembly diverts the incorporation of mycolic acids from the arabinogalactan to free lipids, which accumulate in the wall (Mikusova et al. 1995). Changes in the structure of the wall resulting from ethambutol action are indicated by conversion in cell shape to a more spherical form (Kilburn and Greenberg 1977). Such changes could result in increased permeability and explain its synergistic action with other antimycobacterial agents (Rastogi et al. 1990).

4.2.4 Pyrazinamide.

Pyrazinamide (Fig. 2) is also a pro-drug analogue of nicotinamide, requiring activation after uptake by the enzyme, pyrazinamidase (encoded by the pncA gene of Myco. tuberculosis), to the presumed active species, pyrazinoic acid (Cheng et al. 2000). Its mechanism of action remains to be determined, but might result from internal acidification (Boshoff and Mizrahi 2000). Resistance to pyrazinamide occurs through mutations in the pyrazinamidase gene. Additional mechanisms of resistance involve exclusion of the pro-drug. This has been investigated in a number of naturally sensitive and resistant mycobacterial species (Raynaud et al. 1999). Myco. tuberculosis, a pyrazinamide-susceptible species, exhibited both pyrazinamide uptake and pyrazinamidase activity whereas four naturally resistant species Myco. avium, Myco. bovis BCG, Myco. smegmatis and Myco. kansasii lacked either or both activities. The same study provided evidence that pyrazinamide crosses the cell wall of Myco. tuberculosis via the OmpATb porin channels and that the mycolate content of the cell wall does not influence the uptake. An ATP-dependent uptake system appears to be involved in transporting the drug across the cytoplasmic membrane although interpretation of experiments involving the direct measurement of uptake of labelled drug by whole cells should also consider the potential effects of active efflux systems. The unique sensitivity of Myco. tuberculosis and intrinsic resistance of other mycobacteria may be due to the lack of an efflux system in Myco. tuberculosis (Zhang et al. 1999).

4.2.5 Fluoroquinolones.

The activity of fluoroquinolones against mycobacteria generally increases with the hydrophobicity of the agent (Haemers et al. 1990), supporting the view that these agents enter cells by diffusion across the lipid rich wall. Resistance of the cells is chiefly dependent upon the activity of efflux pumps (Poole 2000) and mutations in the DNA gyrase targets of these agents.

4.2.6 β-Lactams.

Cephalosporin penetration rates through the porin channel of Myco. chelonae are 1000 times lower than those for Escherichia coli and 10 times lower than those for Pseudomonas aeruginosa, one of the least permeable Gram-negative bacteria (Jarlier and Nikaido 1990). Permeabilities vary among different mycobacterial species, Myco. smegmatis (Trias and Benz 1994) and Myco. tuberculosis (Chambers et al. 1995) give cephalosporin penetration rates approximately 10 times higher than those for Myco. chelonae. The overall sensitivity of mycobacteria to β-lactam antibiotics depends on the growth rate of the organism, the expression of β-lactamase and sensitivity of penicillin binding proteins as well as the permeability. Relative contributions of these factors to β-lactam resistance vary with the species investigated (Jarlier et al. 1991; Mukhopadhyay and Chakrabarti 1997; Quinting et al. 1997).

4.2.7 Streptomycin.

Streptomycin is an aminocyclitol glycoside antibiotic which still has a role in the treatment of tuberculosis, despite its toxicity. Its action involves interference with protein synthesis at the proof reading stage of translation (Ruusala and Kurland 1984). High level resistance of clinical Myco. tuberculosis strains results from mutation in the genes encoding the ribosomal protein S12, whilst intermediate level resistance results from mutation in the 16S rRNA genes (Bottger 1994; Honore and Cole 1994). Low level streptomycin resistance shown in about one third of clinically resistant strains is thought to be due to restricted permeability (Meier et al. 1996).

5. CONCLUSIONS

  1. Top of page
  2. 1. SUMMARY
  3. 2. INTRODUCTION
  4. 3. CELL WALL COMPOSITION AND STRUCTURE
  5. 4. BARRIER FUNCTION OF THE CELL WALL
  6. 5. CONCLUSIONS
  7. References

Restricted permeability is undoubtedly one important factor in determining resistance and no agent will be effective unless it can penetrate the cell wall. However, antimicrobial resistance is rarely due to the permeability barrier of the cell wall alone. In many published studies on resistance, restricted permeability is often cited as the likely cause when no evidence can been found for other mechanisms. Such conclusions need to be supported by accurate measurements of uptake, together with a measure of the contribution of efflux activity, and the final cellular location of the drug to distinguish true uptake from surface binding. It is important to view restricted permeability of the cell wall as one contributory factor acting in concert with other mechanisms, including expression of efflux systems, inactivating enzymes and changes in target sites (Chopra 1998). With the increasing availability of genomic sequences the identification of resistance genes in organisms which are either difficult or dangerous to culture is an attractive and exciting prospect (Rosamond and Allsop 2000). This approach has already led to the identification and study of porins (Senaratne et al. 1998) and efflux genes (Silva et al. 2001) in Myco. tuberculosis.

References

  1. Top of page
  2. 1. SUMMARY
  3. 2. INTRODUCTION
  4. 3. CELL WALL COMPOSITION AND STRUCTURE
  5. 4. BARRIER FUNCTION OF THE CELL WALL
  6. 5. CONCLUSIONS
  7. References
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