Mechanisms of antimicrobial action directed against the bacterial cell wall and corresponding resistance mechanisms

Mechanisms of antimicrobial resistance

Subunits for cell wall construction

Cell Wall AssemblyLayer of cell wall with cross links of 5 glycines (gray)Second layer of cell wall cross-linked to the lower layerTranspeptidase (PBP) forms a 5-glycine bridge between peptidesA subunit is added to the growing chain

Transpeptidase, or PBP (orange sunburst)is bound by beta-lactam antibiotic (light blue) and its activity is inhibited (turns gray)

5-glycine crosslinking bridges cannot form in the presence of a beta-lactam, and the cell wall is deformed and weakened

Mechanisms of beta-lactam resistanceDrug-modifying enzymes (beta-lactamases)Gram-positives(e.g., S. aureus) excrete the enzymeGram-negative (e.g., E. coli) retain the enzyme in the periplasmOverexpression of cell wall synthetic enzymese.g., vancomycin-intermediate S. aureus (VISA) Alteration of the PBPs so antibiotic cannot binde.g., S. pneumoniae, gonococcusExclusion from the site of cell wall synthesisPorin mutations in the outer membrane of Gram-negative bacteria only (e.g., Ps. aeruginosa)

Beta-lactamases (dark orange) bind to the antibiotics (light blue) and cleave the beta-lactam ring.

The antibiotic is no longer able to inhibit the function of PBP (orange sunburst)Beta-lactamases

Beta-lactamase activity

Altered drug targets

Vancomycin-intermediate S. aureusvancomycin MIC = 2 g/mlvancomycin MIC =8 g/mlMRSAVISAProduction of excessive cell wall; the antibiotic cannot keep up

MRSAVISA

Mechanism of vancomycin action

Mechanism of vancomycin resistanceVancomycin is unable to bind to the D-ala-D-lactate structure

June 2002: isolated from the catheter exit site in a chronic dialysis patientThe patient had received multiple courses of abx since April 2001; toe amputation in April 2002 --> MRSA bacteremiaVRSA also found at amputation stump wound (with VRE and Klebsiella); not in the patients noseVancomycin MIC >128mcg/ml!! (contains vanA)Sensitive to trim/sulfa, chloro, tetracyclines, Synercid, linezolid

MRSA and penicillin-resistant S. pneumoniaeThese bacteria are both resistant because they have altered bacterial targets -- penicillin-binding proteins (PBPs or transpeptidases)In MRSA, the altered PBP2 (mecA) gene is acquired by gene transfer from another bacterium.In pneumococci, the alteration in PBP is generated by uptake of DNA released by dead oral streptococci and recombination at the pneumococcal pbp gene to create a new, chimeric protein that does not bind penicillin.depicted on the next slide . . .

Alpha-strep

Outer membrane permeability in Gram-negative bacteriaBacterium