Mechanisms of antimicrobial action directed against the
bacterial cell wall and corresponding resistance
mechanisms
Mechanisms of antimicrobial action directed against the
bacterial cell wall and corresponding resistance
mechanisms
M-4 Advanced Therapeutics Course
Mechanisms of antimicrobial resistanceMechanisms of antimicrobial resistance
Drug-modifying enzymes(e.g., - lactamases, aminoglycoside- modifying enzymes)
Altered drug targets (e.g., PBPs ribosomes, DNA gyrase)
Altered uptake oraccumulation of drug(e.g., altered porins, membrane efflux pumps)
Subunits for cell wall construction
D-ala-D-ala
pentapeptide
N-acetylmuramic acid N-acetylglucosamine
Cell Wall AssemblyCell Wall Assembly
Layer of cell wall with cross links of 5 glycines (gray)
Second layer of cell wall cross-linked to the lower layer
Transpeptidase (PBP) forms a 5-glycine bridge between peptides
A 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 resistance
• Drug-modifying enzymes (beta-lactamases)–Gram-positives(e.g., S. aureus) excrete the enzyme
–Gram-negative (e.g., E. coli) retain the enzyme in the periplasm
• Overexpression of cell wall synthetic enzymes–e.g., vancomycin-intermediate S. aureus (VISA)
• Alteration of the PBPs so antibiotic cannot bind–e.g., S. pneumoniae, gonococcus
• Exclusion from the site of cell wall synthesis–Porin 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 activityBeta-lactamase activity
Altered drug targets
Vancomycin-intermediate S. aureus
vancomycin MIC = 2 µg/ml vancomycin MIC =8 µg/ml
MRSA VISA
Production of excessive cell wall; the antibiotic cannot keep up
MRSA VISA
Mechanism of vancomycin actionMechanism of vancomycin action
D-ala-D-ala
V
Mechanism of vancomycin resistanceMechanism of vancomycin resistance
V
D-ala-D-lactate
Vancomycin is unable to bind to the D-ala-D-lactate structure
·June 2002: isolated from the catheter exit site in a chronic dialysis patient·The patient had received multiple courses of abx since April 2001; toe amputation in April 2002 --> MRSA bacteremia·VRSA also found at amputation stump wound (with VRE and Klebsiella); not in the patient’s nose·Vancomycin MIC >128mcg/ml!! (contains vanA)·Sensitive to trim/sulfa, chloro, tetracyclines, Synercid, linezolid
MRSA and penicillin-resistant S. pneumoniae
• These 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 . . .
S. pneumoniae chromosomal pbp; penicillin-sensitive
alpha-strep pbp
alpha-strep pbp
Chimeric pbp (resistant to penicillin)
Alpha-strep
transformationS. pneumoniae
DNA
Outer membrane permeability in Gram-negative bacteria
Inner membrane
Outer membrane
Cell wall(peptidoglycan)
Cytoplasm
Beta-lactam (blue) enters through an outer membrane porin channel
Altered porin channel prevents access of the antibiotic to the cell wall
Bacterium