Modified penicillin binding proteins as mechanism of resistance to antibiotics.
Penicillin Binding Proteins (Transpeptidases) cross link peptidoglycan which stabilizes the cell wall
 - Beta-lactam antibiotics like PCN and cephalosporins bind PBPS active site and block this function
 - Vancomycin blocks the process by binding the terminal D-Ala, D-Ala
Resistance:
 - Streptococcus pneumoniae will modify the PBPS so that PCN does not bind as well.
THIS CAN BE OVERCOME:
 - Giving more antibiotic (e.g. high dose amox for an ear infection)
 - Switch to cephalosporin (e.g. CTX for a complicated pneumonia or pneumonia in unvaccinated)
 - Add vancomycin to CTX (e.g. pneumococcal meningitis)
RESISTANCE:
 - Methicillin resistant Staphylococcus aureus makes a different transpeptidase which functions even in presence of PCNs or cephalosporins.
 - Vancomycin again remains active as it works at a different site.

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