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An isolate of Streptococcus pneumoniae from a patient with meningitis is incubated with low-dose radiolabeled ceftriaxone and then subjected to protein electrophoresis. Five distinct bands are detected by radioautography.
Item 1 of 2
These bands most likely represent radiolabeled ceftriaxone that is bound to which of the following?
Penicillins and cephalosporins function by irreversibly binding to penicillin-binding proteins. Transpeptidases are one form of penicillin-binding protein that function to cross-link peptidoglycan in the bacterial cell wall. Inhibition of transpeptidase (which occurs with the use of the cephalosporin ceftriaxone) leads to cell wall instability and bacteriolysis. Many bacterial species synthesize multiple different penicillin-binding proteins; the laboratory study described in this question stem identifies 5 such proteins that are produced by this isolate of Streptococcus pneumoniae.
(Choice A) Vancomycin binds to the terminal D-alanine residues of cell wall glycoproteins and prevents transpeptidases from forming cross-links.
(Choice B) Fluoroquinolones interfere with DNA replication by binding to proteins such as DNA gyrase.
(Choice C) Porins are utilized by bacterial organisms as a means of controlling diffusion of small to medium-sized metabolites. Antibiotic resistance can develop through porin mutations that prevent antibiotic diffusion through the outer membrane of gram-negative bacteria. No known antibiotics function by binding to porins.
(Choice D) Ribosomal proteins are bound by multiple antibiotics including macrolides, tetracyclines, and others.
Educational objective:
Penicillins and cephalosporins function by irreversibly binding to penicillin-binding proteins such as transpeptidases.