Penicillin-binding proteins and role of amdinocillin in causing bacterial cell death

The activity of penicillins against bacteria is in large part related to binding to specific receptor proteins involved in cell wall biosynthesis. These proteins have been designated penicillin-binding proteins. They can be separated into distinct entities through the use of acrylamide gel electrophoresis and binding of radioactive 14C-labeled penicillin G. Six major proteins have been defined in the Enterobacteriaceae, penicillin-binding proteins 1 to 6. Selection of mutants has shown that there are three essential proteins: penicillin-binding protein 1, which is divided into penicillin-binding protein 1Bs, a peptidoglycan transpeptidase, and penicillin-binding protein 1A, which acts as a replacement for penicillin-binding protein 1Bs. Penicillin-binding protein 2 is a murein-elongation initiating enzyme and penicillin-binding protein 3 is a septal murein-synthesizing enzyme. Penicillin-binding proteins 4, 5, and 6 are not essential for bacterial survival. Binding of penicillins to penicillin-binding protein 1Bs produces lysis, binding to penicillin-binding protein 2 produces round cells, and binding to penicillin-binding protein 3 produces long filaments. Amdinocillin is a beta-amidino penicillanic acid derivative that binds specifically to penicillin-binding protein 2. The compound is more beta-lactamase stable than ampicillin and has no major delay in entry into the periplasmic space as do some penicillins. Amdinocillin inhibits most of the Enterobacteriaceae, with the exception of some indole-positive Proteus species, but it does not inhibit gram-positive cocci or Pseudomonas aeruginosa. Amdinocillin produces spherical bacterial cells that eventually lyse. Its activity in vitro is markedly affected by ionic content of media. This agent acts synergistically with many penicillins, such as ampicillin, carbenicillin, and the like, and with cephalosporins, cefazolin, cefamandole, or cefoxitin to inhibit gram-negative bacilli, probably on the basis of binding to different proteins needed for the production of the peptidoglycan of the bacterial cell wall. Amdinocillin possesses a number of the essentials for effective antimicrobial activity and, by virtue of its enhancement of the activity of other beta-lactams, may prove to be a useful agent in the chemotherapy of certain infections.