Susceptibility to β-lactam antibiotics of Pseudomonas aeruginosa overproducing penicillin-binding protein 3

By using a broad-host-range vector, pUCP27, the Pseudomonas aeruginosa and Escherichia coli pbpB genes, which encode penicillin-binding protein 3 (PBP3), were separately overexpressed in a P. aeruginosa strain, PAO4089, that is deficient in producing chromosomal β-lactamase. Susceptibility studies indicated that overproduction of the P. aeruginosa PBP3 in PAO4089 resulted in twofold-increased resistance to aztreonam, fourfold-increased resistance to cefepime and cefsulodin, and eightfold-increased resistance to ceftazidime, whereas overproduction of the P. aeruginosa PBP3 in PAO4089 did not affect susceptibility to PBP1-targeted cephaloridine or PBP2-targeted imipenem. Similar results were obtained with PAO4089 overproducing E. coli PBP3, with the exception that there was no influence on the MICs or minimal bactericidal concentrations of cefsulodin and cefepime, which have very low affinities for E. coli PBP3. These data are consistent with the conclusion that PBP3 has to achieve a certain level of saturation, with β-lactams targeted to this protein, to result in cell inhibition or death.