Function of penicillin-binding protein 2 in viability and morphology of Pseudomonas aeruginosa

Objectives: To investigate the function of penicillin-binding protein 2 (PBP 2) in Pseudomonas aeruginosa PAO1. Methods: The growth and morphology of P. aeruginosa cultured in the absence and presence of mecillinam was assessed. The gene encoding PBP 2, pbpA, was identified in the genome of P. aeruginosa PAO1 and both its full-length and an engineered truncated form were cloned and expressed in Escherichia coli. Site-directed mutagenesis was used to confirm Ser-327 as the catalytic nucleophile of its transpeptidase domain. Allelic exchange was used to construct a chromosomal mutant of pbpA in strain PAO1. Results: PAO1 grew with a spherical morphology in the presence of mecillinam at concentrations as high as 2000 mg/L. Both wild-type and truncated, soluble forms of PBP 2 were shown to bind penicillins and a competition assay demonstrated their specificity for mecillinam. The PAO1 ΔpbpA insertional mutant also grew as spheres, and complementation with a plasmid encoding active pbpA, but not with an inactive Ser-327 → Ala derivative, restored rod-shape morphology. MIC values of a variety of β-lactams were significantly lower for the insertional mutant compared with wild-type PAO1. The muropeptide profile of peptidoglycan from PAO1 ΔpbpA analysed by HPLC/MALDI TOF MS indicated wild-type levels of cross-linking despite the loss of PBP 2 transpeptidase activity. Conclusions: PBP 2 in P. aeruginosa is responsible for the rod-shape morphology of the cells and contributes significantly to β-lactam resistance. The viability of cells lacking an active PBP 2 suggests that the organization of the peptidoglycan biosynthetic machinery is different in this pathogen compared with E. coli.