Molecular and biochemical characterization of novel PAM-like MBL variants, PAM-2 and PAM-3, from clinical isolates of Pseudomonas tohonis

Background: There is no comprehensive study on PAM-like MBLs.

Objectives: Our aim was to characterize novel B3 MBL variants, PAM-2 and PAM-3, from Pseudomonas tohonis clinical isolates.

Methods: We evaluated the antimicrobial susceptibility and the MBL gene composition of three novel P. tohonis clinical isolates identified at a Japanese hospital, using the broth microdilution method and WGS, respectively. We characterized the PAM-2 and PAM-3 proteins using recombinant protein expression and biochemical evaluations.

Results: Low carbapenem MICs (meropenem MIC = 0.125-1 mg/L) were observed for all three P. tohonis isolates; however, the isolates produced MBLs. We identified blaPAM-2 and blaPAM-3 as potential genes, belonging to a novel subclass of B3 MBLs. Their genomic sequence was similar to that of blaPAM-1 from Pseudomonas alcaligenes. PAM-2 and PAM-3 comprised 287 amino acids and exhibited 90% amino acid identity with PAM-1, 73% identity with POM-1 from Pseudomonas otitidis and 61% identity with L1 from Stenotrophomonas maltophilia. Biochemical evaluations of recombinant PAM-2 and PAM-3 revealed similar kcat/Km ratios and demonstrated catalytic activity against all the tested β-lactams, except for aztreonam. In addition, the kcat/Km ratio for imipenem was 40-fold lower than that for meropenem.

Conclusions: P. tohonis harbours a species-specific PAM-family MBL gene. This enzyme has higher hydrolytic activity against meropenem compared with that against imipenem.