In the Philippines, data are scarce on the co-occurrence of multiple β-lactamases (BLs) in clinically isolated Gram-negative bacilli. To investigate this phenomenon, we characterized BLs from various β-lactam-resistant Klebsiella pneumoniae, Escherichia coli, Acinetobacter baumannii, and Pseudomonas aeruginosa isolated from a Philippine tertiary care hospital. The selected Gram-negative bacilli (n = 29) were resistant to either third-generation cephalosporins (resistance category 1 (RC1)), cephalosporins and penicillin-β-lactamase inhibitors (RC2), or carbapenems (RC3). Isolates resistant to other classes of antibiotics but susceptible to early-generation β-lactams were also selected (RC4). All isolates underwent antibiotic susceptibility testing, disk-diffusion-based BL detection assays, and PCR with sequence analysis of extended-spectrum BLs (ESBLs), metallo-BLs, AmpC BLs, and oxacillinases. Among the study isolates, 26/29 harbored multi-class BLs. All RC1 isolates produced ESBLs, with bla_CTX-M as the dominant (19/29) gene. RC2 isolates produced ESBLs, four of which harbored bla_TEM plus bla_OXA-1 or other ESBL genes. RC3 isolates carried bla_NDM and bla_IMP_, particularly in three of the metallo-BL producers. RC4 Enterobacteriaceae carried bla_CTX-M, bla_TEM, and bla_OXA-24-like, while A. baumannii and P. aeruginosa in this category carried either bla_IMP or bla_OXA-24. Genotypic profiling, in complement with phenotypic characterization, revealed multi-class BLs and cryptic metallo-BLs among β-lactam-resistant Gram-negative bacilli.

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