Molecular characteristics of extended-spectrum β-lactamases in clinical isolates of Enterobacteriaceae from the Philippines

β-Lactamases, including extended-spectrum β-lactamases (ESBLs) and AmpC β-lactamases, are major resistance mechanisms of Enterobacteriaceae. Emergence of plasmid-mediated quinolone resistance (PMQR) determinants in ESBL-producing isolates poses a global threat. The molecular characterisitcs of ESBL and PMQR determinants in the Philippines are not well characterized. In this study, we investigated ESBLs and AmpC β-lactamases in clinical isolates of Enterobacteriaceae from the Philippines, and analyzed the association between ESBL and PMQR genes. A total of 91 amoxicilin non-susceptible Enterobacteriaceae were collected at the Research Institute for Tropical Medicine of the Philippines from 2006 to 2008. AmpC- or ESBL-producing isolates were screened by detecting a zone diameter for cefoxitin ≤ 14 mm or cefpodoxime ≤ 20 mm, respectively. Possible ESBL-producing strains were assessed by the ESBL confirmation test of the Clinical and Laboratory Standards Institute. PCR and sequencing were performed to detect the ESBL and PMQR genes. The number of ESBL-producers and AmpC-producers confirmed phenotypically was 17 (18.7%) and 61 (67.0%), respectively. Of 17 phenotypic ESBL-producers, 14 isolates had ESBL genes, including 6 of Escherichia coli, 3 of Enterobacter cloacae, 2 of Enterobacter aerogenes, 2 of Klebsiella pneumoniae, and 1 of Klebsiella ozaenae. Among these isolates, there were 13, 4, and 12 with blaCTX-M, blaSHV, and blaTEM, respectively. Of the blaCTX-M-positive isolates, blaCTX-M-15 shows the highest prevalence, followed by blaCTX-M-3 and blaCTX-M-14. Of 14 ESBL-producers identified by PCR, 4, 6, and 7 isolates were positive for qnrB, qnrS, and aac(6′)-Ib-cr, respectively. The frequency of aac(6′)-Ib-cr positivity was significantly higher among CTX-M-15-producing isolates. Thus, we identified blaCTX-M, aac(6′)-Ib-cr, and qnr in ESBL-producing Enterobacteriaceae from the Philippines, and revealed a significant association between blaCTX-M-15 and aac(6′)-Ib-cr. Local epidemiological data are important for implementing appropriate antimicrobial therapy and effective infection control measures. Continuous monitoring of antimicrobial resistance genes in the Philippines will be required.