Role of inducers in detection of bla(PDC)-mediated oxyimino-cephalosporin resistance in Pseudomonas aeruginosa

BACKGROUND & OBJECTIVES: Pseudomonas aeruginosa possessing chromosomally inducible bla(PDC) along with other intrinsic mechanism causes infection with high mortality rate. It is difficult to detect inducible AmpC enzymes in this organism and is usually overlooked by routine testing that may lead to therapeutic failure. Therefore, three different inducers were evaluated in the present study to assess their ability of induction of bla(PDC) in P. aeruginosa. METHODS: A total of 189 consecutive Pseudomonas isolates recovered from different clinical specimens (November 2011-April 2013) were selected for the study. Isolates were screened with cefoxitin for AmpC β-lactamases and confirmed by modified three-dimensional extract test (M3DET). Inductions were checked using three inducers, namely, clavulanic acid, cefoxitin and imipenem along with ceftazidime. Molecular screening of AmpC β-lactamase genes was performed by PCR assay. Antimicrobial susceptibility and minimum inhibitory concentrations (MICs) were determined, and repetitive extragenic palindromic-PCR of all bla(PDC) harbouring isolates was performed. RESULTS: Inducible phenotype was observed in 42 (24.3%) of 97 (56%) isolates confirmed by M3DET. Among these, 22 isolates harboured chromosomal bla(PDC) gene, and cocarriage of both chromosomal and plasmid-mediated bla(AmpC) genes was observed in seven isolates. Cefoxitin-ceftazidime-based test gave good sensitivity and specificity for detecting inducible AmpC enzymes. Isolates harbouring bla(PDC) showed high MIC against all tested cephalosporins and monobactam. DNA fingerprinting of these isolates showed 22 different clones of P. aeruginosa. INTERPRETATION & CONCLUSIONS: P. aeruginosa harbouring inducible (chromosomal) and plasmid-mediated AmpC β-lactamase is a matter of concern as it may limit therapeutic option. Using cefoxitin-ceftazidime-based test is simple and may be used for detecting inducible AmpC β-lactamase amongst P. aeruginosa.