Occurrence of extended-spectrum and AmpC β-lactamases in multiple drug resistant Salmonella isolates from clinical samples in Lagos, Nigeria

PURPOSE Salmonella spp. are important foodborne pathogens exhibiting increasing resistance to antimicrobial drugs. Resistance to broad-spectrum β-lactams, mediated by extended-spectrum β-lactamase (ESBL) and AmpC β-lactamase enzymes is fast spreading and has had negative impacts on the clinical outcomes, particularly on third-generation cephalosporins. This study investigated the carriage of AmpC gene among multidrug-resistant Salmonella spp. from Lagos, Nigeria. METHODS Forty Salmonella spp. from clinical samples (S. typhi = 13; S. typhimurium = 10; S. enteritidis = 8; S. choleraesuis = 5; S. paratyphi = 4) were subjected to in vitro susceptibility test by disk diffusion methods. Isolates that were resistant to cefoxitin and third-generation cephalosporins were screened for ESBL (Double Disk Synergy Test Method) and AmpC enzyme (AmpC disk test) production. Detection of AmpC fox gene was carried out by polymerase chain reaction. RESULTS Thirty-two (80%) of the Salmonella isolates were cefoxitin resistant. Plasmid-mediated AmpC β-lactamase and ESBL enzymes were recorded in 10/40 (25%) and 16/40 (40%) of the Salmonella isolates, respectively. Specifically, 16/40 (40%) of the Salmonella isolates possessed 380 bp AmpC fox gene, with the highest occurrence found in S. typhi strains (43.8%) followed by S. typhimurium (25%). There was no AmpC fox gene detected in S. paratyphi strains. Interestingly, coproduction of enzymes occurred in some of the isolates, raising fears of resistance to a multitude of antibiotics in the treatment of bacterial infections. CONCLUSION Emergence of AmpC β-lactamase-producing Salmonella isolates in our environment was recorded for the first time, raising concern on increased antibiotic resistance among strains of Salmonella serovars in Lagos. Further genotypic study of the isolates could answer the questions on strain sources, clonal relatedness, and mechanism of spread.