bla TEM-1 and bla TEM-176 overexpression underlies atypical ampicillin/sulbactam resistance in non-ESBL Salmonella from raw chicken

AIMS: This study was designed to investigate the basis of atypical ampicillin/sulbactam (SAM) resistance in Salmonella isolates from raw chicken that lacked extended-spectrum or inhibitor-resistant TEM β-lactamase variants. METHODS AND RESULTS: The ampicillin (AMP) minimum inhibitory concentrations (MICs) of all Salmonella isolates and blaTEM expression and copy numbers of select isolates were determined. Plasmids from select Salmonella isolates were conjugated into Escherichia coli recipients, and their AMP MICs, SAM resistance phenotypes, and blaTEM expression were quantified. SAM-resistant Salmonella isolates displayed ∼5.4-fold higher average AMP MIC and significantly elevated blaTEM expression levels compared to SAM-susceptible control. Conjugation experiments revealed differences in AMP MICs and SAM resistance phenotypes between Salmonella donors and E. coli transconjugants. An AMP MIC breakpoint of 1024 ppm ostensibly confers resistance to SAM in Salmonella. CONCLUSION: Host-dependent blaTEM overexpression and resulting β-lactamase hyperproduction can lead to atypical SAM resistance in Salmonella. This study furthers our understanding of the understudied SAM resistance in Salmonella spp.