Characterization of a novel extended-spectrum β-lactamase phenotype from OXA-1 expression in Salmonella Typhimurium strains from Africa and Ireland

Salmonella enterica (S. Typhimurium) strains from Kenya, Malawi, and Ireland showing elevated cefepime MIC values carried blaOXA-1. These strains were not detected by current guidelines for extended-spectrum β-lactamase production based on MIC values or clavulanate inhibition, since cefepime is a preferred substrate for OXA-1 and this enzyme is reported to be resistant to clavulanate inhibition. blaOXA-1 was located within a class I integron and an activated P2 promoter was identified upstream of blaOXA-1. P2 was activated by the insertion of a triplet 'GGG' upstream of the −10 signal. All African strains were ST313, prevalent in this continent, and the Irish isolate belonged to ST19.