Antibiotic Resistance/Susceptibility Profiles of <em>Staphylococcus equorum </em>Strains from Cheese, and Genome Analysis for Antibiotic Resistance Genes

Abstract

In this work, the resistance/susceptibility (R/S) profile of Staphylococcus equorum strains (n=30) from cheese to 16 antibiotics was determined by broth microdilution. The minimum inhibitory concentration (MIC) for all antibiotics was low in most strains, although higher MICs compatible with acquired genes were also noted. Genome analysis of 13 strains showed the S. equorum resistome to be composed of intrinsic mechanisms, acquired mutations, and acquired genes. However, the genetic data did not always correlate with the phenotype. As such, a cat gene providing resistance to chloramphenicol was found on a plasmid in one strain; this was able to provide resistance to Staphylococcus aureus after electroporation. An msr(A) polymorphic gene was identified in five strains. The Mrs(A) variants were associated with variable resistance to erythromycin. All strains harboured a polymorphic fosB/fosD gene, although only one acquired copy was associated with strong resistance to fosfomycin. Similarly, a plasmid-associated blaR1-blaZI operon encoding a penicillinase was identified in five ampicillin- and penicillin G-susceptible strains. Identified genes not associated with resistance further included mph(C) in two strains and norA in all strains. The antibiotic R/S status and gene content of S. equorum strains intended to be employed in food systems should be carefully determined.