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

In food, bacteria carrying antibiotic resistance genes could play a prominent role in the spread of resistance. Staphylococcus equorum populations can become large in a number of fermented foods, yet the antibiotic resistance properties of this species have been little studied. In this work, the resistance/susceptibility (R/S) profile of S. 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. As such, a plasmidic cat gene providing resistance to chloramphenicol was found 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. However, the genetic data did not always correlate with the phenotype. As such, all strains harbored 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 system was identified in five ampicillin- and penicillin G-susceptible strains. Identified genes not associated with phenotypic 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.

Automated summary

Bacteria carrying antibiotic resistance genes in food can contribute to the spread of resistance. This study focused on the antibiotic resistance properties of Staphylococcus equorum strains from cheese. The resistance/susceptibility profile of 30 strains to 16 antibiotics was determined, revealing low minimum inhibitory concentrations (MICs) for most antibiotics but also higher MICs indicating acquired resistance genes. Genomic analysis identified intrinsic mechanisms, acquired mutations, and acquired genes in the S. equorum resistome. Notably, a plasmidic cat gene providing resistance to chloramphenicol was found, and certain genetic variants were associated with varying resistance to erythromycin. However, there were inconsistencies between genetic data and phenotype. The presence of certain genes not associated with resistance was also noted. These findings emphasize the importance of understanding the antibiotic resistance status and gene content of S. equorum strains intended for use in food systems.