Antimicrobial Resistance and Molecular Characterization of Methicillin-Resistant Staphylococcus aureus Isolates from Artisanal Cheeses in Southern and Southeastern Brazil

Abstract

Brazilian artisanal cheeses are widely valued for their sensory quality and cultural relevance; however, their production frequently involves raw milk and extensive handling, conditions that may favor contamination by Staphylococcus aureus. This retrospective baseline study analyzed artisanal raw-milk cheeses collected between 2014 and 2016 in Southern and Southeastern Brazil and characterized the isolates regarding antimicrobial resistance, biofilm-forming ability, and the presence of virulence and resistance genes. A total of 147 cheese samples were collected from street vendors and markets in five Brazilian states; notably, 93% of the sampling and all S. aureus-positive samples were concentrated in Paraná, São Paulo, and Minas Gerais, Brazil. S. aureus was isolated using selective media and biochemical identification, followed by antimicrobial susceptibility testing according to CLSI guidelines. Molecular analyses were performed to detect enterotoxin genes, the tst gene, the mecA gene, and SCCmec types. Biofilm production was assessed using a microtiter plate assay. Thirteen samples (8.84%) were positive for S. aureus, yielding 25 isolates. Resistance to cefoxitin (FOX) was the most frequent phenotype (20%), followed by resistance to ampicillin, ciprofloxacin, chloramphenicol, and streptomycin (8% each). Two isolates were classified as multidrug-resistant (MDR). Genotypic analysis revealed one mecA-positive isolate (methicillin-resistant S. aureus [MRSA]; SCCmec nontypable), while four FOX-resistant mecA-negative isolates (including three SCCmec type II carriers) were classified as borderline-oxacillin-resistant S. aureus. Biofilm formation was observed among resistant isolates. Among the virulence genes investigated, only one isolate carried the sea gene, and none were positive for tstH. Although the prevalence of mecA-positive MRSA and enterotoxigenic strains was low, the detection of MDR, biofilm-producing S. aureus highlights potential risks to food safety and public health. These findings reinforce the importance of continuous surveillance, molecular monitoring of resistance determinants, and strict hygienic practices in artisanal cheese production.