Phenotypic and Genotypic Characterization of Methicillin, Vancomycin, and Erythromycin-resistant Staphylococcus aureus Isolated from Milk and Dairy Products

Abstract

Detailed information on the resistance patterns of Staphylococcus aureus (S. aureus) in milk and cheese is strongly required to facilitate risk assessment analysis in case of food poisoning context and to improve therapeutic approaches used in dairy farms. The present study aimed to perform phenotypic and genotypic antimicrobial characterizations of methicillin, vancomycin, and erythromycin-resistant S. aureus isolated from milk and dairy products through screening mecA, vanA, and ermC using molecular PCR amplification technology. Moreover, the association between each genotypic and its related antibiotic resistance phenotypic features within the isolated S. aureus strains were analyzed. Moreover, the current study aimed to study MRSA's ability to form biofilms. Out of 226 milk and dairy product samples collected from different retailers in Giza Governorate, 69.5% of the samples were positive for the presence of S. aureus. The isolation rate of S. aureus strains from cattle milk, sheep milk, white cheese, flamenco, and mesh samples were 79.7%, 76.5%, 56.0%, 40.0%, and 94.7%, respectively. Multidrug-resistant S. aureus (MDR) was detected in 51% of all isolated S. aureus strains. All tested S. aureus strains were sensitive to trimethoprim-sulfamethoxazole, linezolid, ciprofloxacin, and gentamycin. However, their resistance rates against penicillin, oxacillin, vancomycin, erythromycin, tetracycline, clindamycin and chloramphenicol were 62.4%, 65.0%, 44.6%, 45.9%, 21.0%, 14.0%, and 2.5%, respectively. Of the isolated S. aureus strains, 72.6%, 40.1%, and 48.4% were carriers for mecA, vanA, and ermC genes and the amplified products were at 310, 1030, and 295 bp, respectively. Methicillin-resistant S. aureus isolates were detected in 47.1% of all isolated S. aureus strains. The results indicated that 35.0% of the tested S. aureus strains were genotypic vanA gene carriers and phenotypic resistant to vancomycin representing vancomycin-resistant S. aureus strains. Moreover, 42.7% of all isolated S. aureus strains were carriers for ermC gene and were phenotypic resistant to erythromycin representing erythromycin-resistant S. aureus. The presence of mecA, vanA, and ermC genes in S. aureus was statistically associated with their related phenotypic resistance patterns against both penicillin and oxacillin, vancomycin, and erythromycin, respectively. Moreover, along with an increase in the frequency of mecA, vanA, and ermC genes, their phenotypic antibiotic resistance patterns sharply increased with an odd ratio >1. Of MRSA isolates, 6.8% indicated weak biofilm-formation ability, while 93.2% exhibit no biofilm-forming ability.