Prevalence of multidrug-resistant Staphylococcus aureus isolates with strong biofilm formation ability among animal-based food in Shanghai

Antimicrobial resistance has gradually become a serious problem threatening public health and food safety throughout the world. Biofilm is one of the important factors affecting the antimicrobial resistance of bacteria. Staphylococcus aureus usually has strong biofilm formation ability, and it is widely found in animal-based food. The purpose of this study is to determine the prevalence, antimicrobial resistance and biofilm formation of S. aureus in animal-based food. Total 959 samples representing eight types of animal-based foods were collected from randomly selected locations (21 supermarkets and 18 wet markets) throughout the Shanghai city. The overall isolation rate of S. aureus was 17.2% (165/959). For each food category, the isolation rate was 21.8% for chicken (45/206), 21.5% for pork (71/331), 15.2% for beef (16/105), 13.8% for duck (9/65), 12.1% for aquatic products (17/141), 8.6% for egg (5/58), and 7.1% for lamb (2/28), respectively. No isolate was found from pasteurized milk (n = 25). Antimicrobial susceptibility test showed that among all the S. aureus isolates, 90.3% were resistant to at least one antimicrobial, 39.4% were multi-drug resistant, and 23 isolates were methicillin-resistant S. aureus (MRSA). Comparing the resistance rates to different antimicrobials, S. aureus had the highest resistance rate to penicillin, up to 82.4% (136/165); followed by erythromycin (57.6%, 95/165) and tetracycline (27.9%, 46/165). All isolates were sensitive to vancomycin. With the microtiter plate and crystal violet staining assay, 64.8% of all the 165 isolates had strong biofilm formation ability and 20.0% were moderate producers. Remarkably, significant difference was found in biofilm formation ability between those isolates from supermarkets and wet markets (p < 0.01). According to WGS analysis of 19 multi-drug resistant isolates with strong biofilm formation ability, ST7 was the dominant sequence type. Combined analysis showed that S. aureus isolates with drug resistance phenotypes usually had stronger ability to form biofilm.