Egg yolk and tellurite

To isolate and detect the S. aureus, each sample was diluted 10-fold in sterile peptone water and homogenized. Aliquots (1 mL) were placed onto Baird-Parker agar supplemented with 5% egg yolk and tellurite (Beijing Land Bridge Technology Ltd., Beijing, China). The plates were incubated for 24–48 h at 37°C. Colonies with typical black appearance and surrounded by a clear zone were enumerated as S. aureus.
Presumptive colonies were confirmed by polymerase chain reaction (PCR) (Bio-Rad S1000, United States) detection of the thermonuclease gene (nuc, S. aureus specific) (Supplementary Table 1). DNA of the strains was extracted using the InstaGene Matrix DNA extraction kit (Bio-Rad Laboratories, Hercules, California, United States) following the manufacturer’s instructions. The amplification conditions and reagents for the PCR assays were those described by Liu et al. (2017) (link). Negative control (without DNA template) and positive control (S. aureus ATCC 6538) templates were included in all PCR assays. After identification, one to three colonies per sample were randomly selected for subsequent analysis. All strains were stored with sterile magnetic beads at −80°C until further analysis.