Characterization of Staphylococcus aureus isolated from powdered infant formula milk and infant rice cereal in China

Dry infant foods are not sterile and could be contaminated with various bacteria including certain pathogens. The aim of this study was to investigate the prevalence of Staphylococcus aureus in infant foods and to characterize these strains. A total of 367 infant food samples, including 143 samples of powdered infant formula milk (PIF) and 224 samples of infant rice cereal (IRC), were collected in the Shaanxi Province of China during the period of July to August 2010 and screened for S. aureus. All S. aureus isolates were characterized by antimicrobial susceptibility testing, pulsed-field gel electrophoresis (PFGE), and detection of genes encoding enterotoxins, exfoliative toxins, Panton-Valentine leukocidin (PVL), and toxic shock syndrome toxin 1. Among all the samples examined, sixteen of 143 PIF samples (11.2%) and 14 of 224 IRC samples (6.3%) were positive for S. aureus. From these positive samples, 29 S. aureus strains were isolated from PIF and 25 from IRC. Of these S. aureus isolates, 83.3% were resistant to at least one antimicrobial, 35.2% to three or more antimicrobials. Resistance was most frequently observed to erythromycin (75.9%), followed by ciprofloxacin (51.9%) and trimethoprim/sulfamethoxazole (27.8%), while significantly fewer isolates were resistant to gentamicin (22.2%), tetracycline (18.5%), or cefoxitin (3.7%). In addition, 63.0% of isolates were positive for one or more toxin genes tested. The three most predominant toxin genes were pvl (40.7%), seg (38.9%), and sec (18.5%), followed by sea (7.4%), seb (7.4%), sed (5.6%), and see (5.6%). The ets, tsst-1, seh, sei, and sej genes were not detected. A total of 39 PFGE patterns were generated among 51 selected food isolates. Our findings indicate that PIF and IRC in the Shaanxi province were contaminated with S. aureus, and many S. aureus isolates harbored multiple toxin genes and exhibited multiple antimicrobial resistance. In addition, these S. aureus isolates were genetically diverse. The presence of S. aureus strains in these infant foods poses a potential threat to infant health.