Prevalence, Molecular Characterization, and Antibiotic Susceptibility of Isolates from Powdered Infant Formula Collected from Chinese Retail Markets.

is an opportunistic pathogen that causes severe infections in neonates and infants through contaminated powdered infant formula (PIF). Therefore, the aim of this study was a large-scale study on determine the prevalence, molecular characterization and antibiotic susceptibility of isolates from PIF purchased from Chinese retail markets. Two thousand and twenty PIF samples were collected from different institutions. Fifty-six strains were isolated, and identified using sequencing analysis, giving a contamination rate of 2.8%. Multilocus sequence typing (MLST) was more discriminatory than other genotyping methods. The isolates were divided into 14 sequence types (STs) by MLST, compared with only seven clusters by and sequence analysis, and four serotypes by PCR-based O-antigen serotyping. ST4 (19/56, 33.9%), ST1 (12/56, 21.4%), and ST64 (11/56, 16.1%) were the dominant sequence types isolated. serotype O2 (34/56, 60.7%) was the primary serotype, along with o6 and 1 as the main allele profiles, respectively. Antibiotic susceptibility testing indicated that all isolates were susceptible to ampicillin-sulbactam, cefotaxime, ciprofloxacin, meropenem, tetracycline, piperacillin-tazobactam, and trimethoprim-sulfamethoxazole. The majority of strains were susceptible to chloramphenicol and gentamicin (87.5 and 92.9%, respectively). In contrast, 55.4% strains were resistant to cephalothin. In conclusion, this large-scale study revealed the prevalence and characteristics of from PIF in Chinese retail markets, demonstrating a potential risk for neonates and infants, and provide a guided to effective control the contamination of in production process.