Purpose: The aim of this study is to determine the toxin gene profiles of S. aureus isolates from different clinic and food samples in order to understand the genetic and pathogenic relatedness, as well as its epidemiology, as well as to monitor the antimicrobial resistance profiles of this bacterium through the food supply chain.
Methods: In this study, a total of 108 S. aureus isolates from 16 major hospitals located in 14 different provinces in China were characterized for the profiles of 18 staphylococcal enterotoxin (SE) genes, 3 exfoliatin genes (eta, etb and etd), and the toxic shock syndrome toxin gene (tsst) by PCR. The genomic diversity of each isolate was also evaluated by pulsed-field gel electrophoresis (PFGE), multilocus sequence typing (MLST), and accessory gene regulator (agr) typing. Furthermore, the disk diffusion method and E-test were performed to determine the prevalence of ARSA in 135 foodborne S. aureus isolates using 18 antibiotics. PCR screening for the presence of 14 genes conferring antibiotic resistance was conducted.
Results: Of these clinic isolates, 90.7% (98/108) harbored toxin genes, in which tsst was the most prevalent toxin gene (48.1%), followed by sea (44.4%), sek (42.6%) and seq (40.7%). The see and etb genes were not found in any of the isolates tested. Because of high-frequency transfer of toxin genes contained in mobile genetic elements among S. aureus isolates, a total of 47 different toxin gene combinations were detected, including a complete egc cluster in 19 isolates, co-occurrence of sea, sek and seq in 38 isolates, and sec and sel together in 11 isolates. Genetic typing by PFGE grouped all the isolates into 25 clusters based on 80% similarity. MLST revealed 25 sequence types (ST) which were assigned into 16 clonal complexes (CCs) including 2 new singletons. Among these, 11 new and 6 known STs were first reported in the S. aureus isolates from China. Overall, the genotyping results showed high genetic diversity of the isolates regardless of their geographical distributions, and no strong correlation between genetic background and toxin genotypes of the isolates. For genotyping S. aureus, PFGE appears to be more discriminatory than MLST. However, toxin gene typing combined with PFGE or MLST could increase the discriminatory power of genotyping S. aureus isolates. Among the antimicrobial resistance profiles of 135 foodborne isolates, the highest resistance frequency was found for penicillin G (74.8%), followed by erythromycin (52.6%) and ciprofloxacin (32.6%), whereas no vancomycin-resistant isolates were found. Eight MRSA isolates were found in this study. These isolates could be subtyped into 62 resistance profiles and 26 clusters based on their antimicrobial susceptibility. The presence of resistance genes was relatively high: blaTEM (78.5%), ermB (38.5%), ermC (37.8%) and aac6’/aph2” (36.3%). The incidence of antibiotic resistance was significantly correlated to food types (P = 0.018), with isolates from meat and raw milk more resistant to antibiotics than those from frozen food and vegetables.
Significance: The toxin and antimicrobial resistance of S. aureus have become a serious concern in clinics and foodstuffs. The results of this study will assist the better understanding of the toxin and antimicrobial resistance profiles of those isolates from different sources. The appropriate genotyping method for the S. aureus isolates has also been recommended.