Prevalence and Characteristics of Antibiotic Resistant Bacteria in Selected Ready-to-Consume Deli and Restaurant Foods

One of the major public health challenges nowadays is the rapid emergence of antibiotic resistant (ART) pathogens. In the past couple of decades, despite the extensive studies on ART pathogens in clinical settings, the lack of proper understanding on major routes and mechanisms of antibiotic resistance (AR) emergence, dissemination and persistence likely contributed to the incompetence in the battle against AR. Particularly, the impact of commensal bacteria, which account for a majority of the microbial population, on the development of AR has been overlooked. Since horizontal gene transfer is crucial to the rapid dissemination of AR, these dominant organisms in microbial ecosystems likely have played a key role in the emergence of ART pathogens. Recently a large amount of ART commensal bacteria were detected in many food items, suggesting that the food chain may serve as an important avenue of disseminating AR to human. Therefore, the objectives of this study were to reveal the AR status of commensal bacteria from selected ready-to-consume (RTC) food products collected from local stores and restaurants and to analyze the genetic elements of ART commensals for an improved understanding of the AR risk associated with the food chain.

ART bacteria, screened by selective media with tetracycline, were detected in 20 out of 26 food samples collected from salad bars from local grocery stores and restaurants. The total counts varied among samples, with 46% ranging from 10⁵CFU/g to 10⁹CFU/g. Tetracycline resistant (Tet^r) bacteria counts of no less than 10³ CFU/g were detected in seven samples. Noticeably, four samples, including mushroom salad, spicy shrimp, chicken salad, carried Tet^r counts about 10⁴ CFU/g. Out of 740 Tet^r isolates examined, 114 (15.4%) carried one or more of tetM, tetL, tetS, tetK genes by conventional PCR, 27 harbored more than one Tet^r determinants. The most prevalent genotype was tetM, which was detected in 57.0% of the Tet^r gene carriers, followed by tetL (37.8%), tetS (9.6%) and tetK (3.0%). The genera identified by 16S rRNA gene analysis included Enterococcus, Lactococcus, Staphylococcus, Brochothrix, Carnobacterium, Sphingobacterium. Out of 15 Tet^r isolates examined, 12 had the Tet^r gene(s) located on both the chromosome and plasmid, and 2 only on chromosome and 1 on plasmid by Southern hybridization. AR determinants from selective samples were successfully transmitted into Streptococcus mutans via natural gene transformation and Enterococcus faecalis via electroporation. Isolates which carry plasmids with AR determinants were consecutively transferred for more than 400 generations in bacterial media without tetracycline to study the stability of resistance. Result showed that among 13 isolates tested, only 2 lost the Tet^r phenotype in 1% of the progenies. These findings suggest that restaurant and deli foods serve as an avenue in transmitting AR to humans, the AR genes can potentially be transmitted to other bacteria including pathogens, and the AR trait is quite stable even at the absence of the selective pressure. A comprehensive understanding of the AR risk will assist in the development of proper control strategies.