Ecology of Antibiotic Resistance Genes: Characterization of Enterococci from Houseflies Collected in Food Settings

Abstract

In this project, enterococci from the digestive tracts of 260 houseflies ( Musca domestica L.) collected from five restaurants were characterized. Houseflies frequently (97% of the flies were positive) carried enterococci (mean, 3.1 × 10 ³ CFU/fly). Using multiplex PCR, 205 of 355 randomly selected enterococcal isolates were identified and characterized. The majority of these isolates were Enterococcus faecalis (88.2%); in addition, 6.8% were E. faecium , and 4.9% were E. casseliflavus. E. faecalis isolates were phenotypically resistant to tetracycline (66.3%), erythromycin (23.8%), streptomycin (11.6%), ciprofloxacin (9.9%), and kanamycin (8.3%). Tetracycline resistance in E. faecalis was encoded by tet (M) (65.8%), tet (O) (1.7%), and tet (W) (0.8%). The majority (78.3%) of the erythromycin-resistant E. faecalis isolates carried erm (B). The conjugative transposon Tn 916 and members of the Tn 916/ Tn 1545 family were detected in 30.2% and 34.6% of the identified isolates, respectively. E. faecalis carried virulence genes, including a gelatinase gene ( gelE ; 70.7%), an aggregation substance gene ( asa1 ; 33.2%), an enterococcus surface protein gene ( esp ; 8.8%), and a cytolysin gene ( cylA ; 8.8%). Phenotypic assays showed that 91.4% of the isolates with the gelE gene were gelatinolytic and that 46.7% of the isolates with the asa1 gene aggregated. All isolates with the cylA gene were hemolytic on human blood. This study showed that houseflies in food-handling and -serving facilities carry antibiotic-resistant and potentially virulent enterococci that have the capacity for horizontal transfer of antibiotic resistance genes to other bacteria.