A geospatial analysis of flies and the spread of antimicrobial resistant bacteria

Livestock is often colonized with ESBL-producing Enterobacteriaceae (ESBL-E) and Staphylococcus aureus. There is a risk that flies spread antimicrobial resistant bacteria from livestock to humans. Here, we screened flies from urban and rural areas near the city of Münster, Germany, for the presence of ESBL-E and S. aureus and compared molecular characteristics of these isolates with those isolated from humans in the same region. In total, 1346 single flies were grinded and cultured overnight in BHI-broth. The broth was cultured on Columbia blood agar and selective media for the detection of S. aureus and ESBL-E. Human isolates from routine diagnostics at the University Hospital Münster were used for comparison. Antimicrobial susceptibility, phylogroups (Escherichia coli), spa types/multilocus sequence types (S. aureus) and selected antimicrobial resistance genes were determined for each isolate. GPS data of the sampling sites were used to map flies carrying ESBL-E and S. aureus. Overall, Serratia fonticola (123/1346; 9.1%) was the most prevalent ESBL-E in flies, followed by E. coli (44/1346; 3.3%). A high proportion of flies was positive for ESBL-producing E. coli (15.0-22.2%) in a rural area. CTX-M-1 was the most prevalent beta-lactamase in E. coli (38.6%). One livestock-associated methicillin resistant S. aureus (LA-MRSA, t011/ST398) was found in the city centre of Münster. Overall, a substantial part of ESBL-producing E. coli and S. aureus from flies and humans showed a similar genetic background. In conclusion, flies can carry ESBL-E and LA-MRSA in urban and rural areas. The similar genetic background of isolates from humans and flies points towards a common source.