House Flies (Musca domestica) from Swine and Poultry Farms Carrying Antimicrobial Resistant Enterobacteriaceae and Salmonella

Simple Summary House flies (Musca domestica) are ubiquitous insects living in close contact with humans and farmed animals. Due to their behavior and life cycle, these insects could be easily contaminated by bacteria, becoming mechanical vectors of potentially pathogenic microorganisms. In the present investigation, house flies were captured in poultry and swine farms, and they were microbiologically evaluated. Enterobacteriaceae were abundantly detected in analyzed samples; high levels of resistance were found against commonly used antimicrobials, such as beta-lactams and tetracyclines. One extended spectrum beta-lactamase producer strain was identified among Enterobacteriaceae, carrying the gene blaTEM-1. Salmonella spp. was detected in samples from about one third of farms; most of the tested antimicrobials were effective against more than 80% of isolated salmonellae. House flies could be important vectors of antimicrobial resistant bacteria and Salmonella, representing a potential source of infection for farmed animals and practitioners. The house fly (Musca domestica) is a very common insect, abundantly present in farm settings. These insects are attracted by organic substrates and can easily be contaminated by several pathogenic and nonpathogenic bacteria. The aim of this survey was to evaluate the presence of Salmonella spp. and other Enterobacteriaceae in house flies captured in small-medium size farms, located in Northwest Tuscany, Central Italy, and to evaluate their antimicrobial resistance; furthermore, isolates were tested for extended spectrum beta-lactamase and carbapenems resistance, considering the importance these antimicrobials have in human therapy. A total of 35 traps were placed in seven poultry and 15 swine farms; three different kinds of samples were analyzed from each trap, representing attractant substrate, insect body surface, and insect whole bodies. Enterobacteriaceae were isolated from 86.36% of farms, 82.87% of traps, and 60.95% of samples; high levels of resistance were detected for ampicillin (61.25% of resistant isolates) and tetracycline (42.5% of resistant isolates). One extended spectrum beta-lactamase producer strain was isolated, carrying the bla(TEM-1) gene. Salmonella spp. was detected in 36.36% of farms, 25.71% of traps, and 15.24% of samples. Five different serovars were identified: Kentucky, Kisarawe, London, Napoli, and Rubislaw; some isolates were in R phase. Resistance was detected mainly for ampicillin (31.21%) and tetracycline (31.21%). House flies could represent a serious hazard for biosecurity plans at the farm level, carrying and sharing relevant pathogenic and antimicrobial resistant bacteria.