Diversity and antibiotic resistance of enterococci associated with stored-product insects collected from feed mills

Diversity and antibiotic resistance of enterococci associated with stored-product insects were assessed by screening 298 live adults from nine insect species collected from six feed mills in the midwestern United States. The eight antibiotics tested were tetracycline, ampicillin, erythromycin, vancomycin, chloramphenicol, ciprofloxacin, streptomycin, and neomycin. Enterococci were isolated from seven out of nine insect species collected. A majority of enterococci and aerococci were isolated from the red flour beetle, Tribolium castaneum (Herbst). Approximately 18% of the insects tested positive for bacteria on mEnterococcus agar, and the mean colony forming units ranged from 2×10¹ to 1.3×10⁵ per insect. Bacteria isolated on mEnterococcus agar included Enterococcus faecium, Enterococcus gallinarum, and Aerococcus viridans 3. Isolates of E. faecium displayed complete or intermediate resistance, most frequently to neomycin, tetracycline, and erythromycin. In addition, many E. faecium isolates exhibited intermediate resistance to vancomycin. The majority of A. viridans 3 isolates were susceptible to all antibiotics, while isolates of E. gallinarum displayed resistance to neomycin. Enterococci and aerococci are nosocomial human pathogens and are implicated in secondary infections, mainly in immuno-compromised individuals. Additionally, enterococci are considered an important reservoir of antibiotic resistance genes that can be horizontally transferred to other bacteria, including serious human pathogens. Our data reinforce the need for pest management to reduce the availability of vectors (insects) for dissemination of microorganisms carrying antibiotic resistance genes in the feed mill environment.