Identification, antimicrobial resistance profiles, and virulence of members from the family Enterobacteriaceae from the feces of yellow-headed blackbirds (Xanthocephalus xanthocephalus) in North Dakota

Public pressure to reduce or eliminate antimicrobials as ingredients of feed for poultry and other agricultural animals is mounting, primarily due to the fear of multidrug-resistant bacteria in clinical infections in both animals and humans. Exploration of the occurrence of antibiotic resistance in the gut flora of wildlife avian flocks that presumptively do not receive antimicrobials will determine the rate of resistance in a naïve population. Fecal samples collected from a healthy population of the yellow-headed blackbirds (YHB) (Xanthocephalus xanthocephalus) in North Dakota were cultured to determine what genera and species of gram-negative facultative anaerobic bacteria these wild birds carry in their intestinal flora and to evaluate the antimicrobial susceptibility profiles. Isolates of Escherichia coli were further characterized for the presence of putative virulence factors and for pathogenic potential using the chicken embryo lethality assay (ELA). The ELA was performed in chicken embryos with challenges at both 12 days and 16 days of incubation to determine whether the 16-day-old embryos were better able to fight the infection and subsequent disease and also to determine whether the ELA could distinguish between primary and secondary avian Escherichia coli pathogens. After screening 33 isolates from the 21 fecal samples, only two E. coli isolates were identified. The predominant genus and species of bacterium identified was Pantoea agglomerans. Collectively, 12 of the 33 isolates (36%) exhibited no resistance to any antimicrobial tested. However, several multidrug-resistant isolates of varying genera were identified. Among the antimicrobial resistances observed, the most common was to ampicillin (60%), followed by cephalothin (33%). Neither E. coli isolate belonged to serogroups that are notorious for causing major outbreaks of colibacillosis in poultry, and only one E. coli isolate retained resistance to any antibiotics; nevertheless, the ELA results indicate that at least one of these E. coli may be a primary pathogen of chickens. This study demonstrates that antibiotic resistance occurs in the gut flora of natural populations of YHB despite the absence of antibiotic pressure. In addition, these results indicate that YHB will harbor E. coli isolates that are potentially pathogenic in poultry. However, these E. coli isolates are not a significant reservoir for multiple antibiotic resistances nor are they widespread in the population of YHB surveyed in North Dakota.