Date Published: July 3, 2017
Publisher: Public Library of Science
Author(s): Zachary R. Stromberg, James R. Johnson, John M. Fairbrother, Jacquelyn Kilbourne, Angelica Van Goor, Roy Curtiss, Melha Mellata, Chitrita DebRoy.
Extraintestinal pathogenic Escherichia coli (ExPEC) strains are important pathogens that cause diverse diseases in humans and poultry. Some E. coli isolates from chicken feces contain ExPEC-associated virulence genes, so appear potentially pathogenic; they conceivably could be transmitted to humans through handling and/or consumption of contaminated meat. However, the actual extraintestinal virulence potential of chicken-source fecal E. coli is poorly understood. Here, we assessed whether fecal E. coli isolates from healthy production chickens could cause diseases in a chicken model of avian colibacillosis and three rodent models of ExPEC-associated human infections. From 304 E. coli isolates from chicken fecal samples, 175 E. coli isolates were screened by PCR for virulence genes associated with human-source ExPEC or avian pathogenic E. coli (APEC), an ExPEC subset that causes extraintestinal infections in poultry. Selected isolates genetically identified as ExPEC and non-ExPEC isolates were assessed in vitro for virulence-associated phenotypes, and in vivo for disease-causing ability in animal models of colibacillosis, sepsis, meningitis, and urinary tract infection. Among the study isolates, 13% (40/304) were identified as ExPEC; the majority of these were classified as APEC and uropathogenic E. coli, but none as neonatal meningitis E. coli. Multiple chicken-source fecal ExPEC isolates resembled avian and human clinical ExPEC isolates in causing one or more ExPEC-associated illnesses in experimental animal infection models. Additionally, some isolates that were classified as non-ExPEC were able to cause ExPEC-associated illnesses in animal models, and thus future studies are needed to elucidate their mechanisms of virulence. These findings show that E. coli isolates from chicken feces contain ExPEC-associated genes, exhibit ExPEC-associated in vitro phenotypes, and can cause ExPEC-associated infections in animal models, and thus may pose a health threat to poultry and consumers.
The primary and secondary habitats of Escherichia coli are the intestinal tract of warm-blooded animals and the environment, respectively. In poultry, as in humans, E. coli resides in the lower digestive tract, which it colonizes in the first 24 h after hatching  or birth . Although many E. coli strains are harmless commensals, a subset have acquired the ability to cause intestinal or extraintestinal diseases. Extraintestinal pathogenic E. coli (ExPEC) strains cause diverse infections outside of the intestinal tract in humans and animals [3–5]. Based on the host and the site of infection, different ExPEC strains are subclassified as neonatal meningitis E. coli (NMEC), sepsis-associated E. coli (SEPEC), uropathogenic E. coli (UPEC), which cause newborn meningitis, sepsis, and urinary tract infections (UTI), respectively; and avian pathogenic E. coli (APEC), which mainly causes respiratory and systemic disease in poultry.
The presence and characteristics of pathogenic E. coli colonizing healthy production chickens could be important to both animal and human health. Here, we characterized E. coli isolates from the feces of healthy production chickens both genotypically and phenotypically, including for their ability to cause disease in animal models of chicken and human infections. Based on the molecular criteria of Johnson et al. , 13% (40/304) of the present chicken fecal E. coli isolates qualified as ExPEC. Varying isolation methods, classification methods, geographic locations, and management practices likely contribute to differences in frequency of ExPEC isolation between studies. In a previous study using methods different than that of the current study, 10% of E. coli isolates from feces of commercial egg layer and meat chickens qualified molecularly as ExPEC . These findings indicate that commercial chickens can harbor E. coli isolates with virulence characteristics of ExPEC that could be transmitted to other chickens in the production house or contaminate carcasses during processing. Notably, one study recovered E. coli from 87% (691/798) of post-chill chicken carcasses at large commercial harvest facilities . Although concentrations decreased with subsequent processing steps, low counts persisted, suggesting the possibility of contaminated retail poultry products, as documented in multiple retail market surveys [32, 40].
Our study provides an in-depth assessment of virulence-related genotypes and phenotypes, including in vivo virulence, of fecal ExPEC isolates from healthy production chickens. Multiple methods were used to identify isolates with presumptive zoonotic potential. Some isolates were able to cause one or several diseases in animal models of septicemia, meningitis, UTI, and avian colibacillosis. Thus, this study provides the strongest evidence to date that chicken feces could be a source of virulent ExPEC that are able to infect humans and poultry. Interventions that reduce these pathogens in the chicken intestine and on carcasses and meat products could help to reduce transmission via poultry products and thus prevent clinical ExPEC infections and humans.