Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 4, 2014
Publication Date: N/A
Interpretive Summary: Escherichia coli O157:H7 (O157) are Shiga toxin-producing bacteria that infect humans through the consumption of contaminated meats, such as ground beef, unpasteurized dairy products, fresh produce and water. Human infections with O157 generally produce mild diarrhea but in children and elderly, the Shiga toxins produced by O157 could lead to bloody diarrhea, kidney malfunction, and even death. Cattle generally carry these bacteria in their intestines without experiencing any damage or disease to themselves, but can shed O157 in their feces at variable magnitudes and duration. Cattle feces are the major risk factor for the contamination of animal hides in feedlots and carcass contamination at slaughter plants leading to the downstream contamination of meat products. Cattle manure is also a major risk factor for the contamination of water resources and environmental spread of O157. According to CDC estimates, O157 causes over 73,000 human illnesses per year with over 2,000 cases of hospitalizations and 61 deaths. Combined economic losses due to human illnesses and food contamination are estimated at almost a billion dollars. Thus, a concerted approach is needed to identify factors that promote intestinal colonization and fecal shedding of O157 in cattle. By engineering and testing specific mutants of O157 that we hypothesized to be compromised in sensing stress hormones, which are produced by cattle and all mammals, we discovered that these mutants were better in colonizing the digestive system (established by many studies to be terminal portion of the large intestine) of cattle. The better colonization capacity of mutant O157 bacteria were indicated by their increased shedding in the feces of cattle experimentally fed these mutant bacteria. These findings have facilitated identification/confirmation of O157 factors for developing novel or improving existing vaccines for reducing colonization and persistence of O157 in cattle that is critical for reducing the risks of downstream contamination of meats, produce, water, and human infections.
Technical Abstract: The qseBC encoded quorum-sensing system (QS) regulates motility of enterohemorrhagic Escherichia coli (EHEC) O157:H7 in response to bacterial autoinducer-3 (AI-3) and mammalian stress hormones epinephrine (E) and norepinephrine (NE). The qseC gene encodes a sensory kinase that post-autophosphorylation in response to AI-3, E or NE phosphorylates its cognate response regulator QseB. In the absence of QseC, QseB down-regulates bacterial motility and virulence of the qseC mutant in animal models. In this study, we determined if the qseBC encoded QS provides competitive advantage to EHEC O157:H7 in colonization and fecal shedding in calves. Eight to ten months old weaned calves inoculated orally with a mixture containing equivalent numbers of the wild-type EHEC O157:H7 and its isogenic qseBC mutant showed significantly higher fecal shedding of the qseBC mutant. In vitro analysis revealed similar growth profiles and relative motilities of the qseBC mutant and the wild-type strain in the presence or absence of NE. The magnitude of response to NE in the qseBC mutant, as indicated by the enhancement in its motility, was similar to the wild-type strain. Transcriptional analysis showing no significant differences in the expression of flagellar genes flhD and fliC correlated with similar motilities of the qseBC mutant and the wild-type strain on media with or without NE. The expression of ler, the positive regulator of locus of enterocyte effacement (LEE), the ler-regulated secreted protein EspA, and csgA encoding curli fimbriae were increased in the qseBC mutant compared to the wild-type strain. On the other hand, growth, motility, and transcriptional levels of flagellar, LEE, and csgA genes were significantly reduced in the complemented qseBC mutant. Thus, in vitro gene expression data indicate that the near-wild-type motility and enhanced expression of LEE, and curli genes might in part be responsible for the increased colonization and fecal shedding of the qseBC mutant in calves.