Department of Veterinary and Biomedical Sciences
Penn State University, PA 16802 USA
Shiga-toxin producing E. coli (STEC) is responsible for a significant portion of foodborne illness, and can result in serious complications and death. The notorious enterohemorrhagic E. coli serotype O157:H7 is the best-studied, but other STEC have emerged in outbreaks all over the world. Most human infections occur through food contamination, most commonly in beef or produce, which can most often be traced back to cattle and other ruminant animals. Adult cattle and other ruminants can carry STEC bacteria asymptomatically, allowing the bacteria to persist and spread throughout the environment through fecal shedding for weeks or sometimes months at a time. Contamination of beef occurs through hide contamination, and produce is often contaminated through water or fertilizer. In humans, STEC infections can be difficult to treat, and can produce serious and permanent complications such as hemolytic uremic syndrome (HUS) leading to kidney damage, and causes the most serious morbidity and mortality among children under the age of 5, the elderly, and the immunocompromised.
Understanding of STEC behavior during initial adherence to bovine intestinal epithelial cells will provide potential targets for therapeutic intervention at the source, eliminating the need for costly interventions during food processing, and greatly reducing the probability of outbreaks. Understanding STEC behavior during initial adherence to human intestinal epithelial cells provides potential for new treatments or preventions during outbreaks, and may also be related to the mechanisms of other extracellular intestinal pathogens.
Although the ability of STEC to bind very tightly to intestinal epithelial cells, termed ‘intimate adherence,’ is a very well understood mechanism, initial attachment to epithelial cells before intimate adherence is not well characterized. In both human disease and asymptomatic bovine carriage, initial adherence is a great opportunity for intervention, and preventing colonization and spread. In other pathogens, there is a precedent for the use of eukaryotic cell-surface proteins and receptors to bind or internalize bacteria. E. coli O157:H7 and other STEC strains were investigated for this possibility.
To observe adhesion of STEC to the polymeric Ig receptor (pIgR), human colonic cell lines and primary bovine intestinal epithelial cells were observed using fluorescent microscopy to quantify colocalization of bacteria with the pIgR at different timepoints post infection. Additionally, under the same growth and infection conditions, select virulence genes and pIgR expression were calculated using quantitative PCR, by the comparative Ct method.
Although no colocalization of bacteria and pIgR was observed in bovine cells, pIgR expression was increased as much or more than in human cell lines, which may indicate a specific immune response and not adherence. A significantly higher proportion of STEC bacterial strains were found to be colocalized with the human pIgR at all timepoints, as compared to a non-pathogenic control strain. Human pIgR gene expression also increased during this timeline, with peak expression occurring at 1 hour post infection or less. Peak pIgR expression at early timepoints indicates a possibility that the pIgR may be involved with initial adherence of STEC to human intestinal epithelial cells during colonization.