Submitted to: Vaccine
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/10/2011
Publication Date: 5/6/2011
Citation: Sharma, V.K., Nystrom, E.A., Casey, T. 2011. Evaluation of hha and hha sepB mutant strains of Escherichia coli O157:H7 as bacterins for reducing E. coli O157:H7 shedding in cattle. Vaccine. 29(31):5078-5086. Available: http://www.sciencedirect.com/science/article/pii/S0264410X11006189. Interpretive Summary: Escherichia coli O157:H7, a Shiga toxin-producing E. coli, is the most frequent cause of bloody diarrhea in humans. In certain age groups, O157:H7 infections may result in damage to the kidneys and nervous system, a potentially fatal form of the disease. Cattle are considered the major source for O157:H7 and these animals can secrete O157:H7 in their feces for extended periods of time. Most human infections result from consumption of undercooked ground beef. In addition, milk, produce and water that inadvertently become contaminated with cattle feces containing O157:H7 have also frequently been linked to human infections. Thus, application of strategies for reducing pre-harvest prevalence of O157:H7 at the farm is important for reducing contamination of hides and carcasses in order to enhance post-harvest safety of bovine food products and to prevent human disease outbreaks due to O157:H7, and to mitigate economic losses to the public health system ($405 million per annum) and to the food industry (around $3 billion). One such strategy would be to develop appropriate vaccines that could easily be incorporated into the routine management systems of cattle producers. These vaccines must be capable of enhancing immune response in cattle for specifically blocking colonization of O157:H7 in cattle intestines to reduce fecal shedding of these bacteria. In this study, we describe for the first time the potential application of heat-killed whole cell vaccines (bacterins), which are constructed from specific genetic mutants of O157:H7, for reducing the duration of fecal shedding and number of calves shedding O157:H7. The major advantage of bacterins that we have described in this study is the ease and low-cost of preparation compared to protein-based vaccines that require laborious, expensive, time consuming purification and concentration procedures. In addition, the bacterins provide a platform for constructing specific mutants of O157:H7 as a convenient source for delivering large quantities of desired proteins for inducing immune responses to multiple colonization factors for effective inhibition of O157 colonization of cattle intestines.
Technical Abstract: Escherichia coli O157:H7 colonizes cattle intestines by using locus of enterocyte effacement (LEE)-encoded proteins. Induction of systemic immune response against LEE-encoded proteins, therefore, will prove effective in reducing E. coli O157:H7 colonization in cattle. Previous studies have demonstrated that hha (encodes for a hemolysin expression modulating protein) deletion enhanced the expression of LEE-encoded proteins and sepB (encodes an ATPase required for the secretion of LEE-encoded proteins) deletion resulted in intracellular accumulation of LEE proteins. In this study, we demonstrate efficacy of hha and hha sepB deletion mutant strains as bacterins for reducing fecal shedding of E. coli O157:H7 in experimentally inoculated weaned calves. Weaned calves were injected intramuscularly with 1-ml of a heat-killed suspension (10**9 cells/ml) of the hha**+ parent or hha or hha sepB mutant strains, and boosted with the same doses 2- and 4-weeks later. Evaluation of the immune response two weeks after the last booster immunization showed the higher levels of antibody titers against LEE proteins in calves vaccinated with the hha mutant bacterin compared to the titers in calves vaccinated with hha sepB mutant or the parent strain bacterin. Following oral inoculation with 10**10 CFU of wild-type E. coli O157:H7, greater number of calves in the groups vaccinated with hha or hha sepB bacterin stopped shedding the inoculum strain within a few days after inoculation compared to groups of calves vaccinated with the hha**+ parent strain bacterin or PBS sham vaccine. Thus, the use of bacterins prepared from hha and hha sepB mutants for reducing colonization of E. coli O157:H7 in cattle will be an important pre-harvest strategy to enhance post-harvest safety of bovine food products, water and produce.