Shiga toxin-producing Escherichia coli: an overview

A network model of E. coli O157 transmission within a typical UK dairy herd: the effect of heterogeneity and clustering on the prevalence of infection

Cattle are considered to be the main reservoir for Vero cytotoxin-producing Escherichia coli (VTEC) O157, a cause of food-poisoning (and even death) in humans. Here, the transmission of E. coli O157 within a typical UK dairy herd is modelled using a semi-stochastic network model. The model incorporates demographic as well as infection processes. Indirect transmission is modelled homogeneously, while direct transmission is modelled via a dynamic contact network. The aim was to investigate the effects of heterogeneity and clustering on the prevalence of infection within the herd and discover whether, particularly in terms of choosing an intervention strategy, it is necessary to include heterogeneity in direct contacts when modelling this sort of system. Results show that heterogeneity in direct contacts can make it more difficult for the pathogen to persist, particularly when the average number of contacts (per animal) in each group is small. They also show that the relationship between clustering and prevalence is not simple. For example, increasing the average number of contacts can increase clustering and prevalence. However, when the average number of contacts in each group is sufficiently high, higher clustering leads to lower prevalence. It would seem that clustering can aid the flow of infection under certain circumstances, but hinder it under others (probably by preventing wider dissemination). Further results show that indirect transmission (as it is modelled here) effectively removes the effect of heterogeneity in direct contacts. In terms of investigating proposed interventions, the results suggest that a network model would only be required if there was evidence to suggest that direct transmission was the major source of infection.

Associations between bovine, human, and raw milk, and beef isolates of non-O157 Shiga toxigenic Escherichia coli within a restricted geographic area of the United States

A survey for Shiga toxigenic Escherichia coli in raw milk and beef was conducted within a defined geographic region of the United States. Prevalence rates based on detection of Shiga toxin gene (stx) were 36% for retail beef, 23% for beef carcasses, and 21% for raw milk samples, which were significantly higher than were Shiga toxigenic E. coli isolation rates of 7.5, 5.8, and 3.2%, respectively. Seasonal prevalence differences were significant for stx positivity among ground beef and milk samples. Distribution of stx subtypes among isolates varied according to sample type, with stx1 predominating in milk, stx2 on carcasses, and the combination of both stx1 and stx2 in beef. Ancillary virulence markers eae and ehx were evident in 23 and 15% of isolates, respectively. Pulsed-field gel electrophoresis demonstrated associations between food isolates and sympatric bovine fecal, and human clinical isolates. These data demonstrate that non-O157 Shiga toxigenic E. coli is present in the food chain in the Pacific Northwest, and its risk to health warrants critical assessment.

The biology and future prospects of antivirulence therapies

The emergence and increasing prevalence of bacterial strains that are resistant to available antibiotics demand the discovery of new therapeutic approaches. Targeting bacterial virulence is an alternative approach to antimicrobial therapy that offers promising opportunities to inhibit pathogenesis and its consequences without placing immediate life-or-death pressure on the target bacterium. Certain virulence factors have been shown to be potential targets for drug design and therapeutic intervention, whereas new insights are crucial for exploiting others. Targeting virulence represents a new paradigm to empower the clinician to prevent and treat infectious diseases.

Effects of dried distillers' grain on fecal prevalence and growth of Escherichia coli O157 in batch culture fermentations from cattle

Distillers' grains (DG), a by-product of ethanol production, are fed to cattle. Associations between Escherichia coli O157 prevalence and feeding of DG were investigated in feedlot cattle (n = 379) given one of three diets: steam-flaked corn (SFC) and 15% corn silage with 0 or 25% dried distillers' grains (DDG) or SFC with 5% corn silage and 25% DDG. Ten fecal samples were collected from each pen weekly for 12 weeks to isolate E. coli O157. Cattle fed 25% DDG with 5 or 15% silage had a higher (P = 0.01) prevalence of E. coli O157 than cattle fed a diet without DDG. Batch culture ruminal or fecal microbial fermentations were conducted to evaluate the effect of DDG on E. coli O157 growth. The first study utilized microbial inocula from steers fed SFC or dry-rolled corn with 0 or 25% DDG and included their diet as the substrate. Ruminal microbial fermentations from steers fed DDG had higher E. coli O157 contents than ruminal microbial fermentations from steers fed no DDG (P < 0.05) when no substrate was included. Fecal fermentations showed no DDG effect on E. coli O157 growth. In the second study with DDG as a substrate, ruminal fermentations with 0.5 g DDG had higher (P < 0.01) E. coli O157 concentrations at 24 h than ruminal fermentations with 0, 1, or 2 g DDG. In fecal fermentations, 2 g DDG resulted in a higher concentration (P < 0.05) at 24 h than 0, 0.5, or 1 g DDG. The results indicate that there is a positive association between DDG and E. coli O157 in cattle, and the findings should have important ramifications for food safety.

Stx-phages: drivers and mediators of the evolution of STEC and STEC-like pathogens

Bacteriophages, also known as phages, are viruses that infect bacteria. Until recently they have been ignored by most of the scientific community, but their impact upon our world is enormous. They are the most abundant lifeform on the globe and drive the diversity and abundance of bacteria around us, including, in many instances, the pathogenic profiles of many of mankind's most feared bacterial pathogens. This article focuses on how a group of bacteriophages, Stx-phages, which carry the genes encoding Shiga toxin, have driven and are driving the emergence of Shiga toxin-producing pathogens such as the infamous Escherichia coli O157:H7. Since the emergence of this foodborne pathogen as a cause of significant human disease in 1982, more than 500 different serogroups of E. coli have been reported to produce Shiga toxin, as well as a few other organisms. These events and many more are all controlled by the biology of Stx-phages.

Effect of a vaccine product containing type III secreted proteins on the probability of Escherichia coli O157:H7 fecal shedding and mucosal colonization in feedlot cattle

Preharvest intervention strategies to reduce Escherichia coli O157:H7 in cattle have been sought as a means to reduce human foodborne illness. A blinded clinical trial was conducted to test the effect of a vaccine product on the probability that feedlot steers, under conditions of natural exposure, shed E. coli O157:H7 in feces, are colonized by this organism in the terminal rectum, or develop a humoral response to the respective antigens. Steers (n = 288) were assigned randomly to 36 pens (eight head per pen), and pens were randomized to vaccination treatment in a balanced fashion within six dietary treatments of an unrelated nutrition study. Treatments included vaccination or placebo (three doses at 3-week intervals). Fecal samples for culture (n = 1,410) were collected from the rectum of each steer on pretreatment day 0 and posttreatment days 14, 28, 42, and 56. Terminal rectum mucosal (TRM) cells were aseptically collected for culture at harvest (day 57 posttreatment) by scraping the mucosa 3.0 to 5.5 cm proximal to the rectoanal junction. E. coli O157:H7 was isolated and identified with selective enrichment, immunomagnetic separation, and PCR confirmation. Vaccinated cattle were 98.3% less likely to be colonized by E. coli O157:H7 in TRM cells (odds ratio = 0.014, P < 0.0001). Diet was also associated with the probability of cattle being colonized (P = 0.04). Vaccinated cattle demonstrated significant humoral responses to Tir and O157 lipopolysaccharide. These results provide evidence that this vaccine product reduces E. coli O157:H7 colonization of the terminal rectum of feedlot beef cattle under conditions of natural exposure, a first step in its evaluation as an effective intervention for food and environmental safety.

Dietary inclusion of colicin e1 is effective in preventing postweaning diarrhea caused by F18-positive Escherichia coli in pigs

With worldwide concern over the use of antibiotics in animal agriculture and their contribution to the spread of antibiotic resistance, alternatives to conventional antibiotics are needed. Previous research in our laboratories has shown that colicin E1 is effective against some Escherichia coli strains responsible for postweaning diarrhea (PWD) in vitro. In this study we examined the efficacy of the dietary inclusion of colicin E1 in preventing experimentally induced PWD caused by F18-positive enterotoxigenic E. coli in young pigs. Twenty-four weaned pigs (23 days of age), identified by genotyping to be susceptible to F18-positive E. coli infections, were individually housed and fed diets containing 0, 11, or 16.5 mg colicin E1/kg diet. Two days after the start of the trial, all animals were orally inoculated with 1 x 10(9) CFU of each of two F18-positive E. coli strains isolated from pigs with PWD. The dietary inclusion of colicin E1 decreased the incidence and severity of PWD caused by F18-positive enterotoxigenic E. coli and improved the growth performance of the piglets. Additionally, the reduced incidence of PWD due to dietary colicin E1, lowered the levels of expression of the genes for interleukin 1beta and tumor necrosis factor beta in ileal tissues from these animals. The dietary inclusion of colicin E1 may be an effective alternative to conventional antibiotics in the diets of weaning pigs for the prevention of PWD caused by F18-positive enterotoxigenic E. coli.

Aspects of genome plasticity in pathogenic Escherichia coli

The species Escherichia coli comprises not only non-pathogenic or commensal variants that belong to the normal intestinal flora of most mammals, but also various pathogenic strains causing diverse intestinal and extraintestinal infections in man and animals. Virulence factors and mechanisms involved in pathogenesis have been successfully analyzed for many years resulting in a wealth of knowledge about many E. coli pathotypes. However, our knowledge on the genome content, diversity and variability between pathogenic and also non-pathogenic subtypes is only slowly accumulating. Pathotypes have been largely defined by the presence or absence of particular DNA segments that in most cases appear to have been acquired via horizontal gene transfer events. As these regions are frequently subjected to excisions, rearrangements, and transfers they contribute to the previously unexpected and underestimated rapid evolution of E. coli variants resulting in the development of novel strains and even pathotypes. In these studies various novel aspects of genome diversity and plasticity in extraintestinal and intestinal pathogenic E. coli pathotypes have been addressed and the results have been directly applied for the improvement of diagnostic methods.