Super-shedding and the link between human infection and livestock carriage of Escherichia coli O157

Dietary interactions and interventions affecting Escherichia coli O157 colonization and shedding in cattle

Escherichia coli O157 is an important foodborne pathogen affecting human health and the beef cattle industry. Contamination of carcasses at slaughter is correlated to the prevalence of E. coli O157 in cattle feces. Many associations have been made between dietary factors and E. coli O157 prevalence in cattle feces. Preharvest interventions, such as diet management, could reduce the fecal prevalence and diminish the impact of this adulterant. Dietary influences, including grain type and processing method, forage quality, and distillers grains have all been associated with E. coli O157 prevalence. In addition, several plant compounds, including phenolic acids and essential oils, have been proposed as in-feed intervention strategies. The specific mechanisms responsible for increased or decreased E. coli O157 shedding or survival are not known but are often attributed to changes in hindgut ecology induced by diet types. Some interventions may have a direct bacterial effect. Frequently, results of studies are conflicting or not repeatable, which speaks to the complexity of the hindgut ecosystem, variation in animal feed utilization, and variation within feed products. Still, understanding specific mechanisms, driven by diet influences, responsible for E. coli O157 shedding will aid in the development and implementation of better and practical preharvest intervention strategies.

Immunization of cattle with a combination of purified intimin-531, EspA and Tir significantly reduces shedding of Escherichia coli O157:H7 following oral challenge

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a human pathogen that can cause gastrointestinal disease with potentially fatal consequences as a result of systemic Shiga toxin activity. Cattle are the main reservoir host of EHEC O157 and interventions need to be developed that prevent cattle colonization or limit shedding of the organism from this host. EHEC O157 predominately colonizes the bovine terminal rectum and requires a type III secretion system (T3SS) for adherence and persistence at this site. A vaccine based on concentrated bacterial supernatant that contains T3S proteins has shown some efficacy. Here we have demonstrated that vaccination with a combination of antigens associated with T3S-mediated adherence; the translocon filament protein, EspA, the extracellular region of the outer membrane adhesin, intimin, and the translocated intimin receptor (Tir) significantly reduced shedding of EHEC O157 from experimentally infected animals. Furthermore, this protection may be augmented by addition of H7 flagellin to the vaccine preparation that has been previously demonstrated to be partially protective in cattle. Protection correlates with systemic and mucosal antibody responses to the defined antigens and validates the targeting of these colonization factors.

Experimental infection in calves with a specific subtype of verocytotoxin-producing Escherichia coli O157:H7 of bovine origin

BACKGROUND

In Sweden, a particular subtype of verocytotoxin-producing Escherichia coli (VTEC) O157:H7, originally defined as being of phage type 4, and carrying two vtx2 genes, has been found to cause the majority of reported human infections during the past 15 years, including both sporadic cases and outbreaks. One plausible explanation for this could be that this particular subtype is better adapted to colonise cattle, and thereby may be excreted in greater concentrations and for longer periods than other VTEC O157:H7 subtypes.

METHODS

In an experimental study, 4 calves were inoculated with 109 colony forming units (cfu) of strain CCUG 53931, representative of the subtype VTEC O157:H7 (PT4;vtx2;vtx2c). Two un-inoculated calves were co-housed with the inoculated calves. Initially, the VTEC O157:H7 strain had been isolated from a dairy herd with naturally occurring infection and the farm had previously also been linked to human infection with the same strain. Faecal samples were collected over up to a 2-month period and analysed for VTEC O157 by immuno-magnetic separation (IMS), and IMS positive samples were further analysed by direct plating to elucidate the shedding pattern. Samples were also collected from the pharynx.

RESULTS

All inoculated calves proved culture-positive in faeces within 24 hours after inoculation and the un-inoculated calves similarly on days 1 and 3 post-inoculation. One calf was persistently culture-positive for 43 days; in the remainder, the VTEC O157:H7 count in faeces decreased over the first 2 weeks. All pharyngeal samples were culture-negative for VTEC O157:H7.

CONCLUSION

This study contributes with information concerning the dynamics of a specific subtype of VTEC O157:H7 colonisation in dairy calves. This subtype, VTEC O157:H7 (PT4;vtx2;vtx2c), is frequently isolated from Swedish cattle and has also been found to cause the majority of reported human infections in Sweden during the past 15 years. In most calves, inoculated with a representative strain of this specific subtype, the numbers of shed bacteria declined over the first two weeks. One calf could possibly be classified as a high-shedder, excreting high levels of the bacterium for a prolonged period.

Exploiting strain diversity to expose transmission heterogeneities and predict the impact of targeting supershedding

When a few individuals generate disproportionately many secondary cases, targeted interventions can theoretically lead to highly efficient control of the spread of infection. Practical exploitation of heterogeneous transmission requires the sources of variability to be quantified, yet it is unusual to have empirical data of sufficient resolution to distinguish their effects. Here, we exploit extensive data on pathogen shedding densities and the distribution of cases, collected from the same population within the same spatio-temporal window, to expose the comparative epidemiology of independent Escherichia coli O157 strains. For this zoonotic pathogen, which exhibits high-density shedding (supershedding) and heterogeneous transmission in its cattle reservoir, whether targeting supershedding could be an effective control depends critically on the proposed link between shedding density and transmissibility. We substantiate this link by showing that our supershedder strain has nearly triple the R(0) of our non-supershedder strain. We show that observed transmission heterogeneities are strongly driven by superspreading in addition to supershedding, but that for the supershedder strain, the dominant strain in our study population, there remains sufficient heterogeneity in contribution to R(0) from different shedding densities to allow exploitation for control. However, in the presence of substantial within-host variability, our results indicate that rather than seek out supershedders themselves, the most effective controls would directly target the phenomenon of pathogen supershedding with the aim of interrupting or preventing high shedding densities. In this system, multiple sources of heterogeneity have masked the role of shedding densities-our potential targets for control. This analysis demonstrates the critical importance of disentangling the effects of multiple sources of heterogeneity when designing targeted interventions.

Longitudinal study of Escherichia coli O157:H7 in a beef cattle feedlot and role of high-level shedders in hide contamination

The objectives of the study described here were (i) to investigate the dynamics of Escherichia coli O157:H7 fecal and hide prevalence over a 9-month period in a feedlot setting and (ii) to determine how animals shedding E. coli O157:H7 at high levels affect the prevalence and levels of E. coli O157:H7 on the hides of other animals in the same pen. Cattle (n = 319) were distributed in 10 adjacent pens, and fecal and hide levels of E. coli O157:H7 were monitored. When the fecal pen prevalence exceeded 20%, the hide pen prevalence was usually (25 of 27 pens) greater than 80%. Sixteen of 19 (84.2%) supershedder (>10(4) CFU/g) pens had a fecal prevalence greater than 20%. Significant associations with hide and high-level hide (>/=40 CFU/100 cm(2)) contamination were identified for (i) a fecal prevalence greater than 20%, (ii) the presence of one or more high-density shedders (>/=200 CFU/g) in a pen, and (iii) the presence of one or more supershedders in a pen. The results presented here suggest that the E. coli O157:H7 fecal prevalence should be reduced below 20% and the levels of shedding should be kept below 200 CFU/g to minimize the contamination of cattle hides. Also, large and unpredictable fluctuations within and between pens in both fecal and hide prevalence of E. coli O157:H7 were detected and should be used as a guide when preharvest studies, particularly preharvest intervention studies, are designed.

Characterization of the effects of salicylidene acylhydrazide compounds on type III secretion in Escherichia coli O157:H7

Recent work has highlighted a number of compounds that target bacterial virulence by affecting gene regulation. In this work, we show that small-molecule inhibitors affect the expression of the type III secretion system (T3SS) of Escherichia coli O157:H7 in liquid culture and when this bacterium is attached to bovine epithelial cells. Inhibition of T3SS expression resulted in a reduction in the capacity of the bacteria to form attaching and effacing lesions. Our results show that there is marked variation in the abilities of four structurally related compounds to inhibit the T3SS of a panel of isolates. Using transcriptomics, we performed a comprehensive analysis of the conserved and inhibitor-specific transcriptional responses to these four compounds. These analyses of gene expression show that numerous virulence genes, located on horizontally acquired DNA elements, are affected by the compounds, but the number of genes significantly affected varied markedly for the different compounds. Overall, we highlight the importance of assessing the effect of such "antivirulence" agents on a range of isolates and discuss the possible mechanisms which may lead to the coordinate downregulation of horizontally acquired virulence genes.

Recent advances in the microbial safety of fresh fruits and vegetables

Foodborne illness outbreaks linked to fresh produce are becoming more frequent and widespread. High impact outbreaks, such as that associated with spinach contaminated with Escherichia coli O157:H7, resulted in almost 200 cases of foodborne illness across North America and >$300 m market losses. Over the last decade there has been intensive research into gaining an understanding on the interactions of human pathogens with plants and how microbiological safety of fresh produce can be improved. The following review will provide an update on the food safety issues linked to fresh produce. An overview of recent foodborne illness outbreaks linked to fresh produce. The types of human pathogens encountered will be described and how they can be transferred from their normal animal or human host to fresh produce. The interaction of human pathogens with growing plants will be discussed, in addition to novel intervention methods to enhance the microbiological safety of fresh produce.