Sensitivity of an immunomagnetic-separation-based test for detecting Escherichia coli O26 in bovine feces

Intimin gene (eae) subtype-based real-time PCR strategy for specific detection of Shiga toxin-producing Escherichia coli serotypes O157:H7, O26:H11, O103:H2, O111:H8, and O145:H28 in cattle feces

Shiga toxin-producing Escherichia coli (STEC) strains belonging to serotypes O157:H7, O26:H11, O103:H2, O111:H8, and O145:H28 are known to be associated with particular subtypes of the intimin gene (eae), namely, γ1, β1, ε, θ, and γ1, respectively. This study aimed at evaluating the usefulness of their detection for the specific detection of these five main pathogenic STEC serotypes in cattle feces. Using real-time PCR assays, 58.7% of 150 fecal samples were found positive for at least one of the four targeted eae subtypes. The simultaneous presence of stx, eae, and one of the five O group markers was found in 58.0% of the samples, and the five targeted stx plus eae plus O genetic combinations were detected 143 times. However, taking into consideration the association between eae subtypes and O group markers, the resulting stx plus eae subtype plus O combinations were detected only 46 times. The 46 isolation assays performed allowed recovery of 22 E. coli strains belonging to one of the five targeted STEC serogroups. In contrast, only 2 of 39 isolation assays performed on samples that were positive for stx, eae and an O group marker, but that were negative for the corresponding eae subtype, were successful. Characterization of the 24 E. coli isolates showed that 6 were STEC, including 1 O157:H7, 3 O26:H11, and 2 O145:H28. The remaining 18 strains corresponded to atypical enteropathogenic E. coli (aEPEC). Finally, the more discriminating eae subtype-based PCR strategy described here may be helpful for the specific screening of the five major STEC in cattle feces.

Rapid and specific detection of escherichia coli serogroups O26, O45, O103, O111, O121, O145, and O157 in ground beef, beef trim, and produce by loop-mediated isothermal amplification

Escherichia coli O157 and six additional serogroups of Shiga toxin-producing E. coli (STEC) (O26, O45, O103, O111, O121, and O145) account for the majority of STEC infections in the United States. In this study, O serogroup-specific genes (wzx or wzy) were used to design loop-mediated isothermal amplification (LAMP) assays for the rapid and specific detection of these leading STEC serogroups. The assays were evaluated in pure culture and spiked food samples (ground beef, beef trim, lettuce, and spinach) and compared with real-time quantitative PCR (qPCR). No false-positive or false-negative results were observed among 120 bacterial strains used to evaluate assay specificity. The limits of detection of various STEC strains belonging to these target serogroups were approximately 1 to 20 CFU/reaction mixture in pure culture and 10(3) to 10(4) CFU/g in spiked food samples, which were comparable to those of qPCR. Standard curves generated suggested good linear relationships between STEC cell numbers and LAMP turbidity signals. In various beef and produce samples spiked with two low levels (1 to 2 and 10 to 20 CFU/25 g) of respective STEC strains, the LAMP assays consistently achieved accurate detection after 6 to 8 h of enrichment. In conclusion, these newly developed LAMP assays may facilitate rapid and reliable detection of the seven major STEC serogroups in ground beef, beef trim, and produce during routine sample testing.

Dynamics of shiga-toxin producing Escherichia coli (STEC) and their virulence factors in cattle

Starting at birth, twenty Holstein calves were housed individually, in groups of five and finally in one large freestall while fecal samples were collected weekly for 25 weeks. From each sample, twenty isolates of Escherichia coli were screened for 6 virulence markers including shiga-toxin 1, 2, intimin, enterohemolysin, the fimbrial antigen efa1 and the adhesin saa. Dynamic models of transmission of E. coli were used to model the transmission of different virulotypes between calves and the loss of the same virulotypes from the calves. It was found that, once E. coli encoding shiga-toxins in combination with enterohemolysin were transmitted and established in a calf, they tended to be eliminated less efficiently compared to E. coli without this combination of virulence markers. It was concluded that the presence of certain combinations of virulence markers coincided with persistence of E. coli in the bovine gastrointestinal tract. In addition, the combinations of stx with either eae or ehxA in E. coli have a greater impact on the loss rates than on the transmission rates.

Associations between the presence of virulence determinants and the epidemiology and ecology of zoonotic Escherichia coli

The severity of human infection with pathogenic Escherichia coli depends on two major virulence determinants (eae and stx) that, respectively, produce intimin and Shiga toxin. In cattle, both may enhance colonization, but whether this increases fitness by enhancing cattle-to-cattle transmission in the field is unknown. In E. coli O157, the almost uniform presence of the virulence determinants in cattle isolates prevents comparative analysis. The availability to this study of extensive non-O157 E. coli data, with much greater diversity in carriage of virulence determinants, provides the opportunity to gain insight into their potential impact on transmission. Dynamic models were used to simulate expected prevalence distributions for serogroups O26 and O103. Transmission parameters were estimated by fitting model outputs to prevalence data from Scottish cattle using a Bayesian Markov chain Monte Carlo (MCMC) approach. Despite similar prevalence distributions for O26 and O103, their transmission dynamics were distinct. Serogroup O26 strains appear well adapted to the cattle host. The dynamics are characterized by a basic reproduction ratio (R(0)) of >1 (allowing sustained cattle-to-cattle transmission), a relatively low transmission rate from environmental reservoirs, and substantial association with eae on transmission. The presence of stx(2) was associated with reduced transmission. In contrast, serogroup O103 appears better adapted to the noncattle environment, characterized by an R(0) value of <1 for plausible test sensitivities, a significantly higher transmission rate from noncattle sources than serogroup O26, and an absence of fitness benefits associated with the carriage of eae. Thus, the association of eae with enhanced transmission depends on the E. coli serogroup. Our results suggest that the capacity of E. coli strains to derive fitness benefits from virulence determinants influences the prevalence in the cattle population and the ecology and epidemiology of the host organism.

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.