Faster, Cheaper, More Accurate Detection of STEC

Background

Most E. coli strains are not pathogenic, but Shiga toxin-producing E. coli (STEC) can cause foodborne illness in humans. STEC can be found in any undercooked or contaminated foods but is mostly associated with beef. Because of its potential to cause severe disease, STEC is monitored by beef processors across Canada. Current methods of testing (culture and PCR testing) easily identify E. coli O157 and genes that code for toxins. These genes can also be found in other bacteria, limiting the specificity of these tests despite their widespread adoption. Refining tests to only identify E. coli that can actively produce Shiga toxin could save time and money, while reducing food waste.

An experimental media has been developed to identify STEC that actively produce Shiga toxin. It needs to be tested under Canadian conditions to evaluate if it could be the future for STEC testing in Canadian beef processing plants.

Objectives

• To simplify, reduce costs and improve the specificity for monitoring Shiga toxin-producing E. coli (STEC) in a way that meets current standards set for Canadian beef and improves food safety.

What they did

Fecal samples collected from a Lethbridge feedlot were mixed with a broth and incubated to encourage the growth of STEC bacteria. Samples of the broth were plated on both the experimental media and the industry standard media. Potential STEC colonies were tested using polymerase chain reaction (PCR) to identify virulence factors that are consistently present in human outbreaks of STEC (stx1, stx2, and eae).

DNA was also extracted from the industry standard media and the original fecal broth to test for the same virulence factors using PCR, using traditional testing methods. This allowed the team to compare the ability of each method (experimental media and traditional media/PCR) to identify the samples contaminated with STEC.

What they learned

The experimental media produced halos around colonies of STEC, which made the STEC much easier to identify than using the traditional media (which also requires PCR validation). All colonies which had halos using the experimental media were validated as STEC by PCR, which is encouraging. However, there were some STEC colonies that failed to produce halos on the experimental media. The experimental media seemed to work better in freshly collected samples than it did when previously isolated STEC were evaluated. Potentially, the older STEC had been regrown and frozen many times and they may not be producing as much Shiga toxin as the STEC which were freshly isolated from the feces of feedlot cattle. Further work will be required to confirm if this is the case.

Alternatively, some of the older STEC included isolates which had less-common subtypes of Shiga toxin which the experimental media may not be able to detect. For the newly collected feedlot samples, STEC having a Shiga toxin + intimin (eae) greatly increased the likelihood that a colony would produce a halo, but the same was not true for the previously collected isolates. There needs to be a better understanding of the reasons why STEC are not identified by the experimental media before it could be used by food processors to detect STEC contamination.

Another finding was that a particular kind of STEC, E. coli O157:H7, was much less common at this feedlot than it was 20 years ago when an earlier study was conducted. In the earlier study, super-shedders which had up to 1 million cells of STEC per gram of feces were easy to find. In the current study, perhaps due to changes in diet, low-stress management and/or evolution of the population of microorganisms present in the cattle, STEC of any kind were much less common.

What it means

The STEC which produced halos using the experimental medium were quick and easy to identify compared to the two-step procedure (plating + PCR) required to confirm STEC using standard media. However, some STEC were not identified by the experimental media, and we need to understand the reasons for this. The experimental media is a work in progress, and its formulation was modified during this relatively short study. More investigation will be required to separate STEC-related effects such as lack of Shiga toxin production from media-related effects such as inability of the media to detect some STEC subtypes before the experimental medium could replace existing methods for detecting STEC contamination in commercial beef processing facilities.