University of California-Davis researchers who recently evaluated the practical functionality of rapid E. coli tests weren’t seeking to discover which test was best.

Study compares test procedures for E. coli on leafy greens

Courtesy Trevor Suslow

The UC-Davis department of plant sciences compared four rapid test kits for E. coli to determine their strengths and weaknesses.

Rather, the aim was to look at the tests and measure their limitations and strengths within needs identified by major lettuce and leafy green handlers and processors. Key test formats also were compared against a recently released regulatory “gold standard methodology.”

“Shortly after the 2006 outbreak of E. coli in spinach, the California Department of Public Health and (Food and Drug Administration) got together and modified the primary existing regulatory protocol to adopt better measures and methods for produce,” said Trevor Suslow, extension research specialist with the department of plant sciences at UC-Davis and an author of the study. “We were comparing four commercial tests tests to that one.”

The four primary rapid test kits Suslow and his research partner, Carol D’Lima, compared were Neogen Reveal, SDI RapidChek, BioControl GDS O157 and Qualicon BAX O157. Nine fresh leafy greens were inoculated with E. coli.

“We found it came down to two things,” Suslow said. “In the right conditions, (the tests) do work.”

In fact, Suslow said that the kits based on Polymerase Chain Reaction — or PCR — method, a very specific test to look for this specific pathogen, was most consistent for identifying the presence of pathogen in the shortest period of time.

However, Suslow said the study also showed that the four tests were susceptible to producing false-negative readings when detection conditions were most time-sensitive, or when there was interference from the leafy green samples in some form during testing. The study showed that red-pigmented leafy vegetables, such as red butter lettuce, curly endive, red lettuce and lollo rosa, especially were susceptible to false-negative readings. Again, PCR detection was minimally affected.

But other tests, known as immunoassays, were negatively affected by delays in achieving critical threshold populations of the target pathogen during the allowed enrichment period. In other words, pathogens could be missed when testing in a limited timeframe to meet shipping demands, before bacteria had a chance to grow.

“The end result is that awareness is building in that, if you’re testing within a timeframe of a single-production shift and limited pre-shipping cold storage options, you could miss pathogens that are present,” Suslow said. “One answer is to take more growth time to allow the E. coli to reach critical numbers for each type of kit.”

Suslow said it also shows the need to develop more sensitive methods of detecting pathogens.

“Those (developments) are well underway in many public labs and by private developers,” he said.