Peter Feng, Ph.D.
Food and Drug Administration-Center for Food Safety and Applied Nutrition (FDA-CFSAN)
5100 Paint Branch Parkway
College Park, MD 20740
Complexities in determining the health risk of STEC in fresh produce
Fresh produce, including bagged salads, can have high microbial loads, and occasionally contain, Shiga toxin-producing E. coli (STEC). STEC have been isolated from many types of produce in the U.S. but most often from fresh spinach and cilantro. There are estimated to be ~300 known STEC serotypes that include highly virulent strains but, also, strains that do not appear to be pathogenic. Hence, in terms of food safety, it is critical to determine which STEC strains have the potential to cause severe diseases. Most health risk criteria proposed for STEC are based on common factors and traits carried by STEC strains that have caused severe infections. These include the ability to adhere to intestinal cells, the production of certain Shiga toxin (Stx) subtypes and STEC strains of certain serotypes. These criteria are commonly tested by PCR but testing can be labor and time intensive as there are various adherence factors, some with multiple alleles, three Stx1 and seven Stx2 subtypes. Genetic assays are also used to test for selected O types, but the coverage is limited and not inclusive of the various pathogenic STEC serotypes that have been found in produce. Traditional serology provides broader coverage but only reference laboratories have the capability to fully serotype STEC and the process can take 1 to 2 weeks, which is too long for short shelf life product like fresh produce. Furthermore, many STEC isolated from produce often yield only partial serotypes or are not typeable. As a result, critical data on STEC are not timely available and often incomplete, hence, the health risks of most STEC strains isolated from fresh produce remains undetermined.