Isolation and Characterization of Shiga toxin-Producing Escherichia coli from Minced Beef and Minced Pork on the Norwegian Market

Shiga toxin-producing Escherichia coli (STEC) is a zoonotic pathogen and a common cause of foodborne infection. The stx genes are the main virulence factors and are classified into different subtypes. Some subtypes are more associated with severe infections than others, i.e. the HUS-associated stx_2a. Other virulence factors, such as eae, are also commonly observed, particularly from severe infections. Ruminants, in particular cattle, are recognized as the main reservoir for STEC; however, STEC is also isolated from pigs. In the present study, minced beef and pork were analyzed for STEC using two different approaches. Minced beef was analyzed for STEC of specific serogroups (O26, O91, O103, O121, O145, and O157), while for minced pork, all STECs were of interest. A specific real-time PCR screening for stx_2a was also employed. In total, 308 samples of minced beef and 157 of minced pork were collected and analyzed using ISO TS 13136:2012. After screening of minced beef, 28 samples met the criteria for isolation and STEC was isolated from two samples. Furthermore, after screening for and isolation of potentially stx_2a-positive STEC, STEC was isolated from two more samples. The isolates were characterized by whole genome sequencing as STEC O26:H11 (stx_1a, eae), O91:H21 (stx_2b), O22:H8 (stx_1a, stx_2a), and O178:H19 (stx_1a, stx_2a). This suggests that the occurrence of STEC in minced beef is most likely underestimated when only looking for STEC of specific serogroups. While 46 of 157 samples were positive for stx₁ and/or stx₂ in the screening of minced pork, STEC was not isolated from any of the samples. No samples were positive for stx_2a. While the occurrence of STEC of the specific serogroups in minced beef is low, it is difficult to draw any firm conclusions about the occurrence of STEC in minced pork. The results highlight the issue with PCR-positive/culture-negative results as observed for both sample types.