Isolation and molecular characterization of Shiga toxin-producing Escherichia coli (STEC) from bovine and porcine carcasses in Poland during 2019-2023 and comparison with strains from years 2014-2018

The presence of Shiga toxin-producing Escherichia coli (STEC) on bovine and porcine carcasses during 2019-2023 was investigated. A total of 368 bovine and 87 porcine carcasses were tested using the ISO/TS 13136 standard and the STEC isolates were further characterized with whole genomic sequencing (WGS). It was found that 119 (32.3 %) of bovine and 14 (16.1 %) of porcine carcasses were positive for the stx Shiga toxin gene. Further analysis of the stx-positive samples allowed to isolate 32 (26.9 %) bovine and two (14.3 %) porcine STEC, respectively. Bovine isolates were classified into 21 different serotypes with the most prevalent O168:H8 (3 isolates), whereas two porcine STEC belonged to two serotypes that were not identified in bovine strains. Isolates of bovine carcass origin were mainly positive for the stx2 Shiga toxin gene, either alone or in combination with stx1 type (26 of 32; 81.2 % isolates). Two STEC from porcine carcasses were positive for the stx2e variant only. All STEC, irrespective of the origin, were negative for the eae intimin gene. MLST and cgMLST analyses of all strains tested revealed that they were diverse. However, a close molecular relationship between some bovine isolates based on cgMLST schemes was observed. Comparison of the current bovine STEC with those isolated between 2014 and 2018 showed that some of them consisted of the same MLST sequence types. However, based on cgMLST analysis only two cases of three genomes of STEC isolates each (two from period 2019-2023 and one isolated between 2014 and 2018) revealed up to 50 allelic differences.