Shiga-toxin-producing (STEC) of different serotypes are known to harbour large plasmids. The aim of this study was to investigate, using the example of the plasmid-encoded serine protease EspP, whether these plasmids are a uniform genetic element present in STEC. Examination of 201 diarrhoeagenic strains using a newly developed -specific PCR showed that is specific for STEC and present in 57% of STEC belonging to 16 different serotypes. The genes of the 16 STEC serotypes varied to a certain extent, as shown by nucleotide sequence and restriction enzyme analyses, but the DNA regions adjacent to the gene were completely different. When two further STEC-plasmid markers, the catalase-peroxidase gene and the enterohaemorrhagic -haemolysin gene EHEC- were included, many combinations of the three markers were found, depending in part on the serotype. In addition, strains possessing none of the three markers still harboured large plasmids. In the most prevalent STEC serogroup, O157, it was observed that the plasmid of sorbitol-fermenting STEC O157:H lacks the and genes although both genes are present in the plasmid of the non-sorbitol-fermenting STEC O157:H7. The EHEC- gene, however, is present in both. In conclusion, this study shows that the large plasmids of STEC are not uniform genetic elements but heterogeneous in both their gene composition and arrangement.


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