Genetic profiling of enterohemorrhagic Escherichia coli strains in relation to clonality and clinical signs of infection

Abstract

Sixty-seven human strains of enterohemorrhagic Escherichia coli (EHEC) (from patients with more or less severe symptoms) were serogrouped and arranged according to pulsed-field gel electrophoresis (PFGE) patterns. We used PCR to investigate the strains according to known or putative virulence factors, and associations with disease were studied. All EHEC strains with the same PFGE pattern belonged to the same serogroup. On the contrary, two serogroups (O157 and O8) included strains with different PFGE patterns. We found several different combinations of chromosomal and plasmid-borne determinants, encoding the putative virulence factors, among the strains. As judged from clinical symptoms, there was no marked difference in pathogenicity among the strains and their combinations of virulence traits. All strains of O157 had the genes coding for verocytotoxin (VT) 2, intimin (eaeA), E. coli hemolysin (E-hly), and secreted serine protease (espP). Among EHEC non-O157 strains, the genes coding for VT1 and VT2 were equally dispersed. EaeA positivity was just as common among VT1- as VT2-positive strains. Among the plasmid-borne determinants, E-hly and espP were the most common and E-hly might be a pathogenicity marker among EHEC non-O157 strains. The conclusion is that PFGE is a very useful tool in epidemiological studies. The EHEC plasmids are heterogeneous in their gene composition, with the four plasmid-borne determinants found in many combinations. There was no reliable correlation between chromosomal and plasmid-borne virulence factors and human disease.

FIG. 1.

Dendrogram of EHEC strains, both O157 and non-O157, isolated from routine fecal specimens sent to the Bacteriological Laboratory of the Sahlgrenska University Hospital in Göteborg. The dendrogram was constructed with the use of the unweighted pair group method with arithmetic averages to a matrix by comparison of XbaI PFGE patterns. A, E, and S are examples of genetically related clusters of strains, while AB, AH, and AC are examples of strains not related.

FIG. 2.

Electrophoretic analysis of the PCR products obtained by amplification of the plasmid-borne DNA sequences coding for E-hly (lane 2), etpD (lane 4), katP (lane 6), and espP (lane 8). The target DNA was obtained from an EHEC strain with PFGE pattern A isolated from a patient. Lanes 3, 5, 7, and 9 are amplifications with primers without template DNA. Lanes 1 and 10 are a 1-kb DNA ladder (New England BioLabs, Inc.).