Clonal Analysis of STEC
The diversity of E. coli strains that carry stx genes is enormous. For example, Lior (Lior, 1994) reports 36 different O:H serotypes, including O157:H7, that have been isolated from humans cases of disease, and more than 60 distinct serotypes have been cultured from cattle and food (Lior, 1994). To provide a clonal framework for studying the diversity of STEC, we have assembled a collection of strains and characterized them by multilocus enzyme electrophoresis. The sample includes Stx-positive isolates from cases of HC and HUS in the U.S.A. (supplied by CDC) and Canada (40 different serotypes, many of which were nontypeable with standard O antisera, supplied by H. Lior of LCDC), Stx-positive strains from adults with diarrhea from Bangkok, Thailand (Bettelheim et al., 1990), non-O157 strains from children with diarrhea in Seattle, Washington (Bokete et al., 1997), probe-positive Stx strains isolated from various food sources in Seattle area supermarkets (Samadpour et al., 1993), and an O104:H21 (G5506) from an outbreak of HUS in Montana (CDC, 1995). For comparison, both the ETs from Fig. 1 (Whittam et al., 1988) and the DEC (diarrheagenic E. coli) ET designations (Whittam et al., 1993) are shown.
There are four groups of Stx-positive strains where common ETs (represented by independent isolates recovered at separate times from different places) are concentrated. EHEC 1 includes O157:H7 and its nonmotile relatives. The only O type found in the O157:H7 complex other than O157 is O55:H7 which typically in non-cytotoxigenic.
Dendrogram of EHEC 1 (O157:H7) and STEC 2 (O103) clonal groups
EHEC 2 is the most common group of non-O157 Stx-producing strains. It includes strains of serotype O111:H8 or the nonmotile relatives (O111:H-) and a variety of serotypes such as O26:H11 and O111:H11 (Whittam, et al., 1993). Many EHEC 2 strains are nonmotile or nontypeable with standard antisera. Because this group has the same virulence factors as E. coli O157:H7 and are recovered from patients with hemmorhagic colitis and hemolytic uremic syndrome, they are classified together with O157:H7 as EHEC. However, evolutionary genetic analysis indicates that this group is substantially divergent from E. coli O157:H7 and for this reason we refer to these non-O157 EHEC strains as the EHEC 2 group (Whittam & McGraw, 1996). EHEC 2 includes a common nonmotile O111 clone (ET 8 of Campos et al., 1994) which occurs in both North and South America. Members of this clone are eaeA+, and produces both Stx-I and enterohemolysin (Campos, et al., 1994). Elsewhere we have shown that the EHEC 2 group includes nonmotile O111 isolate 3007-85, an Stx-producing strain originally isolated from a case of hemorrhagic colitis (Bopp et al., 1987), which is highly virulent to gnotobiotic pigs (Tzipori et al., 1989), and RDEC-1, an O15:NM isolate from a case of rabbit diarrhea (Cantey & Blake, 1977), which has been used as a model organism for human EPEC infection. Because EHEC 2 strains share the prominent virulence factors of O157:H7 and cause similar disease, and are also common in the bovine reservoir, it is this organism that is most likely to emerge as an important foodborne pathogen.
Dendrogram of EHEC 2 (O26/O111) and STEC 1 (H21) clonal groups
The third group is designated STEC 1 and includes many different O types, usually associated with H21 flagellar antigen. These strains typically do not express intimin and do not carry the LEE pathogencity island. The most common serotypes are O113:H21, OX3:H21, and O91:H21. Strains in this groups include B2F1 (O91:H21), CL-3 (O113:H21), and 87-307 (O113:H21). Members of this clonal group are commonly isolated from humans cases of disease and bovids in North America, Europe, and Asia.
The fourth group of STEC is composed of three ETs with serotype O103:H2, O103:H6, or O45:H2. Little is known about the virulence of this group, but the clonal analysis indicates that the chromosomal background is highly divergent from other STEC groups. Further study of structural variation in the Shiga toxins, expression and regulation of toxin genes, and the identification of ancillary virulence factors, should be conducted with reference to the clonal framework of STEC.
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