Extraintestinal pathogenic Escherichia coli carrying the Shiga Toxin gene stx2.

The 2011 outbreak of infections with Escherichia coli with characteristics of both enteroaggregative E. coli (EAEC) and Shiga toxin-producing E. coli (STEC) caused a paradigm shift with regard to the human pathogenicity of STEC (1). Diarrheagenic E. coli (DEC) usually does not cause extraintestinal diseases such as urinary tract infections and bacteremia (2). The DNA of 193 E. coli isolates from adult bacteremic patients described previously (3) was analyzed for the presence of stx1 and stx2 (4). All of the strains were stx1 negative, but nine (4.7%) were stx2 positive. According to the Scheutz subtyping protocol (5), seven belonged to subtype stx2c and two belonged to stx2d. Table 1 shows the associations between the clinical presentations of the patients and their stx2 statuses. Arbitrarily chosen, DNA from 2 of the stx2-postive strains and 26 of the stx-negative strains was analyzed for additional DEC genes (4, 6) and extraintestinal pathogenic E. coli (ExPEC) genes by PCR assay as selected by B. A. Lindstedt et al. (unpublished data) (Table 2). One of the two stx2-positive strains that were further analyzed for additional virulence markers carried both stx2c and aggR. Seven of the 26 stx-negative strains carried other DEC genes; with or without concomitant ExPEC genes. The stx2-positive strains were recultured and tested for toxin production by ImmunoCard STAT! EHEC (Meridian Bioscience, Inc.). All nine stx2-positive strains tested negative for toxin production. Therefore, another isolation of DNA from the viable strains was performed and new PCR assays detecting stx and aggR were run. Surprisingly, the toxin gene was lost by all nine but aggR was reproduced in the relevant strain, indicating no mix-up of strains or DNA. Loss of stx2-carrying bacteriophages is not a novel phenomenon (7, 8), but to our knowledge, such a frequent loss has not been described previously. Reports of bacteremia or sepsis in patients with hemolyticuremic syndrome (HUS) caused by STEC do exist (2, 9–11). In our study, we found that almost 5% of the strains found in adult patients with bacteremia caused by E. coli carried stx2 subtypes associated with HUS (12–14). Not having symptoms of gastroenteritis was associated with stx2-positive status, but this finding seems rather implausible. Further characterization of two stx2ccarrying isolates showed features of STEC and EAEC, as well as ExPEC, which, to our knowledge, is a novel finding. If reproduced, these stx findings may have consequences for infection control. And if an association with clinical presentation is found, differential diagnoses of bacteremia with E. coli should include STEC colitis, as well as HUS. This is particularly relevant for elderly patients, who may have vague symptoms and comorbidity complicating the clinical picture. Furthermore, elderly, institutionalized patients have a unique susceptibility to STEC infection and its sequelae (15). Microbiological analyses of E. coli blood culture isolates to detect stx should therefore be encouraged, and infection control measures and contact tracing should be implemented when stx-carrying E. coli bacteremia is confirmed.