Shiga toxin-negative attaching and effacing Escherichia coli:: Distinct clinical associations with bacterial phylogeny and virulence traits and inferred in-host pathogen evolution

Background. Attaching and effacing Escherichia coli (AEEC) that lack Shiga toxin genes (stx) and the enteropathogenic E. coli adherence factor (EAF) plasmid (stx-/EAF-) are classified as atypical enteropathogenic E. coli and cause diarrhea worldwide. However, it is unknown whether there are bacterial lineage-specific human disease phenotypes. We compared stx-/EAF- AEEC recovered from patients (mostly children) with bloody and nonbloody diarrhea. Methods. stx-/EAF- AEEC were isolated using eae colony blot hybridization and were serotyped, tested (by polymerase chain reaction) for putative virulence genes, and analyzed for phylogenetic relationships by use of multilocus sequence typing. Results. During the period 1995-2007, stx-/EAF- AEEC were isolated as the only bacterial pathogens from stool specimens obtained from 18 (15.3%) of 118 patients with bloody diarrhea and from 141 (1.3%) of 10,550 patients with nonbloody diarrhea (P<.001). All but 1 of 18 strains recovered from patients with bloody diarrhea resembled enterohemorrhagic E. coli (EHEC) on the basis of serotypes, non-stx virulence profiles, and multilocus sequence types. In contrast, most (75.9%) of 141 stx-/EAF- AEEC recovered from patients with nonbloody diarrhea belonged to other serotypes and differed from the former strains phylogenetically and with regard to virulence genes. Three of 18 patients with bloody diarrhea and none of 141 patients with nonbloody diarrhea who shedded stx-/EAF- AEEC developed hemolytic uremic syndrome. Conclusions. Most stx-/EAF- AEEC associated with bloody diarrhea are plausibly EHEC that lost Shiga toxin during infection (EHEC-LST). To prevent serious complications of such infections, Shiga toxin-independent diagnostic strategies to accurately and rapidly identify such patients should be developed and applied. Multilocus sequence typing has potential to distinguish EHEC-LST from less pathogenic stx-/EAF- AEEC.