Association of cytolethal distending toxin-II gene-positive Escherichia coli with Escherichia albertii, an emerging enteropathogen.

Cytolethal distending toxin (CDT)-producing Escherichia coli have been isolated from patients with diarrhea, sepsis and urinary tract infection. CDT of E. coli is divided into five types (CDT-I through CDT-V) based on differences in amino acid sequences and its genomic location. However, in our recent studies, a few strains of cdt-II gene-positive bacteria, initially identified as atypical E. coli, were re-identified as Escherichia albertii, an emerging enteropathogen, by extensive characterization including multilocus sequence (MLS) analysis and sugar utilization tests. This finding prompted us to investigate if bacteria previously identified as cdt-II gene-positive E. coli might be E. albertii. In the present study, we therefore re-examined the identity of 20 cdt-II gene-positive bacteria isolated from children with diarrhea, which were initially identified as atypical E. coli. By extensive sugar utilization tests, these bacteria showed a closer relatedness to E. albertii than E. coli, because they did not ferment any of the tested sugars including dulcitol, lactose, d-melibiose, l-rhamnose and d-xylose. Further, both phylogenetic analyses based on nucleotide sequences of 7 housekeeping genes (MLS analysis) and rpoB gene showed that all the cdt-II gene-positive bacteria belonged to a distinct lineage of E. albertii from those of E. coli and Shigella boydii. They were also positive by an E. albertii-specific PCR. Taken together, these data suggest that cdt-II gene-positive bacteria previously identified as E. coli are actually E. albertii. Therefore, we suggest a new definition for cdt-II gene-positive E. coli as E. albertii with the inclusion of CDT-II in E. albertii CDT.