Discovery and Phylogenetic Analysis of Novel Members of Class b Enterotoxigenic Escherichia coli Adhesive Fimbriae

Abstract

Enterotoxigenic Escherichia coli (ETEC) is recognized to be a common cause of acute watery diarrhea in children from developing countries. Colonization factors (CFAs) have been identified predominantly in ETEC isolates secreting heat-stable enterotoxin (ST) or cosecreting ST with a heat-labile toxin (LT). We hypothesized that LT-only-secreting ETEC produces unique colonization factors not previously described in ST and LTST-secreting ETEC. A set of degenerate primers based on nucleotide sequence similarities between the major structural genes of CS20 (csnA), CS18 (fotA), CS12 (cswA), and porcine antigen 987 (fasA) was developed and used to screen a collection of 266 LT-secreting ETEC isolates in which no known CFA was detected. PCR-amplified products of different molecular masses were obtained from 49 (18.4%) isolates. Nucleotide sequence analysis of the PCR amplicons followed by GenBank nucleotide BLASTn analysis revealed five novel DNA sequences; translated amino acid BLASTx analysis confirmed sequence similarity to class 1b major structural proteins encoded by csnA, fotA, and fasA. Strains expressing the novel CFAs were phylotyped and analyzed using multilocus sequence typing (MLST; Achtman scheme), and the types detected were compared to those of a collection of archived global E. coli strains. In conclusion, application of the degenerate primer sets to ETEC isolates from surveillance studies increased the total number of ETEC isolates with detectable CFAs by almost 20%. Additionally, MLST analysis suggests that for many CFAs, there may be a requirement for certain genetic backgrounds to acquire and maintain plasmids carrying genes encoding CFAs.