Infection and Immunity, December 2000, p. 7028-7038, Vol. 68, No. 12
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Division of Infectious Diseases, Department of Medicine, University of Maryland School of Medicine, Baltimore, Maryland 21201,1 and Institute of Molecular Evolutionary Genetics and Department of Biology, Pennsylvania State University, University Park, Pennsylvania 168022
Received 12 June 2000/Returned for modification 7 September 2000/Accepted 13 September 2000
Typical enteropathogenic Escherichia coli (EPEC)
strains produce bundle-forming pili (BFP), type IVB fimbriae that have
been implicated in EPEC virulence, antigenicity, autoaggregation, and localized adherence to epithelial cells (LA). BFP are polymers of
bundlin, a pilin protein that is encoded by the bfpA gene
found on a large EPEC plasmid. Striking sequence variation has
previously been observed among type IV pilin genes of other
gram-negative bacterial pathogens (e.g., Pseudomonas and
Neisseria spp.). In contrast, the established sequences of
bfpA genes from two distantly related prototype EPEC
strains vary by only a single base pair. To determine whether bundlin
sequences vary more extensively, we used PCR to amplify the
bfpA genes from 19 EPEC strains chosen for their various
serotypes and sites and years of isolation. Eight different
bfpA alleles were identified by sequencing of the PCR
products. These alleles can be classified into two major groups. The
group contains three alleles derived from strains carrying O55,
O86, O111, O119, O127, or O128 somatic antigens. The
group contains
five alleles derived from strains carrying O55, O110, O128ab, O142, or
nontypeable antigens. Sequence comparisons show that bundlin has highly
conserved and variable regions, with most of the variation occurring in
the C-terminal two-thirds of the protein. The results of multilocus
enzyme electrophoresis support the hypothesis that bfpA
sequences have spread horizontally across distantly related clonal
lineages. Strains with divergent bundlin sequences express bundlin
protein, produce BFP, and carry out autoaggregation and LA. However,
four strains lack most or all of these phenotypes despite having an
intact bfpA gene. These results have important implications
for our understanding of bundlin structure, transmission of the
bfp gene cluster among EPEC strains, and the role of
bundlin variation in the evasion of host immune system responses.
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