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Sequence diversity of flagellin (fliC) alleles in pathogenic Escherichia coli
S D Reid1, R K Selander, T S Whittam
1Institute of Molecular Evolutionary Genetics and Department of Biology, Pennsylvania State University, University Park, Pennsylvania 16802, USA.
Abstract:
To study the molecular evolution of flagellin, the protein subunit specifying flagellar (H) antigens, the fliC genes from 15 pathogenic strains of Escherichia coli were amplified by PCR and sequenced. Comparison of fliC sequences of H6 and H7 strains revealed that alleles have a mosaic structure indicating the occurrence of past horizontal transfer of DNA segments between strains. The close similarity of H7 sequences also indicates the exchange of an entire fliC H7 allele between distant clonal lineages. In addition, the ratio of silent substitutions to amino acid replacements suggests that a short segment in the central region of fliC has been under positive selection in the divergence of H6 and H7 alleles. Phylogenetic analysis demonstrates that the fliC sequences of O157:H7 and O55:H7 serotypes are nearly identical and highly divergent from those of E. coli strains expressing H6 and H2 flagellar antigens. A nonmotile clone of sorbitol-fermenting O157 has rapidly accumulated multiple mutations in fliC, presumably as a result of the silencing of flagellin expression.
Insights
Molecular evolution of Escherichia coli flagellin (fliC) reveals horizontal gene transfer and positive selection. Pathogenic E. coli strains exchange fliC alleles, shaping antigen diversity and evolution.
Area of Science:
- Microbiology
- Molecular Biology
- Evolutionary Biology
Background:
- Flagellin (H antigens) are crucial for bacterial motility and immune recognition in pathogenic Escherichia coli.
- Understanding the molecular evolution of flagellin genes (fliC) provides insights into bacterial adaptation and virulence.
Purpose of the Study:
- To investigate the molecular evolution of the fliC gene in pathogenic Escherichia coli strains.
- To identify mechanisms driving the diversity of flagellar antigens.
Main Methods:
- PCR amplification and sequencing of fliC genes from 15 pathogenic E. coli strains.
- Comparative sequence analysis of fliC alleles.
- Phylogenetic analysis to determine evolutionary relationships.
Main Results:
- E. coli fliC alleles exhibit mosaic structures, indicating horizontal DNA transfer between strains.
- Evidence of entire fliC H7 allele exchange among distant clonal lineages.
- Positive selection identified in a central fliC region during H6 and H7 allele divergence.
- O157:H7 and O55:H7 serotypes show nearly identical fliC sequences, divergent from H6 and H2 strains.
- A nonmotile O157 clone accumulated fliC mutations after silencing flagellin expression.
Conclusions:
- Horizontal gene transfer and recombination are significant drivers of flagellin diversity in E. coli.
- Positive selection acts on specific regions of the fliC gene, contributing to antigen evolution.
- Phylogenetic analysis highlights distinct evolutionary trajectories for different E. coli serotypes and clones.