Evolution of Salmonella enterica Virulence via Point Mutations in the Fimbrial Adhesin

Whereas the majority of pathogenic Salmonella serovars are capable of infecting many different animal species, typically producing a self-limited gastroenteritis, serovars with narrow host-specificity exhibit increased virulence and their infections frequently result in fatal systemic diseases. In our study, a genetic and functional analysis of the mannose-specific type 1 fimbrial adhesin FimH from a variety of serovars of Salmonella enterica revealed that specific mutant variants of FimH are common in host-adapted (systemically invasive) serovars. We have found that while the low-binding shear-dependent phenotype of the adhesin is preserved in broad host-range (usually systemically non-invasive) Salmonella, the majority of host-adapted serovars express FimH variants with one of two alternative phenotypes: a significantly increased binding to mannose (as in S. Typhi, S. Paratyphi C, S. Dublin and some isolates of S. Choleraesuis), or complete loss of the mannose-binding activity (as in S. Paratyphi B, S. Choleraesuis and S. Gallinarum). The functional diversification of FimH in host-adapted Salmonella results from recently acquired structural mutations. Many of the mutations are of a convergent nature indicative of strong positive selection. The high-binding phenotype of FimH that leads to increased bacterial adhesiveness to and invasiveness of epithelial cells and macrophages usually precedes acquisition of the non-binding phenotype. Collectively these observations suggest that activation or inactivation of mannose-specific adhesive properties in different systemically invasive serovars of Salmonella reflects their dynamic trajectories of adaptation to a life style in specific hosts. In conclusion, our study demonstrates that point mutations are the target of positive selection and, in addition to horizontal gene transfer and genome degradation events, can contribute to the differential pathoadaptive evolution of Salmonella

Analysis of the Genome of the Escherichia coli O157:H7 2006 Spinach-Associated Outbreak Isolate Indicates Candidate Genes That May Enhance Virulence ▿ †

In addition to causing diarrhea, Escherichia coli O157:H7 infection can lead to hemolytic-uremic syndrome (HUS), a severe disease characterized by hemolysis and renal failure. Differences in HUS frequency among E. coli O157:H7 outbreaks have been noted, but our understanding of bacterial factors that promote HUS is incomplete. In 2006, in an outbreak of E. coli O157:H7 caused by consumption of contaminated spinach, there was a notably high frequency of HUS. We sequenced the genome of the strain responsible (TW14359) with the goal of identifying candidate genetic factors that contribute to an enhanced ability to cause HUS. The TW14359 genome contains 70 kb of DNA segments not present in either of the two reference O157:H7 genomes. We identified seven putative virulence determinants, including two putative type III secretion system effector proteins, candidate genes that could result in increased pathogenicity or, alternatively, adaptation to plants, and an intact anaerobic nitric oxide reductase gene, norV. We surveyed 100 O157:H7 isolates for the presence of these putative virulence determinants. A norV deletion was found in over one-half of the strains surveyed and correlated strikingly with the absence of stx1. The other putative virulence factors were found in 8 to 35% of the O157:H7 isolates surveyed, and their presence also correlated with the presence of norV and the absence of stx1, indicating that the presence of norV may serve as a marker of a greater propensity for HUS, similar to the correlation between the absence of stx1 and a propensity for HUS

List of bacterial strains used in this study.

1<p>Harborview Medical Center, Seattle, WA, USA.</p>2<p>Institute for Environmental Health, Lake Forest Park, WA, USA.</p>3<p>University of Iowa, Iowa City, IA, USA.</p>4<p>Wroclaw University of Environmental and Life Sciences, Wroclaw, Poland.</p>5<p>Ohio State University, Columbus, OH, USA.</p>6<p>University of Illinois, Urbana, IL, USA.</p

Maximum-likelihood DNA phylograms of <i>S. enterica fimH</i> and concatenated MLST loci (<i>aroC</i>, <i>hisD</i> and <i>thrA</i>).

<p>The <i>fimH</i> tree (A) was built based on an alignment of <i>fimH</i> sequences amplified from 55 isolates including 45 different strains of subspecies I and 10 strains of subspecies II–VI (for details see <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1002733#ppat-1002733-t001" target="_blank">Table 1</a>). Five additional alleles of <i>fimH</i> obtained from GenBank (<i>S.</i> Typhimurium AJB3 (Thm3), <i>S.</i> Typhimurium LB5010 (Thm4), <i>S.</i> Gallinarum 287/91 (Gal1) and 589/02 (Gal2), and <i>S.</i> Paratyphi C 49 [RKS4594] (PaC1) were included. The MLST loci tree (B) was built on an alignment of concatenated sequences of three genes (<i>aroC</i>, <i>hisD</i> and <i>thrA</i>) obtained for 57 study strains. The trees shown were rooted using <i>S. enterica</i> subsp. II (2993). The italicized values along the branches denote % bootstrap values based on 1000 runs (the bootstrap proportions along the terminal branches separating isolates within single serovars as well as the ones below 50% are not shown). Systemically invasive serovars are shown in red and non-invasive serovars are shown in black. Strain tags are as used in the text.</p

Schematic representation of evolutionary changes in the FimH binding phenotype of selected <i>S. enterica</i> serovars.

<p>The evolutionary changes in the FimH of <i>S.</i> Enteritidis, <i>S.</i> Pullorum, <i>S.</i> Gallinarum and <i>S.</i> Dublin (A), <i>S.</i> Indiana, <i>S.</i> Paratyphi C and <i>S.</i> Choleraesuis (B), <i>S.</i> Paratyphi B (C) and <i>S.</i> Paratyphi A, <i>S.</i> Sendai and <i>S.</i> Typhi (D). Low (green)-FimH with low-binding phenotype; High (blue)-FimH with high-binding phenotype; None (orange)-inactive variant of FimH. The strain tags of systemically non-invasive serovars are in black and the systemically invasive serovars in red. Structural mutations are given along each arrow. Structural hot-spot mutations are underlined. The activating mutations are in blue and the inactivating mutations are in orange. The FimH variants from strains with phylogenetic relatedness supported by MLST are shown in tan boxes.</p