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

Molecular comparison of Gasterophilus intestinalis and Gasterophilus nasalis from two distinct areas of Poland and Italy based on cox1 sequence analysis

In this study, Gasterophilus intestinalis and Gasterophilus nasalis collected from horses in northeastern Poland and southern Italy were genetically compared. The cox1 sequences of the Polish and Italian G. nasalis larvae revealed a higher degree of geographic genetic diversity, with an intra-specific variation rate of 1.27%, than the G. nasalis specimens collected in Poland (intra-specific variation rate: 0.49%) and those collected in Italy (intra-specific variation rate: 0.58%). However, the level of genetic homology of the Polish and Italian G. intestinalis specimens (intra-specific variation rate: 1.27%) was similar to that of the G. intestinalis larvae collected in northeastern Poland (intra-specific variation rate: 0.94%) and those collected in southern Italy (intra-specific variation rate: 1.16%). Analysis of the restriction enzyme sites in the coxI gene of G. nasalis and G. intestinalis showed that the nucleotide polymorphism (NP) at position 1050 of this gene determines cleavage by MnlI only in G. nasalis, making it possible to differentiate the two species using PCR-RFLP. Interestingly, comparison of the nucleotide sequences of the PCR-amplified coxI gene fragments from the Italian specimens of G. nasalis with other analyzed cox1 genes revealed an additional NP at position 1236 of cox1 gene, recognized by MnlI. The present study shows that G. nasalis specimens from different geographical areas display a level of genetic diversity which can influence PCR-RFLP analysis