Influence of the major nitrite transporter NirC on the virulence of a Swollen Head Syndrome Avian Pathogenic E. coli (APEC) strain

Avian Pathogenic Escherichia coli (APEC) strains are extra-intestinal E. coli that infect poultry and cause diseases. Nitrite is a central branch-point in bacterial nitrogen metabolism and is used as a cytotoxin by macrophages. Unlike nitric oxide (NO), nitrite cannot diffuse across bacterial membrane cells. The NirC protein acts as a specific channel to facilitate the transport of nitrite into Salmonella and E. coli cells for nitrogen metabolism and cytoplasmic detoxification. NirC is also required for the pathogenicity of Salmonella by downregulating the production of NO by the host macrophages. Based on an in vitro microarray that revealed the overexpression of the nirC gene in APEC strain SCI-07, we constructed a nirC-deficient SCI-07 strain (δ. nirC) and evaluated its virulence potential using in vivo and in vitro assays. The final cumulative mortalities caused by mutant and wild-type (WT) were similar; while the δ. nirC caused a gradual increase in the mortality rate during the seven days recorded, the WT caused mortality up to 24. h post-infection (hpi). Counts of the δ. nirC cells in the spleen, lung and liver were higher than those of the WT after 48. hpi but similar at 24. hpi. Although similar number of δ. nirC and WT cells was observed in macrophages at 3. hpi, there was higher number of δ. nirC cells at 16. hpi. The cell adhesion ability of the δ. nirC strain was about half the WT level in the presence and absence of alpha-. d-mannopyranoside. These results indicate that the nirC gene influences the pathogenicity of SCI-07 strain.