Virulence potential of five major pathogenicity islands (SPI-1 to SPI-5) of Salmonella enterica serovar Enteritidis for chickens

Abstract Background Salmonella is a highly successful parasite of reptiles, birds and mammals. Its ability to infect and colonise such a broad range of hosts coincided with the introduction of new genetic determinants, among them 5 major pathogenicity islands (SPI1-5), into the Salmonella genome. However, only limited information is available on how each of these pathogenicity islands influences the ability of Salmonella to infect chickens. In this study, we therefore constructed Salmonella Enteritidis mutants with each SPI deleted separately, with single individual SPIs (i.e. with the remaining four deleted) and a mutant with all 5 SPIs deleted, and assessed their virulence in one-day-old chickens, together with the innate immune response of this host. Results The mutant lacking all 5 major SPIs was still capable of colonising the caecum while colonisation of the liver and spleen was dependent on the presence of both SPI-1 and SPI-2. In contrast, the absence of SPI-3, SPI-4 or SPI-5 individually did not influence virulence of S. Enteritidis for chickens, but collectively they contributed to the colonisation of the spleen. Proinflammatory signalling and heterophil infiltration was dependent on intact SPI-1 only and not on other SPIs. Conclusions SPI-1 and SPI-2 are the two most important pathogenicity islands of Salmonella Enteritidis required for the colonisation of systemic sites in chickens.