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.

Figure 1

Distribution of S. Enteritidis 147 wild-type strain and SPI mutants in the spleen of orally infected chickens. S. Enteritidis counts in the liver correlated with counts in the spleen except for the fact that ΔSPI2 mutant colonised liver significantly less efficiently than the wild type S. Enteritidis also on day 12 (not shown). Y axis, average log CFU/g of spleen ± SD. a, b - ANOVA different at p < 0.05 in comparison to the group infected with the wild-type S. Enteritidis (a) or ΔSPI1-5 mutant (b). Abbreviations: wt - wild-type S. Enteritidis 147; ΔSPI1-5: mutant from which all major 5 SPI have been removed, ΔSPI1, ΔSPI2, ΔSPI3, ΔSPI4, ΔSPI5: mutants from which the respective SPI has been removed, SPI1o, SPI2o, SPI3o, SPI4o, SPI5o: "SPIonly" mutants, mutants with only the respective SPI retained.

Figure 2

Distribution of S. Enteritidis 147 wild-type strain and ΔSPI1&2 and SPI1&2o, ΔSPI1-5 mutants in the liver and spleen of orally infected chickens. Y axis, average log CFU/g of organ ± SD. a, b - t-test different at p < 0.05 in comparison to the group infected with the wild-type S. Enteritidis (a) or the ΔSPI1-5 mutant (b). Abbreviations: wt - wild-type S. Enteritidis 147; ΔSPI1-5: mutant from which all major 5 SPIs have been removed; ΔSPI1&2: mutant from which SPI1 and SPI2 have been removed; SPI1&2 only: mutant with only SPI1 and SPI2 retained.

Figure 3

Heterophil infiltration in caeca of chickens infected with different SPI mutants of S. Enteritidis. Y axis, average number of heterophils per microscopic view ± SD. a, b, c - ANOVA test different at p < 0.05 in comparison to the group infected with the wild-type S. Enteritidis (a), the ΔSPI1-5 mutant (b), or the non-infected controls (c). Abbreviations: as in Fig. 1.