The role of Salmonella Enteritidis Pathogenicity Island-1 in the colonization of chickens

Abstract

Salmonella enterica serovar Enteritidis (S. Enteritidis) is a major cause of gastrointestinal disease in humans worldwide that is mainly associated with the consumption of contaminated poultry meat and eggs. During the course of infection, S. Enteritidis uses two Type 3 Secretion Systems (T3SS), one of which is encoded by Salmonella Pathogenicity Island-1 (SPI-1). SPI-1 plays a major role in the invasion process.

In order to study the role of SPI-1 in the colonization of chickens, we constructed deletion mutants affecting either the complete SPI-1 region (40 kb) or invG, a single gene located on this pathogenicity island. The mutants were impaired in the secretion of effector proteins and were less invasive compared to the wild type strain in polarized Caco-2 cells. Similarly, when chicken cecal and small intestinal explants were co-infected with the wild type and ÄSPI-1 mutant strains we found that the ÄSPI-1 mutant strain was less invasive relative to the wild type strain. Oral challenge of 1-week-old chickens with the wild type or ÄSPI-1 strains demonstrated that there was no difference in chicken cecal colonization. However, systemic infection, measured as levels of Salmonella in the liver and spleen, was delayed in birds that were challenged with the ÄSPI-1 strain. This demonstrates that SPI-1 facilitates systemic infection but is not essential for invasion and systemic spread of S. Enteritidis in chickens.

Based on the above results, we examined the effect of sera against SPI-1 T3SS components to S. Enteritidis invasion. Anti-SipD serum protected Caco-2 cells against entry of wild type S.Enteritidis, but not against invasion of a mutant strain lacking sipD. On the other hand, sera against InvG, PrgI, SipA, SipC, SopB, SopE and SopE2 did not affect S. Enteritidis entry. To illustrate the specificity of anti-SipD mediated inhibition, SipD specific antibodies were depleted from the serum. Depleted serum restored the invasion of S. Enteritidis, demonstrating that the SipD protein may be an important target in blocking SPI-1 mediated virulence.

To determine if SPI-1 T3SS proteins were protective against S. Enteritidis oral challenge, chickens were vaccinated subcutaneously twice at 14 and 28 days of age with PrgI and SipD. The results indicate that these proteins induce strong IgG antibody responses and confer significant protection against infection of the livers in vaccinated birds. In another study, we vaccinated hens with selected SPI-1 T3SS proteins to determine if their progeny could be protected from S. Enteritidis oral challenge. The proteins induced strong antibody responses but did not affect the levels of the challenge strain in the ceca or internal organs of the vaccinates. Taken together, our results establish that S. Enteritidis SPI-1 is an important virulence factor in chickens and that the proteins associated with this T3SS may form components of a subunit vaccine used for protection against colonization by S. Enteritidis in poultry.