Infection model for hatching chicks infected with Salmonella enteritidis

Salmonella Enteritidis flagellar mutants have a colonization benefit in the chicken oviduct

Egg borne Salmonella Enteritidis is still a major cause of human food poisoning. Eggs can become internally contaminated following colonization of the hen's oviduct. In this paper we aimed to analyze the role of flagella of Salmonella Enteritidis in colonization of the hen's oviduct. Using a transposon library screen we showed that mutants lacking functional flagella are significantly more efficient in colonizing the hen's oviduct in vivo. A micro-array analysis proved that transcription of a number of flagellar genes is down-regulated inside chicken oviduct cells. Flagella contain flagellin, a pathogen associated molecular pattern known to bind to Toll-like receptor 5, activating a pro-inflammatory cascade. In vitro tests using primary oviduct cells showed that flagellin is not involved in invasion. Using a ligated loop model, a diminished inflammatory reaction was seen in the oviduct resulting from injection of an aflagellated mutant compared to the wild-type. It is hypothesized that Salmonella Enteritidis downregulates flagellar gene expression in the oviduct and consequently prevents a flagellin-induced inflammatory response, thereby increasing its oviduct colonization efficiency.

Prevention of egg contamination by Salmonella Enteritidis after oral vaccination of laying hens with Salmonella Enteritidis ΔtolC and ΔacrABacrEFmdtABC mutants

Vaccination of laying hens has been successfully used to reduce egg contamination by Salmonella Enteritidis, decreasing human salmonellosis cases worldwide. Currently used vaccines for layers are either inactivated vaccines or live attenuated strains produced by mutagenesis. Targeted gene deletion mutants hold promise for future vaccines, because specific bacterial functions can be removed that may improve safety and allow differentiation from field strains. In this study, the efficacy of Salmonella Enteritidis ΔtolC and ΔacrABacrEFmdtABC strains in laying hens as live vaccines was evaluated. The mutants are deficient in either the membrane channel TolC (ΔtolC) or the multi-drug efflux systems acrAB, acrEF and mdtABC (ΔacrABacrEFmdtABC). These strains have a decreased ability for gut and tissue colonization and are unable to survive in egg white, the latter preventing transmission of the vaccine strains to humans. Two groups of 30 laying hens were orally inoculated at day 1, 6 weeks and 16 weeks of age with 10⁸ cfu of either vaccine strain, while a third group was left unvaccinated. At 24 weeks of age, the birds were intravenously challenged with 5 × 10⁷ cfu Salmonella Enteritidis PT4 S1400/94. The vaccine strains were not shed or detected in the gut, internal organs or eggs, 2 weeks after the third vaccination. The strains significantly protected against gut and internal organ colonization, and completely prevented egg contamination by Salmonella Enteritidis under the conditions of this study. This indicates that Salmonella Enteritidis ΔtolC and ΔacrABacrEFmdtABC strains might be valuable strains for vaccination of layers against Salmonella Enteritidis.

Role of receptor proteins for enterobactin and 2,3-dihydroxybenzoylserine in virulence of Salmonella enterica

Single, double, and triple mutants of an enterobactin-deficient mutant strain of Salmonella enterica serovar Typhimurium were constructed that were defective in the expression of the iron-regulated outer membrane proteins (IROMPs) FepA, IroN, and Cir, which are proposed to function as catecholate receptors. Uptake of naturally occurring and chemically synthesized catecholate molecules by these mutants was assessed in standard growth promotion assays. Unique patterns of uptake were identified for each IROMP; specifically, FepA and IroN were confirmed to be required for transport of enterobactin, and all three proteins were shown to function as receptors for the enterobactin breakdown product 2,3-dihydroxybenzoylserine. The fepA, iroN, and cir alleles were transduced to enterobactin-proficient strains of S. enterica serovar Typhimurium and S. enterica serovar Enteritidis, and the resulting phenotypes were confirmed by analysis of outer membrane protein profiles, by sensitivity to KP-736, a catecholate-cephalosporin conjugate, and by growth promotion tests on egg white agar. Intragastric infections of mice with the S. enterica serovar Typhimurium strains indicated that the parental strain and the fepA iroN double mutant were similarly virulent but that the fepA iroN cir triple mutant was significantly attenuated. Moreover, in mixed infections, the fepA iroN mutant showed similar cecal colonization and invasion of the liver to the parental strain, while the triple mutant showed significantly reduced cecal colonization and no measurable spread to the liver. Infections of 4-day-old chicks with S. enterica serovar Enteritidis strains also indicated that mutation of the fepA iroN genes did not significantly reduce cecal colonization and systemic spread compared with those of the parental strain. The results indicate that, while enterobactin uptake is not essential for the virulence of S. enterica serovars in mouse and chicken infection models, the ability to take up 2,3-dihydroxybenzoylserine via any of the three catecholate siderophore receptors appears to play an important role, since the S. enterica serovar Typhimurium triple mutant was significantly attenuated in the mouse model. Salmochelins appear not to be involved in the virulence of S. enterica.

Inactivation of the flagellin gene of Salmonella enterica serotype enteritidis strongly reduces invasion into differentiated Caco-2 cells

A nonflagellated mutant of Salmonella enterica serotype Enteritidis was constructed by disrupting the flagellin gene (fliC). Northern blot analysis indicated that the mutation did not affect expression of the downstream fliU gene. Infection experiments with differentiated Caco-2 cells revealed that the mutant was about 50-fold less invasive than the wild-type strain, while bacterial adherence was unaffected. Complementation of the mutant with an intact fliC copy restored flagella formation and efficient bacterial invasion. Our data demonstrate that the fliC gene of S. enterica serotype Enteritidis is essential for the invasion of Caco-2 cells.

Persistence of Salmonella enteritidis from one day of age until maturity in experimentally infected layer chickens

In each of two replicate trials, 1-d-old chicks were inoculated orally with a phage type 13 Salmonella enteritidis isolate (resistant to nalidixic acid). Although S. enteritidis was found in the livers, spleens, and ceca of all sampled chicks at 1 wk postinoculation, colonization generally persisted beyond 4 wk postinoculation only in the ceca. Nearly half of the remaining hens were still shedding S. enteritidis in their feces at 24 wk of age, but only 1 of 62 hens laid eggs that were internally contaminated with S. enteritidis during the initial 4 to 6 wk of egg production. Chickens exposed to S. enteritidis shortly after hatching can apparently remain infected until maturity, at which time they might produce contaminated eggs or spread the infection to other susceptible, previously unexposed hens.

Experimental oral infection of specific pathogen-free laying hens and cocks with Salmonella enteritidis strains

A total of 105 specific pathogen-free (SPF) laying hens and eight cocks were orally infected with strains of Salmonella enteritidis. The organs of these animals were examined bacteriologically for up to 6 weeks post infectionem (p.i.). One-half of the eggs laid on a single day were examined 1 day after laying, and the other half 28 days after laying. The siderophore, ferrioxamine E, was added to the pre-enrichment medium. A high correlation was found to exist between colonization of the caecum by salmonellas and the level of contamination of the egg shell. No correlation was found between the contamination of the egg shell and that of the egg content. The strains used for infection were not recovered from any yolk out of a total of 938 eggs. S. enteritidis was isolated from the albumen in four cases (0.4%). Storage of the eggs for 28 days after laying resulted in a decrease in the frequency of salmonella detection on the egg shell and in the egg content. The strains used for infection could be isolated from various sections of the sexual organs for up to 2 weeks p.i. S. enteritidis was most frequently found in the ovary and rarely in the oviduct, but never in the pre-ovulatory mature follicles. In the groups that included salmonella-infected cocks, the salmonella detection frequency did not increase either for the egg content or for the organs of the laying hens. It seems that primary vertical transmission of salmonellas occurs, in rare cases, as a result of an infection of the albumen but not of the yolk.