Pathogenicity Island 1 Is Expressed in the Chicken Intestine and Promotes Bacterial Proliferation.

serovar Enteritidis is a common cause of foodborne illness in the United States. The bacterium can be transmitted to humans via contaminated chicken meat and eggs, and virulence in humans requires type III secretion system 1 (TTSS-1), encoded on pathogenicity island 1 (SPI-1). Chickens often carry Enteritidis subclinically, obscuring the role of SPI-1 in facilitating bacterial colonization. To evaluate the role of SPI-1 in the infection of chicks by , we created and utilized strains harboring a stable fluorescent reporter fusion designed to quantify SPI-1 expression within the intestinal tracts of animals. Using mutants unable to express TTSS-1, we demonstrated the important role of the secretion system in facilitating bacterial colonization. We further showed that coinoculation of an SPI-1 mutant with the wild-type strain increased the number of mutant organisms in intestinal tissue and contents, suggesting that the wild type rescues the mutant. Our results support the hypothesis that SPI-1 facilitates Enteritidis colonization of the chicken and make SPI-1 an attractive target in preventing carriage and colonization in chickens to reduce contamination of poultry meat and eggs by this foodborne pathogen.