In vitro invasion of laying hen ovarian follicles by Salmonella Enteritidis strains

Salmonella is the major foodborne bacterial pathogen worldwide. Among numerous serotypes, Salmonella Enteritidis (SE) is one of the most common Salmonella serotypes responsible for human infections in the United States. The main source of SE outbreaks is foods associated with raw or undercooked chicken eggs. Salmonella Enteritidis is the only serotype that routinely contaminates eggs. The transovarian transmission of SE and subsequent contamination of the eggs before egg shell formation is considered to be the main route of egg contamination by SE. To evaluate whether invasion of ovarian follicles is an important step during the production of eggs contaminated by SE, we used an in vitro invasion assay to determine ovarian follicle invasion by 5 SE strains. After inoculating the freshly collected ovarian follicles, all 5 SE strains were able to invade into the follicles after 2 h of incubation at 37°C. The mean percentage of SE invasion ranged from 0.016 to 0.034% and no significant difference was found among the SE strains. For Escherichia coli K-12 strain, which was used as a negative control, the mean percentage of invasion was 0.0003%. The in vitro follicle invasion by SE strains demonstrated in this study may reflect the ability of the strains to invade ovarian follicles in laying hens once SE cells reach ovaries through various routes.