Effect of prior serial in vivo passage on the frequency of Salmonella enteritidis contamination in eggs from experimentally infected laying hens.

Experimental infection models are valuable tools for understanding and preventing the deposition of Salmonella enteritidis inside eggs. Oral inoculation is believed to closely simulate naturally occurring S. enteritidis infections of chickens, but oral infection studies have often generated relatively low frequencies of egg contamination. The present study assessed whether repeated in vivo passage of an S. enteritidis strain could affect its ability to cause egg contamination in experimentally infected hens. The incidence of egg contamination was determined in groups of hens inoculated orally with either a phage type 13a S. enteritidis strain or derivatives of this parent strain that were obtained by three successive rounds of passage and reisolation from tissues of infected hens. Passaged S. enteritidis isolates recovered from ovaries and oviducts induced a significantly higher incidence of egg contamination (16.97%) than was attributed to the parent strain (8.27%). However, passaged S. enteritidis isolates recovered from livers and spleens were not associated with a significantly increased frequency of deposition in eggs. By either inducing or selecting for the expression of relevant microbial properties, passage of S. enteritidis through reproductive tissues of chickens may be useful for improving the efficiency at which experimental infection models produce egg contamination.