Leghorn hens over 50 weeks of age were assigned to two treatment groups designated as either unmolted controls or molted. A forced molt was induced by a 9-day feed withdrawal, and each hen was challenged orally with 10(5) Salmonella enteritidis organisms on day 4 of feed withdrawal. On days 4 and 9 of molt, the numbers of lactobacilli and the concentrations of lactate, acetate, propionate, and butyrate, and total volatile fatty acids in the crops decreased while crop pH increased significantly (P < 0.05) in the molted hens compared to the controls. S. enteritidis crop and cecal colonization, in addition to spleen and liver invasion, increased significantly (P < 0.05) in the molted hens compared to the controls. The invasive phenotype of Salmonella spp. is complex and requires several virulence genes which are regulated by the transcriptional activator HilA. Samples of the crop contents from the molted and unmolted birds were pooled separately, centrifuged, and filter sterilized. The sterile crop contents were then used to measure the expression of hilA. By using a lacZY transcriptional fusion to the hilA gene in S. enteritidis, we found that hilA expression was 1.6- to 2.1-fold higher in the crop contents from molted birds than in those from control birds in vitro. The results of the study suggest that the changes in the microenvironment of the crop caused by feed deprivation are important regulators of S. enteritidis survival and influence the susceptibility of molted hens to S. enteritidis infections. Furthermore, our in vitro results on the expression of hilA suggest that the change in crop environment during feed withdrawal has the potential to significantly affect virulence by increasing the expression of genes necessary for intestinal invasion.