A genetically engineered derivative of Salmonella Enteritidis as a novel live vaccine candidate for salmonellosis in chickens

To construct a novel live Salmonella Enteritidis (SE) vaccine candidate, SE was genetically engineered using the allelic exchange method to delete two virulence genes, lon and cpxR. The lon gene deletion is essential to impair Salmonella replication and avoid overwhelming systemic disease in the host. The cpxR gene deletion is needed to enhance the ability of bacteria to adhere and invade the host cell. Scanning electron microscopy revealed that the derivatives JOL917 (Δlon), JOL918 (ΔcpxR), and JOL919 (Δlon/ΔcpxR) had increased surface fimbrial filamentous structures. Significant elevations of extracellular polysaccharide and FimA expression were observed for the derivatives compared to the parental wild type JOL860, while biochemical properties of the derivatives were not altered. In the safety examination by inoculation of the derivatives in chickens, gross lesion scores of the liver, spleen, kidney, small intestine and caecal tonsils were moderate in the JOL917 and JOL918 groups, and significantly lower in the JOL919 group than those of the JOL860. Bacterial counts from the spleen and caeca of the JOL917 and JOL918 groups were moderate, and significantly reduced in the JOL919 group compared to the JOL860 group. In addition, only the JOL919 group showed significantly lower bacterial counts in the faecal samples than those of the JOL860 group. Significant elevations of IgG and secretory IgA levels observed in the derivative groups, while the JOL919 and JOL860 groups showed a potent lymphocyte proliferation response as compared to those of the control group. In the protection efficacy examination, JOL919 immunized group showed significantly lower depression, lower gross lesion in the liver and spleen, and lower number of the SE positive internal organs than those of the control group against a virulent wild type SE challenge.