TABLE 2.

Oral immunization with Salmonella Dam-based vaccines elicits cross-protective immune responses against heterologous serotypes^a

Vaccine strain	Challenge dose (no. of bacteria)	No. of survivors/total no. after oral challenge with serovar:
		Dublin	Typhimurium	Enteritidis
No bacteria	109	0/20	0/19	0/19
dam102::Mud-Cm Enteritidis	109	9/26	7/25	5/26
No bacteria	109	0/25	0/10	0/10
damΔ232 Typhimurium	109	4/19	11/11	0/10
No bacteria	108	0/28	0/38	0/25
damΔ232 Typhimurium	108	11/23	20/20	4/18
No bacteria	108	0/10	0/10	ND
Dam overproducer Typhimurium	108	6/8	6/8	ND

^aBALB/c mice were perorally immunized via gastrointubation with a dose of 10⁹ Dam⁻ or Dam-overproducing Salmonella (18). The mice were challenged with the virulent Salmonella serotypes indicated 11 weeks postimmunization, which was 6 weeks after the vaccine strains were cleared from murine tissues, including Peyer's patches, mesenteric lymph nodes, liver, and spleen. The Dam-overproducing serovar Typhimurium strain (MT2257) contains E. coli dam on recombinant plasmid pTP166 in a damΔ232 background (18, 29). The oral LD₅₀s of challenge strains are as follows: S. enterica serovars Typhimurium (18) and Enteritidis, 10⁵ organisms (data not shown); serovar Dublin, 5 × 10⁴ organisms (6). The cross-protection conferred was significant according to the two-tailed Fisher exact test (P < 0.05). 

^bND, not done.