Live Vaccine Immunization in Experimental Typhoid With a Streptomycin Dependent Strain of Salmonella enteritidis

Abstract

The most effective immunity against mouse typhoid is known to be attained by immunization with “live” vaccine but not with “killed” vaccine.
To clarify the relation between the effectiveness of the immunization and the rate of multiplication or persistence of immunizing microorganisms in the host, the present study was conducted using a strain of streptomycin (SM)-dependent Salmonella enteritidis, lld, as a “live” vaccine.
About 8×10₄ organisms of lld were injected intsaperitoneally into a numper of mice, and the mice were divided into 3 groups according to the schedule of administration of streptomycin.
To the mice of the first group, 8mg of SM was daily injected intraperitoneally for 50 consecutive days starting from the day of infection. To the mice of the second group, the same daily dosis of SM was administered for 12 days. The mice of the third group were not administered with SM at all, but injection with lid was repeated 2 or 3 times during the period for immunization. A number of mice of each group were sacrificed at intervals and the number of viable lld organisms in the mice was calculated. Thus, the homogenates of the lymph nodes, spleen, liver and remaining carcuss, except the digestive tract, were cultivated quantitatively on agar plates containing SM. To evaluate the effectiveness of the immunization, 51 days after the infection with lld a number of mice of each group were challenged intraperitoneally with about 8×10³ organisms of virulent S. enteritidis and their mortality was observed for 21 days.
The results were as follows.
1. In the mice of the first group, lld, the immunizing organisms, increased for 2 weeks and then dicreased gradually. The mice acquired a significant resistance to the challenge with virulent S. enteritidis.
2. In the mice of the second group, lld organisms as well increased for 2 weeks as in teose of the first group, but decreased rapidly thereafter, and they could not be recovered from any parts of body on the 40th day of the immunization or thereafter. The aquired resistance against virulent infection of the mice of this group was inferior to that of the first group, and the mice challenged on the 18th day of the immunization were well protected from the virulent infection but those challenged on the 51 the day were protected at lower grade than those of the first group.
3. In the mice of the third group, no increase but rapid decrease of lld organisms was observed after every injections with lld. The mice were not protected from the virulent challenge at all.