[LIVE VACCINE IMMUNIZATION IN EXPERIMENTAL TYPHOID WITH A STREPTOMYCIN DEPENDENT STRAIN OF SALMONELLA ENTERITIDIS]

Oral Immunization in Experimental Salmonellosis II. Characteristics of the Immune Response to Temperature-Sensitive Mutants Given by Oral and Parenteral Routes

A temperature-sensitive mutant of Salmonella enteritidis, selected because of its inability to proliferate normally at 37 C, has been used as a living vaccine in mice. When given parenterally or orally, it confers a high degree of resistance against otherwise lethal S. enteritidis infections given intraperitoneally or by mouth. In contrast to most other effective living Salmonella vaccines, the temperature-sensitive mutant survives for only short periods in mouse tissues. Although the vaccine provides protection against S. typhimurium infection, possibly because of antigenic relationships between the immunizing and challenge organisms, it is ineffective against the intracellular infection caused by Listeria monocytogenes. A study of the kinetics of S. enteritidis infection in the liver and spleen of normal and immunized mice has suggested that immunity is dependent upon development of a secondary immunological response which arises approximately 7 days after introduction of the challenge infection. Although humoral antibody production forms part of this secondary response, it is not necessarily responsible for control of the infection.

Contribution to the study of live streptomycin-dependent Salmonella vaccines: the problem of reversion to a virulent form

The recovery of virulence by means of reversion of a live streptomycin-dependent (Sm D) Salmonella typhimurium vaccine was studied in CD-1 Swiss mice. Initially, a one-step Sm D mutant was obtained from a virulent streptomycin-sensitive (Sm S) S. typhimurium strain. Afterwards, two pools of streptomycin-independent (Sm I) revertants were prepared from the Sm D strain. The virulence of the Sm D strain and of the Sm I revertants was tested intraperitoneally. In the virulence testing the original suspension of the Sm I revertants, as well as their 1st and 10th passages on plain medium, medium+50 mug. streptomycin/ml. and medium+1000 mug. streptomycin/ml. were used. The results show that the Sm D mutant was avirulent, its avirulence being due to an intrinsic, genetic quality. The Sm I revertants, compared to the original Sm S strain, also displayed a lack of virulence. However, afterwards, the Sm I revertants behaved quite differently, according to their subsequent passages. Indeed, there was an increase in virulence after passages on plain medium, whereas similar passages on medium containing the drug, the virulence not only failed to increase, but disappeared almost completely. Moreover, the passages on medium containing 1000 mug. streptomycin/ml. induced a return to the status of drug-dependence. The danger of recovery of virulence by means of revertants is evaluated.