One hundred and two strains of virulent
S. typhimurium were examined for SPA (Soluble Protective Antigen) producing ability. Experimental methods were identical to those described in the previous paper.
A wide variety of results ranging from strains capable of producing SPA to protect mice against the challeng of 100, 000 times LD
50 of bacteria to non-SPA producing strains incapable of protecting animals, was obtained,
Many high SPA-producing strains of
S. typhimurium, namely, 4 strains with a potency >100, 000 LD
50 (3.9%) and 46 strains >10, 000 LD
50 (45.1%), were found along with several weak and non-SPA producing strains.
Protective ability of formol vaccines (FV) of the above 102 strains was examined. 86 strains (84.4%) capable of rendering a strong protection to mice against 10, 000 LD
50 of bacteria were found among a wide variety of strains rainigng from capable to incapable of imparting a protecteve immunity to animals. These results were similar to those obtained with the SPA immunity test above.
The FV of those high SPA-producing strains also showed a marked protecting effect on animals against infections by imparting an immunity as a killed vaccine. Whereas, those FVs of non-SPA producing strains were unable to protect mice against infection.
Thus, there was a definite relation in producing protective immunity between SPA and killed vaccine with a correlation coefficient of 0.747.
As shown above, most of the strains of
S. typhimurium capable of rendering a protective immunity to animals against infections were found, and killed vaccine tended to give a positive immunity.
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