The Selection of a Strain of Bacillus pestis for the Preparation of Vaccine, with Special Reference to the effect of Animal Passage on Virulence

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Abstract
(1) Strains of B. pestis, kept in culture for 7 and 9 months, showed no loss of virulence.(2) Virulent strains, when subjected to passage through an immunised Cricetomys gambianus (African pouched rat), suffer distinct loss of virulence for this species, and virulence is not regained in culture or on repeated passage through susceptible C. gambianus. The so-called avirulent plague bacillus, obtained in this way, appears to be a rough variant. It differs in micro-scopical appearance from the ordinary virulent forms, tending to the appearance from the ordinary virulent forms, tending to the appearance of involution forms, which develop in salt media.(3) As regards immunising properties:(a) Vaccine prepared from virulent strain gives considerable protection against infection with virulent strain.(b) Vaccine prepared from virulent strain gives no protection against infection with avirulent strain.(c) Vaccine prepared from avirulent strain gives little or no protection against infection with virulent strain.(4) Animals, which recover from infection with an avirulent strain, are immune to subsequent infection with virulent strain.