Minimal Infectious Inoculum of Histoplasma capsulatum for Mouse and Chick Embryo

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Abstract
(1) Individual infectious particles were isolated from the mycelial phase of H. capsulatum using Chambers micromanipulator. The particles consisted of germinated and non-germinated tuberculated macroconidia, germinated and non-germinated microconidia and non-branching mycelial fragments. Each particle was placed into a sterile capillary tube inserted into a 21 gauge sterile inoculating needle. Aseptic technics were followed, in addition, streptomycin and penicillin were added to the collecting and inoculating fluids. Microscopical examination of each capillary tube was made prior to and immediately after inoculation procedure to ascertain the disposition of the particle. Fertile Leghorn eggs and the white Swiss mouse were the experimental animals. (2) Infection resulted from all particles with one exception, mycelial fragments, in the mouse. The total percentage of infectivity in the 182 inoculated mice was 5%. Of the 571 inoculated chick embryos approximately 8% became infected. The most infectious particle was the mycelial fragment. (3) Inoculation of single particles from the mycelial phase of H. capsulatum proved that the minimal infection inoculum can be units other than the macroconidium. In each instance the infectivity rate was relatively small but no physical protection was given the inoculated particle.

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