STUDIES ON THE MILKY DISEASE ORGANISMS

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Abstract
Numerous strains of B. popilliae Dutky were studied for ability to complete their parasitic life cycles of growth and sporulation on artificial media. In addition to typical spores consisting of a spore proper and a refractile parasporal body produced by some strains, 2 other spore types were formed by certain strains. One was similar to the spore of parasitic spores, but lacked the parasporal body, and was formed by a few strains that appeared to become adapted to saprophytic growth. These spores were of low virulence. The second type of atypical spore was "abortive" in the sense that it did not become retractile or heat resistant. The "abortive" spores were formed by abnormally large vegetative cells on the growth medium. Of hundreds of spore suspensions of B. popilliae removed from diseased larvae, only a very small percentage consistently formed typical parasitic spores under saprophytic conditions. The best strain examined failed to produce new parasitic type spores for more than 4 complete life cycles on artificial media. Generally the ability to form typical spores was lost after the second transfer. The vegetative cells of these strains formed spores when injected into the living larval host, even after 100 transfers on artificial media. Typical spores did not form in cells left undisturbed on the medium on which they were grown. In order to reproduce the parasitic life cycle on artificial media, it was necessary to grow the vegetative cells on a nutritionally sufficient medium and to collect the cells into a paste after growth was completed. Sporulation was facilitated by transfer of this paste of vegetative cells to a starvation medium and by raising the incubation temperature. Strains forming spores of parasitic morphology on artificial media varied from trial to trial in ability to sporulate. Sporulation on artificial media was not stimulated by addition of larval extracts to the growth or sporulation media. Some strains grown in the vegetative state on artificial media had an enhanced pathogenicity which interfered with their ability to be parasitic when injected or fed to the living larvae.
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