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Host-free growth and reproduction of a host-dependent strain ofBdellovibrio bacteriovorus incubated with an extract from host cells were studied. The morphological changes occurring in the cells were correlated with deoxyribonucleic acid (DNA) synthesis as measured by labeled nucleotide or orthophosphate incorporation. The host-free developmental cycle ofBdellovibrio is similar to that of the two-membered system; the early loss of flagella, the elongation into filaments, and multiple fission into flagellated progeny are typical for both host-free and intraperiplasmic development of bdellovibrios. Filament length and time of division appear to depend on the concentration of the host extract. Host extract was found to be heat stable and DNase stable, and Pronase sensitive and RNase sensitive. Addition of ribonucleic acid to the extract medium at various times during theBdellovibrio growth cycle demonstrated that host extract is required continuously during the cycle for growth. The observations reported give a unified picture ofBdellovibrio development and allow for the suggestion that wild-type bdellovibrios depend upon the presence of some host factor for induction of DNA synthesis, whereas depletion of host factor triggers division. The ecological implications of such host dependence are discussed.

Growth Cycle of Predacious Bdellovibrios in a Host-Free Extract System and Some Properties of the Host Extract