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SUMMARY: In 18 hr. incubation of broth inoculated with a majority of bacteria of Salmonella typhimurium LT2 col -, i.e. non-colicinogenic, and a minority of strain LT2 (colI), i.e. carrying colicine factor colI, 30--70% of the col - bacteria acquired colI; this increased to 50--90% after 2 hr. of secondary incubation after tenfold dilution with broth. These cultures, containing a high proportion of bacteria newly infected by colI, transmitted colI to about 50% of the bacteria of a col - strain in 1 hr. (whereas pure cultures of LT2 (colI) transmitted to only 0.01%); they are termed HFC (high-frequency colicinogeny-transferring). An inoculum containing 1--3 recently infected bacteria from an HFC preparation of a streptomycin-sensitive strain sufficed to transmit colI to a resistant strain in streptomycin broth. We infer that 30--100% of bacteria newly infected by colI are ‘competent donors’, able to transmit colI. By the same test the proportion of competent donors in LT2 (colI) strains was only 0.02%. Inoculum size, ratio of inoculum components, motility, aeration and secondary incubation affected the HFC property of mixed cultures in a way explicable by the need for the epidemic spread of colI in the col - component to reach a peak at the time of testing. The rate of loss of the HFC property on further growth indicated that for 3--7 generations the progeny of newly infected bacteria are competent donors. Transmission was associated with clumping and pairing. Bacteria of an HFC preparation pair with 10% of an acceptor population within 2 min. of mixing; completion of transfer requires 2 1/2--30 min. Non-availability of nutrients and some growth inhibitors interfere with transfer. We conclude that colI multiplies autonomously in newly infected bacteria and their immediate progeny and enables them to conjugate, but does not confer ability to conjugate in established colicinogenic strains; perhaps it is then integrated into the host chromosome.

High Infectivity of Salmonella typhimurium newly infected by the colI factor