Influence of the State of Fimbriation on Transmission of the Colicinogenic Factor colI between Strains of Shigella flexneri

SUMMARY: The colicinogenic factor colI derived from Shigella sonnei colicine type 2 (Abbott & Shannon, 1958) was transmitted between different strains of Shigella flexneri in serotypes 1a, 1b, 2a, 2b, 3a, 3c, 4a, 4b, 4c, 5a, X and Y when donor and acceptor strains were grown together in mixed culture in aerobic static broth for a period of 20 hr or 48 hr at 37·. The rate of transfer was influenced by the state of fimbriation of the acceptor bacteria, but not by that of the donor bacteria. The proportion of acceptor organisms acquiring colI was very high, e.g. 15-40 % in 20 hr and 50-95 % in 48 hr, when the acceptor was genotypically fimbriate and in the fimbriate phase (Fim+F); but it was much lower, e.g. 0-5 % in 20 hr and 1-15 % in 48 hr, when the acceptor was either genotypically non-fimbriate (Fim−) or genotypically fimbriate but in the non-fimbriate phase (Fim+N). Although it facilitated the transmission of colI, the presence of fimbriae was not essential for transmission, since transmission at a low rate was obtained in most crosses of Fim− donors with Fim− acceptors. Only 1 out of 17 strains of S. flexneri tested as acceptors failed to acquire colI in any test. Transmission did not take place within the first 2 hr of mixed culture; it occurred mainly during the period of slow growth after the end of the exponential phase (i.e. between 8 and 48 hr) when, with fimbriate organisms, a pellicle of bacteria had developed on the surface of the broth. The rate of transmission was very much decreased when pellicle formation was prevented by intermittent or continuous agitation of the culture. Experiments in which the donor bacteria were killed by adding streptomycin at 8 hr showed that an extensive ‘epidemic’ spread of colI took place within the acceptor population during the later stages of culture. When stationary-phase (1-10 days) broth cultures of donor and acceptor bacteria were mixed together and incubated without the addition of fresh broth, the rates of transfer at 20 and 48 hr were as high as in the mixed cultures grown from small inocula. Transmission also occurred with high frequency in a defined medium containing glucose, NH4 + and nicotinic acid.