[Effect of R plasmids on the growth characteristics of Shigella sonnei and Escherichia coli].

Revealing of growth characteristics in plasmid and plasmid-free strains was studied with the use of two different hosts: E. coli 15-3 and Sh. sonnei 11-941 containing conjugative R plasmids differing in the set of the resistance markers. It was shown that the R plasmids had no noticeable effect on the period of the lag phase and the time of the microbial cell generation. It was also shown that the number of the viable cells in separate cultures of the plasmid-free strain of Sh. sonnei 11-941 and its plasmid variants was of the same order. Counting of the viable cells in mixed cultures of the plasmid-free strain of E. coli 15-3 and its plasmid variant on the complete nutrient medium revealed an insignificant increase (by 10 per cent) in the proportion of the bacteria carrying the plasmid after 6-hour growth (during early stationary growth phase) and later, up to 24 hours. The results of the study suggested that development of nutrient media for microbial strains containing R plasmids does not require additional cultivation conditions.