Influence of plasmids on bacterial susceptibility to the bactericidal activity of sera. II. Relation between transfer frequency of R plasmids and their protective activity to the lethal effect of normal rabbit serum.

The plasmid pAM-Ia separated from the aggregate R588 protected E. coli K12 cells from the bactericidal action of normal rabbit serum much stronger when carried alone than in the aggregate. When aggregated with R100.1, its protective effect was also weakened. These decreases in protective effect are correlated qualitatively with inhibition of conjugational fertility. On the other hand the plasmid aggregate causing increase in protective activity showed no influence on their fertility.