Serum resistance encoded by colicin V plasmids in Escherichia coli and its relationship to the plasmid transfer system.

Eight colicin V plasmids were conjugated into a plasmidless Escherichia coli (K-12) strain that was susceptible to the bactericidal effects of normal rabbit serum. The resulting colicin V-positive strains were examined for their capacity to resist the lethal effects of serum. Serum resistance was assessed as growth of the bacterial strain in medium containing 5% normal rabbit serum inoculated from a culture in the early exponential phase. Only three of the eight colicin V plasmids were found to confer the serum resistance phenotype on the host strain. Derepression of the transfer system was associated with serum resistance in two of the plasmids. Two other derepressed plasmids did not confer serum resistance on the host bacterium. Therefore, such derepression alone was insufficient to produce serum resistance. The factor(s) encoded by colicin V plasmids and responsible for the serum resistance of the bacterial strains bearing the plasmids was shown to be a property associated with the cell and not an extracellular factor excreted into the growth medium.