Resistance to colicins E3 and K induced by infection with bacteriophage f1.

When infected with the filamentous phage f1, Escherichia coli K-12 strains are 20- to 40-times more resistant to colicins K and E3 than are uninfected cells. The colicins are adsorbed normally and are blocked in their action at some subsequent stage of their action. Productive infection of virions is not necessary for induction of resistance. The evidence presented implicates the product of gene III of the phage, a minor protein in the virion, as being responsible for this resistance. The simplest interpretation of the results is that this gene product blocks the penetration of colicins which is necessary for their action.