Close association between shape alteration and loss of immunity to superinfection in a wild-type Klebsiella pneumoniae stable lysogen which can be both immune and nonimmune to superinfection.

Klebsiella pneumoniae MirM7 is a wild-type strain which grows as cocci at pH 7 and above and as rods at pH 6.5 and below. Cultures of this strain and an auxotrophic derivative, MirM7b, have been found to undergo spontaneous lysis after purification from possible contaminating viruses. Lysates always contained two phages, FR2 and AP3, most often at high titers. FR2 and AP3 plated with the same efficiency on both MirM7b and K59 (another K. pneumoniae strain sensitive to FR2 and AP3) and lysogenized 45 and 54% of the K59-infected cells, respectively. These findings raise the possibility that MirM7b is lysogenic for FR2 and AP3, although nonimmune to their superinfection. The fact that mitomycin C and N-methyl-N'-nitro-N-nitrosoguanidine can induce phages FR2 and AP3 from MirM7b confirmed this possibility. When MirM7b was infected with FR2 several strains immune to FR2 and AP3, which were all rod shaped, were obtained. Furthermore, 19 derivatives, rod shaped at all pH's have been isolated from MirM7b. They were all immune to both FR2 and AP3. From mating experiments between the MirM7b donor derivative, strain M720, and either K59 or MirCV5, a rod-shaped MirM7b derivative cured from the prophages, cysteine recombinants were obtained which were most often (80%) immune to FR2 and AP3. Nonimmune and still lysogenic recombinants were obtained by mating M720 with a rod-shaped immune MirM7b derivative; the majority of the non-immune strains maintained the rod shape. Five coccus-shaped recombinants were also isolated; they were nonimmune to superinfection. Several physiological properties of strain MirM7b and the other nonimmune coccal recombinants have been studied in comparison with those of the rod-shaped immune derivatives. All of the coccal strains have shown several alterations with respect to the rods. The role of possible derepressed prophage genes in the various physiological alterations of MirM7 is discussed, and the analogies between this system and those of vertebrate cells transformed by proviruses are stressed.