Studies in lysogeny elimination of prophages from a lysogenic strain of streptococcus cremoris 'curing' and lysogenic conversion : a thesis presented presented in partial fulfilment of the requirements for the degree of Master of Science in Genetics at Massey University
The naturally lysogenic strain Streptococcus cremoris R₁, yielded after ultraviolet light and Mitomycin C induction, phages of two types. A collared form, r₁t₁ with a buoyant density 1.50 g/cm³ which differed morphologically and serologically from the collarless type r₁t₂ (buoyant density 1.47 g/cm³). These two phage appear to differ also in their lysogenic immunity specificity. Elimination of these two prophages from their host was accomplished by UV irradiation of a logarithmic culture after reduction of the long chains to mono-or diplococci by blending, and plating on a medium free from Ca²+. A screening procedure for the isolation of the newly cured strain has also been devised which permits examination of as many as 24 cultures at a time. Morphologically and physiologically the lysogenic strain was not different from its cured derivative. They both formed long chains in broth media, they produced lactic acid at the same rate, fermented lactose, glucose, galactose, but not sucrose and maltose, they did not hydrolyse arginine and they did not grow at 40°C. That is, the prophage(s) themselves appeared to be playing no part in determining the bacterial phenotype. Microbiologically, no difference in virulent phages sensitivity between the two strains was observed and no host induced modification/restriction was observed to be due to the presence of the prophage. However when temperate phages liberated from 22 strains of S. cremoris and 2 strains of S. lactis were spotted on lawns of both the R₁ and its cured derivative it was found that the presence of the R₁ prophages conferred immunity against phages liberated by 13 out of 24 strains examined. Relysogenization of the cured R₁ strain with one of the two R₁ phages conferred immunity to the phages liberated by only a few of the other strains. Comparison of the morphology of the various phages involved,showed that the collared phage liberated from R₁ conferred immunity to collared temperate phages liberated by the other strains examined, and were heteroimmune to the collarless temperate phage. During the course of this project two unusual mutants were isolated which had in common the fact that they did not form chains. One of these was physiologically quite different from the parental strain.