[A mutant of Streptococcus lactis with resistance to bacteriophages isolated in Argentina and the United States of America].

Streptococcus lactis and Streptococcus cremoris have an important role in the fermentation of milk during the manufacturing of lactic products. Bacteriophages are spread in the plant environment and they may infect the starters causing technical and economic problems to the dairy industry. It is now known that the usual methods of control are not completely safe against the proliferation of phages. It is necessary therefore to include resistant strains to the phages which infect the plant. This study introduces a simple and direct method to obtain spontaneous mutants which have performed clear resistance to a certain phage. The prolate (ARG) phages used in this work St11, St13 and St15 were isolated in Argentina, on S. lactis C2 while the prolate and isometric (AM) phages phi C2, D59-1, I16-1, and G72-1, F4-1, I37-1 respectively, were isolated in Cornell University. on the corresponding S. lactis strains except the phage D59-1 isolated on an S. cremoris strain. The propagation of phages was performed using M and M17 broth. The phage sensitivity was tested through spot test and overlay plaque plating method expressing the titre of phage suspension. as PFU/ml. In order to obtain spontaneous resistant mutants of S. lactis C2 to phage St15, and active culture of S. lactis C2 (sensitive) was spread on agar M plates, inoculated with drops of a suspension of phage St15 (10(8)PFU/ml) and incubated at 30 degrees C for 48 h. Colonies of mutants grown in the lytic areas caused by the phage were isolated and selected.(ABSTRACT TRUNCATED AT 250 WORDS)