Conjugal Transfer in Lactic Streptococci of Plasmid-Encoded Insensitivity to Prolate- and Small Isometric-Headed Bacteriophages.

Eight of 40 strains of Streptococcus lactis and S. lactis subsp. diacetylactis were able to conjugally transfer a degree of phage insensitivity to Streptococcus lactis LM0230. Transconjugants from one donor strain, S. lactis subsp. diacetylactis 4942, contained a 106-kilobase (kb) cointegrate plasmid, pAJ1106. The plasmid was conjugative (Tra) and conferred phage insensitivity (Hsp) and lactose-fermenting ability (Lac) in S. lactis and Streptococcus cremoris transconjugants. The phage resistance mechanism was effective against prolate- and small isometric-headed phages at 30 degrees C. In S. lactis transconjugants, the phage resistance mechanism was considerably weakened at elevated temperatures. A series of deletion plasmids was isolated from transconjugants in S. cremoris 4854. Deletion plasmids were pAJ2074 (74 kb), Lac, Hsp, Tra; pAJ3060 (60 kb), Lac, Hsp; and pAJ4013 (13 kb), Lac. These plasmids should facilitate mapping Hsp and tra genes, with the aim of constructing phage-insensitive strains useful to the dairy industry.