Abstract
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°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.

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