Streptococcus cremoris M12R transconjugants carrying the conjugal plasmid pTR2030 are insensitive to attack by lytic bacteriophages.

Conjugal transfer of lactose-fermenting ability (Lac+), nisin resistance (Nisr), and phage resistance (Hsp+) was demonstrated in matings between Streptococcus lactis ME2 (donor) and Streptococcus cremoris M43a (recipient), a derivative of M12R. Transconjugants were detected by transfer of Lac+ and were found to exhibit Nisr and harbor a 40-megadalton plasmid (pTR1040). Fifty-six percent of Lac+ transconjugants were resistant to the S. cremoris M12R lytic phage. Efficiency of plaquing for phage m12r . M12 on a phage-resistant transconjugant, T2r-M43a, was less than 4.3 X 10(-10). Five additional phages which were virulent for S. cremoris M12R and isolated from industrial sources failed to plaque on S. cremoris T2r-M43a. Mating experiments with T2r-M43a revealed that phage resistance was accompanied by high-frequency conjugation ability (Tra+) and the appearance of both pTR1040 and pTR2030 encoding Lac+ Nisr and Tra+ Hsp+, respectively, in transconjugants of S. lactis LM2302. Phage-sensitive Lac+ transconjugants of S. cremoris M43a (T2s-M43a) showed no conjugal ability. These observations confirmed that pTR2030 was present and responsible for the phage resistance and conjugal ability exhibited by the S. cremoris transconjugant T2r-M43a. Unlike the S. lactis LM2302 transconjugant carrying pTR2030, resistance of T2r-M43a to phage was not affected at high temperatures (35 to 40 degrees C) or destabilized in repeated transfers through a starter culture activity test. These results demonstrated that phage resistance conferred by pTR2030 in the S. cremoris transconjugant was effective against industrially significant phages under fermentation conditions normally encountered during cheese manufacture.