Inorganic salts resistance associated with a lactose-fermenting plasmid in Streptococcus lactis

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Abstract
Present evidence indicates that lactose metabolism in group N streptococci is linked to plasmid DNA. Lactose-positive (Lac+) S. lactis and lactose-negative (Lac-) derivatives were examined for their resistance to various inorganic ions. Lac+ S. lactis strains ML3, M18 and C2 were more resistant to arsenate (7.5 -60.2-fold), arsenite (2.25 -3.0-fold) and chromate (6.6 -9.4-fold), but more sensitive to Cu (10.0 -13.3-fold) than their Lac- derivatives. Genetic information for resistance and/or sensitivity to these ions resides on the lactose plasmid. Kinetics of UV irradiation inactivation of transducing ability for lactose metabolism and arsenate resistance confirmed the plasmid location of the 2 markers. Lac+ transductants from S. lactis C2 received genetic determinants for resistance to arsenate, arsenite and chromate but not for Cu sensitivity. In this case, resistance markers were lost when the transductants became Lac- but the derivatives remained Cu resistant. The resistance markers for arsenate and arsenite could not be identified as separate genetic loci, but chromate resistance and copper sensitivity markers were independent genetic loci. The lactose plasmid from S. lactis C10 possessed the genetic loci for arsenate and arsenite resistance but not for chromate resistance or Cu sensitivity.