title: Development of high-level streptomycin resistance affected by a plasmid in lactic streptococci.
abstract: Some lactose-negative (Lac-) mutants of Streptococcus lactis C2 and ML3 exhibited development of very high level streptomycin resistance after incubation with subinhibitory concentrations of the drug for 18 to 22 h. These drug-resistant mutants showed no loss of resistance even after 6 months of subculturing in broth without any drug. The parental Lac+ strains did not show mutation to high-level streptomycin resistance. The Lac+ characteristic of the parental strain was conjugally transferred to Lac- derivatives of C2 and ML3, showing the ability to mutate to high-level resistance. When transconjugants were analyzed for this characteristic, they showed both mutable and nonmutable Lac+ types. The results suggested that genetic information for mutation to high-level streptomycin resistance in lactic streptococci resides on the chromosome, and its expression is affected by a plasmid. The plasmid profiles of strains C2, ML3, C2 Lac-, ML3 Lac-, and two kinds of transconjugants confirmed the presence of a plasmid of approximately 5.5 megadaltons in strains showing no mutation to high-level streptomycin resistance, while strains missing such a plasmid exhibited high-level streptomycin resistance after incubation with subinhibitory concentrations of the drug.
mesh_terms: D004352:Drug Resistance, Microbial; D007612:Kanamycin / Q000494:pharmacology; D007700:Kinetics; D007773:Lactates / Q000378:metabolism; D013294:Lactococcus lactis / Q000187:drug effects / Q000235:genetics* / Q000254:growth & development; D009154:Mutation; D009355:Neomycin / Q000494:pharmacology; D010641:Phenotype; D011815:R Factors; D013045:Species Specificity; D013307:Streptomycin / Q000494:pharmacology*; D013752:Tetracycline / Q000494:pharmacology
chemical_list: D007773:Lactates; D007612:Kanamycin; D013752:Tetracycline; D009355:Neomycin; D013307:Streptomycin