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Extrachromosomal elements in group N streptococci.

B R Cords, L L McKay, P Guerry

    Journal of Bacteriology
    |March 1, 1974
    PubMed
    Summary
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    This study investigated plasmid DNA in Streptococcus species, finding that lactose-negative mutants of S. lactis C2 and S. diacetilactis 18-16 retained covalently closed circular DNA, unlike S. cremoris B(1).

    Area of Science:

    • Microbiology
    • Molecular Biology
    • Genetics

    Background:

    • Streptococcus species are crucial in dairy fermentation.
    • Understanding their genetic makeup, including plasmid DNA, is vital for optimizing industrial applications.
    • Plasmid DNA can influence bacterial traits like lactose metabolism.

    Purpose of the Study:

    • To characterize plasmid DNA in Streptococcus lactis C2, S. cremoris B(1), and S. diacetilactis 18-16.
    • To investigate the correlation between lactose metabolism and the presence of plasmid DNA in these species.
    • To identify and quantify plasmids in wild-type and lactose-negative mutant strains.

    Main Methods:

    • Bacterial strains were grown in media containing radiolabeled thymine ((3)H).
    • Chromosomal and plasmid DNA were isolated using enzymatic lysis and differential precipitation.

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  • Cesium chloride-ethidium bromide equilibrium density gradient centrifugation and electron microscopy were employed to detect and analyze plasmid DNA.

    • Main Results:

    • Covalently closed circular DNA (plasmid DNA) was detected in S. lactis C2 and S. diacetilactis 18-16.
    • Lactose-negative mutants of S. lactis C2 and S. diacetilactis 18-16 retained plasmid DNA.
    • S. cremoris B(1) showed no detectable covalently closed circular DNA using cesium chloride-ethidium bromide gradients.
    • Three distinct plasmids with molecular weights of 1.3 x 10(6), 2.1 x 10(6), and 5.1 x 10(6) were identified in S. lactis C2 and its mutant.

    Conclusions:

    • Plasmid DNA is present in S. lactis C2 and S. diacetilactis 18-16, and its presence is not solely linked to lactose metabolism in these strains.
    • The absence of detectable plasmid DNA in S. cremoris B(1) suggests a different genetic organization or plasmid maintenance mechanism.
    • The identified plasmids in S. lactis C2 may play a role in other metabolic functions or cellular processes.