Characterization of plasmids from strains of Mycobacterium avium-intracellulare.

Strains of Mycobacterium avium-intracellulare were examined for the presence of plasmids. Actively growing cultures were treated with cycloserine and ampicillin to sensitize the cells to lysis by detergent, and cleared lysates were prepared by treatment with sodium dodecyl sulfate and NaCl. Purified plasmid DNA was prepared by equilibrium density centrifugation in CsCl gradients containing ethidium bromide and analyzed by agarose gel electrophoresis. Strain LR25 was found to carry three plasmids, including a large plasmid with a molecular weight of greater than 10(8). Examination of the DNA by electron microscopy confirmed that the bands seen in the agarose gels represented circular DNA molecules. The presence of plasmids was demonstrated in four additional strains. Three plasmids were carried in a strain of serovar 2, and single plasmids were detected in strains of serovars 4, 12, and 18. Plasmids were not detected in three strains of serovars 1, 3, and 5.