Isolation and characterization of plasmids from human and environmental isolates of mycobacteria

Human clinical (n=131) and environmental (n=226) isolates of the Mycobacterium avium-intracellulare and M. scrofulaceum (MAIS) complex were screened for plasmids in an effort to increase knowledge about the genetics and epidemiology of these pathogenic bacteria. Approximately 50% of the clinical MAIS isolates from New York, Maryland, Virginia, South Carolina, and Georgia contained one or more plasmids. On the basis of plasmid content, aerosol MAIS isolates more closely resembled human MAIS isolates than did MAIS isolates from the other environmental sources examined (dust, soil, sediment, and water). Plasmid profiles were remarkably heterogenous, and isolates with identical profiles were rarely encountered. However, a 115 megadalton (Md) plasmid was detected in 15 mercury resistant human and environmental isolates. In one of these isolates (M. scrofulaceum W262) the presence of the 115 Md plasmid was shown to correlate with the presence of an NAD(P)H dependent mercuric reductase. Plasmids with molecular weights of 8.8, 11.2, 14.2, 16.9, 17.9, and 18.3 Md were also common among both human and environmental isolates. On the basis of molecular weight, 36 distinct plasmids were detected; their sizes ranged from 7 to 230 Md. It was concluded that human and environmental MAIS isolates share a number of plasmids with identical molecular weights and that plasmids can serve as useful entities in genetic and epidemiologic studies of this group of extremely slow-growing, poorly understood human and animal pathogens.