Deoxyribonucleic acids (DNAs) of selected strains of the Mycobacterium tuberculosis complex, including strains of M . tuberculosis, Mycobacterium bovis, M. bovis BCG, Mycobacterium microti, and Mycobacterium africanum, were isolated and analyzed spectrophotometrically. The genome molecular weight and the guanine-plus-cytosine content of the DNA samples were approximately 1.9 X lo9 and 62.9 mol% , respectively. DNA relatedness among the strains was studied by both spectrophometric DNA hybridization and restriction endonuclease cleavage methods. All strains of the four species showed more than 90% DNA relatedness, except for slightly lower levels (85 to 89%) of relatedness between M . microti and some strains of M. bovis, M . bovts BCG, or M . africanum. The distribution patterns of the DNA fragments produced with restriction enzymes, such as EcoRI, EcoRV, BglII, BcZI, and KpnI, were indistinguishable upon agarose gel electrophoresis. In contrast, the levels of DNA relatedness between M . tuberculosis and other slowly growing mycobacteria, including Mycobacterium nonchromogenicum, Mycobacterium intracellulare, Mycobacterium kansasii, Mycobacterium avium, Mycobacterium scrofulaceum, Mycobacterium haemophilum, Mycobacterium marinum, and Mycobacterium ashticum, were significantly heterogeneous, ranging from 9 to 53%. My data indicate the extremely close evolutionary relationship among four species of the M. tuberculosis complex.Mycohacterium tuberculosis, Mycobacterium bovis, M . bovis BCG, Mycobacterium microti, and Mycohacteriirm africanum are grouped as the M. tirherculosis complex based upon their phenetic similarities (25, 26). Distinctions among these species are defined on the basis of biochemical (9, 21, 25, 26), bacteriological (7,13,21,26), serological (8, 15, 19, 22) and chemical (23, 27) properties, their levels of pathogenicity for experimental animals (10, 13, 20, 21), and their levels of susceptibility to some mycobacteriophages (6, 28). A thorough review of these taxonomic findings has been published by Wayne (26).However, any of the extensive phenotypic analyses available measure less than 20% of the total genomic capability of bacteria (4). Consequently, phenotypic differences should not be emphasized as absolute criteria for definition of species. On the other hand, examination of essential properties of chromosomes, such as genome molecular weight, guanine-plus-cytosine content, and deox yri bonucleic acid (DNA) relatedness, provides a more practical and reliable method to determine the genetic relationships among phenetically similar species. In fact, Bradley (3) and Baess (2) reported 100% DNA homology between M . tirbercirlosis and M . bovis, including M . bovis BCG strains, showing that these organisms belong to the same species.In this report I provide additional information concerning the genome molecular weights, base ratios, and extents of DNA relatedness among selected strains of the species of the M . tubercrrlosis complex. A total of 13 M . hovis BCG strains commonly used for vaccinatio...
