Koch's Tubercle Bacillus - a centenary reappraisal.

The tubercle bacillus discovered by Robert Koch in 1882 was termed Mycobacterium tuberculosis in 1886. This organism was later found to be only one of many acid fast bacilli, some of which caused diseases resembling tuberculosis in various animals and some lived freely as saprophytes. The strains associated with disease were also referred to as tubercle bacilli and in later years those which differed in their properties from Koch's original isolates were given separate species names. Modern taxonomic methods have enable the species within the genus Mycobacterium to be carefully defined. The use of such methods has shown that the strains called M. tuberculosis, M. bovis, M. microti and M. africanum belong to a single evolutionary unit or species. It is therefore recommended that the latter three species names should be discarded and that the variants should be regarded as types of M. tuberculosis. The term "tubercle bacilli" had been applied to the variants of M. tuberculosis and also to quite distinct species of acid fast bacilli such as M. avium. It is therefore necessary to define the term "Tubercle bacillus" whenever it is used. The "classical" subdivisions of M. tuberculosis are based on simple cultural properties and pathogenicity in mammalian hosts. More recent methods enable other important subdivisions to be made. These include three major phage types of the human strains and two variants of the bacillus is therefore one of a heterogeneous group of acid fast bacilli which can, nevertheless, be seen to belong to the evolutionary distinct species Mycobacterium tuberculosis.