FOR many years bacteriologists in culturing sputum specimens for mycobacteria occasionally noticed colonies of organisms differing somewhat from the typical Koch bacillus. In the past ten years these variants have appeared much more frequently, and it has been recognized that these are not atypical tubercle bacilli or mutants, but are truly different species of mycobacteria. Some are discovered fortuitously in the absence of any demonstrable pulmonary disease and are probably nonpathogenic. Other species are definitely pathogenic to man and cause disease which is essentially indistinguishable from the pulmonary infection produced by mycobacteria tuberculosis.
Various classifications have been suggested, but that proposed by Runyon7 has achieved the most universal acceptance. This classification is based on gross cultural characteristics, but certain chemical and physical reactions serve to differentiate them more accurately. The Subcommittee on Unclassified Mycobacteria9 prefer the term "unclassified," yet by common usage the term "atypical" is recognized as