M yc.> bacteriai species are widespread throughout narure. ranging from saprophytic soiiand water-dweiicrs, such as Mycobacterium smcpwtis and Mycobacteriwn gordonae, to obligate mammalian pathogens such as Mycobacteriunl trrberculosis and Mycobacwrium lepae. The predilection cd a given species of mycobacrtiria for a certain environmenr or host probably reflects an ex tensivh process of selection in wl~ ch individual cellular structures have been optimized for survival by exposure to conditions rhat occur in that rric~ oenvironment’. The mycobac-