[6] While most Mycobacterium species are non-pathogenic, the genus' characteristic complex cell wall contributes to evasion from host defenses.

[10] The distinguishing characteristic of all Mycobacterium species is a thick, hydrophobic, and mycolic acid-rich cell wall made of peptidoglycan and arabinogalactan, with these unique components offering targets for new tuberculosis drugs.

[13] Whereas Mycobacterium tuberculosis and M. leprae are pathogenic, most mycobacteria do not cause disease unless they enter skin lesions of those with pulmonary and/or immune dysfunction, despite being widespread across aquatic and terrestrial environments.

Through biofilm formation, cell wall resistance to chlorine, and association with amoebas, mycobacteria can survive a variety of environmental stressors.

The agar media used for most water testing does not support the growth of mycobacteria, allowing it to go undetected in municipal and hospital systems.

[25] M. leprae infection localizes within the skin macrophages and Schwann cells found in peripheral nerve tissue.

Nontuberculosis Mycobacteria (NTM), which exclude M. tuberculosis, M. leprae, and M. lepromatosis, can infect mammalian hosts.

It constantly remodels these layers to survive in stressful environments and avoid host immune defenses.

This cell wall structure results in colony surfaces resembling fungi, leading to the genus' use of the Greek prefix myco-.

[7] M. lepromatosis M. chelonae M. fortuitum M. flavescens M. smegmatis M. avium M. intracellulare M. kansasii M. scrofulaceum M. malmoense M. szulgai M. marinum M. tuberculosis M. gordonae M. simiae M. xenopi M. nonchromogenicum M. terrae Nocardia asteroides Mycobacteria have historically been categorized through phenotypic testing, such as the Runyon classification of analyzing growth rate and production of yellow/orange carotenoid pigments.

[6] Because the International Code of Nomenclature of Prokaryotes (ICNP) currently recognizes 195 Mycobacterium species, classification and identification systems now rely on DNA sequencing and computational phylogenetics.

Due to controversy over complicating clinical diagnoses and treatment, all of the renamed species have retained their original identity in the Mycobacterium genus as a valid taxonomic synonym:[40][41] The two most common methods for visualizing these acid-fast bacilli as bright red against a blue background are the Ziehl-Neelsen stain and modified Kinyoun stain.