What are the common mycobacteria (e.g. Mycobacterium tuberculosis, Mycobacterium avium complex, Mycobacterium kansasii) that cause lung infections in individuals with varying demographics and medical histories, such as immunocompromised conditions (e.g. HIV/AIDS) or chronic lung diseases (e.g. chronic obstructive pulmonary disease)?

Mycobacteria Causing Lung Infections

The mycobacteria that cause lung infections are divided into Mycobacterium tuberculosis complex and nontuberculous mycobacteria (NTM), with the most common NTM pathogens being Mycobacterium avium complex (MAC), M. kansasii, M. xenopi among slowly growing species, and M. abscessus among rapidly growing species. 1

Mycobacterium tuberculosis Complex

• M. tuberculosis complex is the primary cause of tuberculosis and must be distinguished from NTM, as it requires different treatment approaches and has public health implications including human-to-human transmission 1
• M. tuberculosis complex is excluded from the NTM classification, along with M. leprae complex and M. ulcerans 1

Nontuberculous Mycobacteria (NTM): Slowly Growing Species

Mycobacterium avium Complex (MAC)

• MAC is the most common NTM causing pulmonary disease and now consists of 12 separate species 1
• The three most common MAC species causing pulmonary disease are M. avium, M. intracellulare, and M. chimaera 1
• MAC accounts for approximately 90% of disseminated NTM infections in HIV/AIDS patients with CD4 counts below 50 cells/µL 1
• MAC typically affects middle-aged to elderly individuals, particularly those with underlying structural airway disease such as bronchiectasis or chronic obstructive pulmonary disease 1

Mycobacterium kansasii

• M. kansasii is an important slowly growing NTM causing pulmonary disease, with clinical and radiographic presentations that closely resemble tuberculosis 1
• M. kansasii infections are endemic in cities with infected tap water and typically affect patients with underlying chronic lung disease 1, 2, 3
• In HIV/AIDS patients, M. kansasii is the second most common NTM pathogen after MAC 1

Mycobacterium xenopi

• M. xenopi is recognized as another important slowly growing NTM causing pulmonary disease 1
• M. xenopi typically infects middle-aged to elderly persons with preexisting lung disease 2

Other Slowly Growing Species

• M. malmoense causes pulmonary disease and lymphadenitis, particularly in children 1, 2
• M. szulgai and M. simiae are less common slowly growing mycobacteria that can cause pulmonary infection, typically in patients with preexisting lung disease 2

Nontuberculous Mycobacteria: Rapidly Growing Species

Mycobacterium abscessus Complex

• M. abscessus and its subspecies (abscessus, bolletii, and massiliense) are by far the most common causative agents of pulmonary disease due to rapidly growing mycobacteria 1
• M. abscessus is the most pathogenic rapidly growing mycobacterium and presents particularly challenging treatment scenarios due to extensive drug resistance 3
• Subspecies-level identification is clinically important for M. abscessus, as treatment outcomes vary significantly between subspecies 1

Special Populations and Clinical Context

Immunocompromised Patients (HIV/AIDS)

• In patients with advanced HIV infection (CD4 count <50 cells/µL), disseminated MAC disease is the predominant mycobacterial infection, with over 90% of disseminated NTM cases caused by MAC 1
• M. kansasii is the second most frequently reported NTM in HIV patients, though far less common than MAC 1
• Other NTM species reported in AIDS patients include M. scrofulaceum, M. gordonae, M. haemophilum, M. genavense, M. celatum, M. conspicuum, M. xenopi, M. fortuitum, M. marinum, M. malmoense, and M. simiae 1
• Disseminated NTM disease in HIV patients occurs almost exclusively when CD4 counts fall below 50 cells/µL, with average counts at presentation typically less than 25 cells/µL 1, 4

Transplant Recipients

• In lung transplant recipients, MAC and M. kansasii are the most commonly encountered NTM pathogens, with NTM potentially more common than M. tuberculosis as a cause of pulmonary infection 1
• Pulmonary NTM infection in transplant recipients tends to occur late post-transplantation and is frequently associated with chronic rejection 1

Patients with Chronic Lung Disease

• Patients with underlying bronchiectasis or chronic obstructive pulmonary disease are at particularly high risk for NTM pulmonary disease 1
• The incidence and prevalence of NTM pulmonary disease are increasing globally, with rates particularly high in older individuals and those with underlying bronchiectasis 1

Critical Clinical Distinctions

• Only a small number of the over 190 known NTM species actually cause pulmonary disease in humans 1
• Isolation of NTM from respiratory specimens does not always indicate active disease, as environmental contamination and colonization are common 1, 2
• Species-level identification is essential because treatment regimens, drug susceptibility patterns, and prognosis vary significantly between different mycobacterial species 1
• For M. abscessus, subspecies-level identification is required for optimal clinical and epidemiologic management 1