Mycobacterium: Definition and Characteristics
Mycobacteria are a diverse genus of bacteria comprising over 190 species and subspecies, characterized by their unique cell wall structure containing mycolic acids, which distinguishes them from other bacterial genera. 1, 2
Classification of Mycobacteria
Mycobacteria are broadly classified into three main categories:
- Mycobacterium tuberculosis complex - causes tuberculosis
- Mycobacterium leprae complex - causes leprosy
- Nontuberculous mycobacteria (NTM) - all other mycobacterial species found throughout the environment 1
Key Characteristics of Mycobacteria
- Cell Wall Structure: Possess a distinctive cell wall rich in mycolic acids (β-hydroxy-α-fatty acids), making them impermeable to many compounds and contributing to their drug resistance 1, 3
- Growth Rate: Categorized as either:
- Slow-growing (e.g., M. avium, M. intracellulare, M. kansasii)
- Rapid-growing (e.g., M. abscessus and its subspecies) 1
- Acid-Fast Properties: Retain certain dyes even after acid washing due to their waxy cell wall
- Environmental Presence: Most NTM species are found throughout the environment, unlike M. tuberculosis and M. leprae which have no environmental reservoir 4
Clinical Significance
- Pulmonary Disease: The most common clinical presentation of NTM infection, often occurring in patients with underlying structural lung disorders such as bronchiectasis or COPD 1, 4
- Increasing Prevalence: NTM pulmonary disease incidence and prevalence are rising globally, particularly in older individuals and those with bronchiectasis 1
- Biofilm Formation: Mycobacteria can form biofilms, which contribute to their pathogenicity and resistance to antibiotics and disinfectants 5
- Drug Resistance: Mycobacteria either naturally possess or develop high resistance to conventional antibiotics, making treatment challenging 3
Laboratory Identification
Mycobacteria can be identified through various methods:
- Colonial Morphology: Distinctive appearance on specialized media like Middlebrook 7H10 or 7H11 agar 1
- Molecular Techniques:
- Nucleic acid hybridization
- High-performance liquid chromatography (HPLC)
- Gene sequencing
- Polymerase chain reaction restriction analysis (PRA) 1
- Biochemical Testing: Traditional but time-consuming method 1
Treatment Considerations
- Multidrug Regimens: Treatment typically requires multiple antimicrobial agents administered for prolonged periods 1
- Surgical Intervention: In selected patients with NTM pulmonary disease, surgical resection may be considered as an adjunct to medical therapy, particularly for those with treatment failure, cavitary disease, or drug-resistant isolates 1
- Expert Consultation: Due to the complexity of these infections, consultation with experts in mycobacterial diseases is strongly recommended 1, 2
Mycobacteria represent a significant clinical challenge due to their unique biological properties, increasing prevalence, and treatment difficulties. Understanding their characteristics is essential for appropriate diagnosis and management of mycobacterial infections.