Mott bacilli
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MOTT Bacilli (Mycobacteria Other Than Tuberculosis)
MOTT is an acronym for Mycobacteria Other Than Tuberculosis - also called nontuberculous mycobacteria (NTM), atypical mycobacteria, environmental mycobacteria, anonymous mycobacteria, or opportunistic mycobacteria. These are mycobacteria that do not cause tuberculosis or leprosy/Hansen's disease.
Terminology
The term "MOTT bacilli" was coined to distinguish these organisms from Mycobacterium tuberculosis complex and M. leprae. Multiple synonyms exist:
- Nontuberculous mycobacteria (NTM) - now most preferred
- Atypical mycobacteria
- Environmental mycobacterioses
- Anonymous mycobacterioses
- Opportunistic mycobacterioses
(Dermatology 2-Volume Set 5e, Fitzpatrick's Dermatology)
Runyon Classification (1954)
Ernest Runyon proposed the first classification system based on growth rate and pigment production. Despite molecular advances (>200 mycobacterial species now recognized), it remains in use:
| Runyon Group | Characteristic | Key Species |
|---|---|---|
| Group I - Photochromogens | Yellow pigment on light exposure | M. kansasii, M. marinum, M. simiae |
| Group II - Scotochromogens | Yellow/orange pigment in dark | M. scrofulaceum, M. gordonae, M. szulgai (37°C) |
| Group III - Nonchromogens | No significant pigment | M. avium complex (MAC), M. ulcerans, M. xenopi, M. malmoense, M. haemophilum |
| Group IV - Rapid growers | Colonies in <7 days | M. abscessus, M. fortuitum, M. chelonae, M. smegmatis |
(Jawetz Melnick & Adelberg's Medical Microbiology 28E)
Note: In the US, M. tuberculosis complex, MAC, M. kansasii, and M. gordonae account for >95% of clinical mycobacterial isolates.
Epidemiology
- In the US, pulmonary NTM infections are severalfold more common than tuberculosis, and rates continue to rise (especially in the elderly).
- MOTT are environmental organisms - found in soil, water, and natural sources. They are not transmitted person-to-person.
- Among cystic fibrosis patients, NTM infection rates range from 3-15%.
- The predominant species vary geographically: MAC and M. abscessus are most common in North America; M. xenopi and M. malmoense are prominent in northern Europe; M. ulcerans (Buruli ulcer) occurs in tropical zones, mainly West Africa.
(Harrison's Principles of Internal Medicine 22E, 2025)
Pathobiology / Host Susceptibility
Since exposure to MOTT is nearly universal but disease is rare, host defense defects are key:
- HIV/AIDS: Low CD4+ T-cell counts strongly predispose to disseminated MAC infection
- Anti-TNF-α therapy (infliximab, adalimumab, etc.): Inhibits granuloma formation, risking severe mycobacterial infections
- IFN-γ / IL-12 pathway mutations: Autosomal mutations in the IFN-γ / IL-12 synthesis and response pathway account for many cases of disseminated NTM in immunocompetent-appearing hosts
- GATA2 deficiency: Associated with bulky mediastinal M. kansasii lymphadenopathy
The IL-12 → STAT4 → IFN-γ axis is central: macrophages produce IL-12 after phagocytosing mycobacteria; IL-12 drives IFN-γ production from T/NK cells, which then activates macrophage killing. Defects at any step increase susceptibility. (Harrison's 22E)
Clinical Syndromes
Pulmonary Disease (most common)
- Chronic cavitary pulmonary disease - resembles upper-lobe TB; typically in older white male smokers with COPD. Usually MAC or M. kansasii
- Nodular/reticulonodular disease with bronchiectasis - the "Lady Windermere syndrome" - thin postmenopausal women >50 years, sometimes with pectus excavatum or mitral valve prolapse. Usual pathogen: MAC
- Bronchiectasis in cystic fibrosis - MAC or M. abscessus; difficult to distinguish colonization from infection
- Chronic lower lobe pneumonia - aspiration-prone patients; M. fortuitum, M. abscessus, MAC
Extrapulmonary Disease
| Syndrome | Key Organism(s) | Notes |
|---|---|---|
| Cervical lymphadenitis (scrofula) | MAC (80% in US), M. scrofulaceum | Painless, unilateral; children; surgical excision preferred over drainage |
| Lymphocutaneous / sporotrichoid | M. marinum | Fish tank/swimming pool exposure; "fish tank granuloma" |
| Post-surgical/post-traumatic infections | M. fortuitum, M. chelonae, M. abscessus | After cosmetic surgery (breast augmentation), injections |
| Tendon/bone/bursa infection | MAC, M. marinum | Unusual rheumatologic presentations |
| Buruli ulcer | M. ulcerans | Painless progressive skin ulcer; neglected tropical disease; 3rd most common mycobacteriosis worldwide |
| Disseminated disease | MAC | Mainly in HIV/AIDS with CD4 <50 cells/μL |
Key Diagnostic Points
- A single positive culture is NOT sufficient for diagnosis of disease (given ubiquity as commensals/environmental organisms)
- Diagnosis requires: (1) compatible clinical syndrome, (2) exclusion of other diagnoses, and (3) multiple positive culture results
- Transbronchial biopsy may be needed
- The American Thoracic Society (ATS) guidelines are the standard reference for diagnosis and treatment
- Modern identification: DNA probes (16S rRNA hybridization), 16S rRNA gene sequencing, HPLC mycolic acid profiling, multiplex PCR (hsp65)
Treatment by Species
| Species | Key Regimen |
|---|---|
| MAC (pulmonary) | Macrolide (clarithromycin or azithromycin) + ethambutol + rifamycin; thrice-weekly for nodular disease; 18+ months total |
| MAC (disseminated HIV) | Same backbone; rifabutin may replace rifampin; watch rifamycin-PI interactions |
| M. kansasii | Isoniazid + rifampin + ethambutol; continue 1 year after culture conversion |
| M. abscessus | Macrolide + amikacin + cefoxitin or carbapenem; extremely difficult to cure pulmonary disease |
| M. fortuitum | Ciprofloxacin, clarithromycin, TMP-SMX, amikacin, linezolid; surgical excision for localized disease |
| M. chelonae | Clarithromycin; + linezolid, tobramycin, or amikacin for severe disease |
| M. marinum | Clarithromycin + rifampin or ethambutol; minocycline also active; 1-2 months post-resolution |
| M. ulcerans | Rifampin + clarithromycin (or streptomycin) x 8 weeks; local heat (40°C), hyperbaric O2; surgical excision for large ulcers |
| M. haemophilum | Clarithromycin + rifamycin ± ciprofloxacin or amikacin; surgical excision |
| M. scrofulaceum | Isoniazid + rifampin, clarithromycin; surgical excision |
(Dermatology 2-Volume Set 5e, Harrison's 22E, Textbook of Family Medicine 9e)
Key Points Summary
- MOTT = NTM = environmental mycobacteria that do not cause TB or leprosy
- Runyon classification: photochromogens, scotochromogens, nonchromogens, rapid growers
- Most common in US: MAC > M. kansasii > M. abscessus
- Disease is rising in incidence; often involves immunocompromised hosts or those with structural lung disease
- Treatment is prolonged (often 12-18+ months), multidrug, guided by in vitro susceptibility testing
- Inadequate/single-drug therapy leads to resistance and relapse - as in TB
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