What does a sputum culture growing Mycobacterium abscessus complex and Mycobacterium avium complex indicate?

Dual NTM Infection: Mycobacterium abscessus Complex and Mycobacterium avium Complex

This sputum culture result indicates the simultaneous presence of two distinct nontuberculous mycobacterial (NTM) species—both M. abscessus complex and M. avium complex—which represents a polymicrobial NTM infection requiring careful clinical correlation to determine if either or both organisms are causing true pulmonary disease versus environmental contamination. 1

What This Culture Result Means

Species Identification

• M. avium complex (MAC) is a slow-growing NTM that includes M. avium, M. intracellulare, and M. chimaera, accounting for up to 72% of NTM-positive sputum cultures in North America and Europe 1
• M. abscessus complex (MABSC) is a rapid-growing NTM comprising subspecies M. abscessus subsp abscessus, M. a. massiliense, and M. a. bolletii, which has become increasingly common and in many centers is now the most frequently isolated NTM 1
• Both organisms are ubiquitous environmental organisms found in water and soil, and their isolation does not automatically indicate clinical disease 1, 2

Clinical Significance Depends on Multiple Factors

The critical question is whether this represents true pulmonary disease, environmental contamination, or transient colonization. 1

To establish NTM pulmonary disease, ALL of the following criteria must be met 1:

Clinical criteria:

• Pulmonary symptoms (cough, dyspnea, fatigue, weight loss) or systemic symptoms 1

Radiographic criteria:

• Nodular or cavitary opacities on chest radiograph, OR
• High-resolution CT showing bronchiectasis with multiple small nodules 1

Microbiologic criteria (at least one of the following):

• ≥2 positive sputum cultures from separate expectorated specimens (same species required) 1
• ≥1 positive culture from bronchial wash/lavage 1
• Transbronchial or lung biopsy with mycobacterial histologic features plus positive culture 1

Key Diagnostic Considerations for Your Case

A single positive culture—even with two species—has poor predictive value for true disease. 1

• Studies show that only 2% of patients with a single MAC isolation from three specimens developed progressive radiographic disease, while 98% of those with ≥2 MAC isolates had progressive disease 1
• You need repeat sputum cultures on separate days (minimum 3 specimens total) to determine if either or both organisms persist 1, 3
• The same NTM species (or subspecies for M. abscessus) must be isolated in ≥2 cultures to meet diagnostic criteria 1

Species-Specific Virulence Differences

M. abscessus complex is generally more virulent and clinically significant than MAC: 1

• Patients with MABSC-positive cultures are more likely to meet ATS/IDSA criteria for true pulmonary disease 1
• MABSC is more difficult to treat, often requiring prolonged intravenous antibiotics, and has worse clinical outcomes 4
• More than half of MABSC isolates have inducible macrolide resistance 4
• MAC typically causes slower, more indolent disease 1, 5

Common Clinical Pitfalls to Avoid

Do not initiate empiric NTM therapy based on this single culture result alone. 1

• Empiric therapy for suspected NTM lung disease is not recommended 1
• Some NTM species are much more likely to represent contamination than disease (though MAC and MABSC are both recognized pathogens) 1
• Tuberculosis must be excluded first, especially if acid-fast bacilli smears are positive—empiric TB therapy may be necessary pending species confirmation 1

Immediate Next Steps

Collect at least 2 additional sputum specimens on separate days for AFB smear and culture: 1, 3

• Specimens should be collected on different days (not consecutive hours) 3
• At least one should be an early-morning specimen 3
• Both liquid and solid media cultures should be used 1

Obtain high-resolution chest CT if not already done: 1

• Plain chest radiograph may be adequate for fibrocavitary disease, but HRCT is routinely indicated to demonstrate characteristic nodular/bronchiectatic abnormalities 1
• Look for bronchiectasis with clusters of small (≤5mm) nodules, which are characteristic of NTM disease 1

Assess clinical and radiographic criteria: 1

• Document presence/absence of pulmonary symptoms
• Determine if radiographic abnormalities are present and progressive
• Consider whether another diagnosis (malignancy, other infection) could explain findings 1

Special Populations at Higher Risk

Certain patient groups warrant closer monitoring even with a single positive culture: 4

• Cystic fibrosis patients 1, 4
• Older women with bronchiectasis and very low body mass index 4
• Patients with pre-existing structural lung disease 6
• Immunocompromised hosts (though disseminated disease is rare) 5, 6

If Diagnostic Criteria Are Met

Treatment decisions should be based on potential risks and benefits for the individual patient, not solely on meeting diagnostic criteria. 1

• MAC lung disease typically requires 12 months of macrolide-based therapy after culture conversion 1, 2
• MABSC lung disease is very difficult to treat, requiring long-term parenteral agents plus macrolides 2
• Subspecies identification within M. abscessus complex is critical—M. massiliense has much better treatment outcomes than M. abscessus 2
• Drug susceptibility testing should be performed for macrolides (clarithromycin) and amikacin 7

Infection Control Considerations

If the patient has cystic fibrosis, minimize potential for cross-infection of M. abscessus complex between individuals by following national guidelines. 1