What is the significance of Mycobacterium (M.) abscessus in sputum?

Significance of Mycobacterium abscessus in Sputum

The isolation of Mycobacterium abscessus from sputum requires at least two positive cultures of the same subspecies, along with compatible clinical and radiographic findings, to establish a diagnosis of pulmonary disease according to the ATS/ERS/ESCMID/IDSA guidelines.

Diagnostic Criteria for M. abscessus Pulmonary Disease

The 2020 ATS/ERS/ESCMID/IDSA guidelines establish specific criteria for diagnosing nontuberculous mycobacterial (NTM) pulmonary disease, including M. abscessus:

Clinical and Microbiologic Requirements:

• Clinical symptoms: Pulmonary or systemic symptoms
• Radiologic findings: Nodular or cavitary opacities on chest radiograph, or bronchiectasis with multiple small nodules on HRCT
• Microbiologic evidence: At least two positive sputum cultures of the same M. abscessus subspecies 1

Interpretation of Isolated M. abscessus:

• A single positive culture may represent transient colonization or environmental contamination
• Multiple positive cultures increase likelihood of true infection
• The significance must be interpreted in the context of the patient's clinical presentation and radiographic findings 1

Clinical Significance and Disease Patterns

M. abscessus is the third most common NTM respiratory pathogen in the United States and accounts for approximately 80% of rapidly growing mycobacteria (RGM) respiratory disease isolates 1.

Patient Demographics and Presentation:

• Most commonly affects white, female, non-smoking patients over 60 years
• In younger patients (<40 years), typically associated with underlying conditions like:
  • Cystic fibrosis
  • Bronchiectasis
  • Prior mycobacterial infection
  • Gastroesophageal disorders
  • Alpha-1 antitrypsin anomalies 1

Radiographic Patterns:

• Multilobar, patchy, reticulonodular opacities with upper lobe predominance
• Cavitation in only ~15% of cases
• HRCT typically shows cylindrical bronchiectasis and multiple small nodules (<5mm)
• Similar to nodular bronchiectatic form of MAC lung disease 1

Disease Course:

• In patients without underlying disorders (other than bronchiectasis): Indolent and slowly progressive
• In patients with gastroesophageal disorders or CF: Can be more fulminant and rapidly progressive
• Approximately 15% of patients with M. abscessus may also have MAC co-infection 1

Subspecies Significance

M. abscessus comprises three subspecies with different treatment implications:

• M. abscessus subsp. abscessus: Contains functional erm(41) gene causing inducible macrolide resistance
• M. abscessus subsp. massiliense: Contains non-functional erm(41) gene, remains susceptible to macrolides
• M. abscessus subsp. bolletii: Contains functional erm(41) gene similar to subsp. abscessus 2, 3

Treatment response rates differ significantly by subspecies:

• M. a. massiliense: ~88% sputum culture conversion
• M. a. abscessus: ~25% sputum culture conversion 1

Treatment Considerations

If M. abscessus pulmonary disease is diagnosed, treatment is challenging:

• Initial intensive phase: Combination of intravenous drugs (typically 2-4 weeks)

  • Amikacin plus cefoxitin and/or imipenem and/or tigecycline
  • Combined with oral macrolide (if susceptible)

• Continuation phase: Oral drugs plus inhaled amikacin

  • Typically includes macrolide (if susceptible) plus 1-2 additional oral agents
  • Treatment duration is typically 12+ months after culture conversion 1, 2

• Surgical resection: May be beneficial for localized disease, especially with macrolide-resistant strains 1

Important Caveats

• Meeting diagnostic criteria does not automatically necessitate treatment

• Treatment decisions should consider:

  • Pathogenicity of the organism
  • Patient's symptoms
  • Risks and benefits of therapy
  • Patient's wishes and ability to receive treatment
  • Goals of therapy 1

• "Watchful waiting" may be appropriate in some cases

• Drug susceptibility testing is essential, particularly for macrolides and amikacin 2

• Treatment outcomes are generally poor for macrolide-resistant M. abscessus 2

Special Considerations in Cystic Fibrosis

In CF patients, M. abscessus is particularly concerning:

• More common in pediatric CF patients
• More likely to meet microbiologic criteria for disease
• Associated with worse outcomes
• Often requires more aggressive and prolonged therapy
• Eradication is uncommon despite intensive treatment 1

Remember that the significance of M. abscessus in sputum must always be interpreted within the clinical context, and multiple positive cultures over time are needed to establish a diagnosis of true infection rather than transient colonization or contamination.