MICROBIOLOGY

• Human mycobacterial pathogen, occasional environmental contaminant. Present in water, sewerage, vegetation.
• Considered among the most pathogenic and chemotherapy-resistant of rapid-growing Mycobacteria.
  • The organism produces clavulanate-insensitive broad-spectrum β-lactamase that limits the in vivo efficacy of β-lactams[10].
  • Culture on both solid and liquid media preferred to increase sensitivity.
    • CLSI recommends 7H10 and 7H11 solid media at 36°C for slow growers (and 28°C for rapid growers).
      • Some use Löwenstein-Jensen slants as the most sensitive media.
    • Typically, respiratory specimens treated with 0.25% N-Acetyl-L-cysteine and 1% NaOH for decontamination.
  • As a reminder,
    • Mycobacteria spp. divided into rapid growers (< 7 days) and slow growers (>7 days) based on time to mature growth on agar plates.
    • Rapid growers (often 3-7d): M. abscessus, also M. fortuitum: (3-7 days), M. chelonae.
    • Slow growers (> 7-10d): M. gordonae(7-10 days), M. malmoense, M. marinum; M. ulceransandM. xenopi(3-8 wks in Cx), M. terrae, M. nonchromogenicum, (also:M. kansasii, M. avium-intracellulare, M. tuberculosis)
• Formerly part of "M. chelonae-complex", but important to distinguish from M. chelonae as antimycobacterial therapy more difficult with M. abscessus senso strictu.
  • Three human subspecies have been proposed: molecular identification (e.g., hsp65 or rpoB gene-based typing necessary to distinguish) recommended; usual biochemical/phenotypic methods fail. Speciation is difficult and there is not uniformity in recognition, so confusion is not infrequent when applying labels to M. abscessus isolates.
    
    • M. abscessus subsp. abscessus
      
      • Seen more commonly in North America
      • Many isolates have mutational resistance or inducible resistance re: function erm gene.
      • Resistance often is seen after the 3-14d employment of macrolide.
        • Testing of isolates for inducible macrolide resistance suggested.
        • Prior exposure to macrolides heightens the likelihood of macrolide resistance.
      • ~20% of M. abscessus subsp abscessus isolates contain an inactive erm gene.
        • If so, this translates into an improved chance of success the chances of a successful treatment outcome with macrolide-based therapy
    • M. abscessus subsp. massiliense
      • Lacks inducible macrolide resistance or erm gene[15](non-functional)
      • More frequently seen in Korea.
    • M. abscessus subsp. bolletii°
      • May have an inducible erm gene.
    • The first two species represent most human infections.
• Occasionally confused with Corynebacterium spp. (described as diphtheroid growing in broth systems).
• In vitro resistance rates (most studies from Asia):
  • Clarithromycin(0-38%)
  • Cefoxitin(15.1%)
  • Amikacin(7.7%)
  • Higher rates seen with imipenem, doxycycline, moxifloxacin, ciprofloxacin.