Mycobacterium fortuitum : Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes

Introduction

Mycobacterium fortuitum is a rapidly growing, nontuberculous mycobacterium (NTM). It is found ubiquitously in the environment, primarily associated with skin, soft tissue, and post-surgical wound infections. As an opportunistic pathogen that is known for its ability to grow within 7 days, it often causes infections after direct inoculation through trauma, surgical procedures, or contaminated water exposure. 

• Classification: Rapidly growing mycobacteria (RGM), belonging to Runyon Group IV.
• Habitat: Commonly found in soil, water, sewage, and tap water, making it a frequent cause of nosocomial (hospital-acquired) outbreaks.
• Pathogenicity: Opportunistic pathogen that causes infection in both immunocompetent and immunocompromised hosts, often after breaking the skin barrier.
• Complex: M. fortuitum complex includes M. fortuitum, M. peregrinum, and several other related species.

Morphology

• Appearance: Gram-positive (weakly), non-motile, and acid-fast rods (1–3 μm × 0.2–0.4 μm).
• Staining: Inconsistently stains with Gram stain; shows beaded or filamentous branching in tissues, which can be mistaken for Nocardia.
• Colony Characteristics: Smooth, hemispheric, off-white or cream-colored colonies that grow rapidly (within 3–4 days) on culture media like L-J medium, blood agar, and other relevant media.

Pathogenicity

• Transmission: Direct inoculation through trauma, foreign body implantation (catheters, prostheses), or contaminated medical instruments.
• Clinical Spectrum:
  • Skin and Soft Tissue: Most common (abscesses, ulcers, cellulitis, furunculosis).
  • Surgical Site Infections: Common after plastic surgery (mammoplasty) or cardiac surgery (sternal wound).
  • Pulmonary Disease: Often related to pre-existing lung disease or gastroesophageal issues (e.g., aspiration).
  • Miscellaneous: Eye infections (keratitis), catheter-related sepsis, and rare disseminated disease.
• Virulence Factors: Ability to form biofilms, which allows survival in water systems and resistance to standard disinfectants.

Laboratory Diagnosis

• Specimen: Tissue biopsy (preferred), abscess drainage, or sputum.
• Microscopy: Ziehl-Neelsen (ZN) or Fite stain for acid-fast bacilli (AFB).
• Culture: Grows on Lowenstein-Jensen (LJ) medium, Middlebrook 7H10 agar, or blood agar within 3–7 days.
• Biochemical Tests: Nitrate reduction (+), iron uptake (+), Arylsulfatase test (+ in 3 days).
• Molecular Identification: 16S rRNA gene sequencing, PCR-restriction endonuclease analysis (PRA), or Matrix-Assisted Laser Desorption/Ionization Time-of-Flight (MALDI-TOF) mass spectrometry.

Treatment

• Resistance Profile: Highly resistant to standard antituberculous agents (isoniazid, rifampin).
• Antibiotic Therapy: Usually susceptible to amikacin, cefoxitin, fluoroquinolones (ciprofloxacin, moxifloxacin), imipenem, and sulfonamides.
• Regimen: Two-drug therapy based on susceptibility testing (e.g., clarithromycin + ciprofloxacin/doxycycline) for 4–6 months minimum.
• Surgical Management: Debridement, drainage, or removal of foreign bodies (implants) is crucial.

Prevention

• Disinfection: Meticulous cleaning of surgical instruments and water systems; Mycobacterium fortuitum is resistant to chlorine.
• Water Management: Avoiding tap water for washing wounds or surgical sites; using sterile water or normal saline.
• Pedicure Safety: Proper sanitation of salon pedicure footbaths to prevent outbreaks of furunculosis.

Keynotes

• Mycobacterium fortuitum is a “rapidly growing” mycobacterium, growing in < 7 days.
• Most infections are caused by direct inoculation following injury or surgery.
• It is a common “nosocomial” pathogen and is notoriously resistant to chlorine.
• It contains an inducible erm gene, causing resistance to macrolides (e.g., erythromycin), but not always newer ones like clarithromycin.
• It is the most common cause of rapidly growing mycobacterial skin infections.
• Infections are often characterized by suppuration or mixed suppurative-granulomatous inflammation.

Further Readings

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC7526288
2. https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/mycobacterium-fortuitum#
3. https://www.hopkinsguides.com/hopkins/view/Johns_Hopkins_ABX_Guide/540365/all/Mycobacterium_fortuitum
4. https://en.wikipedia.org/wiki/Mycobacterium_fortuitum
5. https://emedicine.medscape.com/article/222918-overview
6. https://academic.oup.com/ajcp/article-pdf/116/2/225/24981305/ajcpath116-0225.pdf
7. https://emedicine.medscape.com/article/222918-treatment
8. https://www.sciencedirect.com/science/article/pii/S002432052400368020tigecycline.
9. https://jidc.org/index.php/journal/article/download/36332221/2949/161490
10. https://www.sciencedirect.com/topics/immunology-and-microbiology/mycobacterium-fortuitum
11. https://pmc.ncbi.nlm.nih.gov/articles/PMC5897959
12. https://pmc.ncbi.nlm.nih.gov/articles/PMC1785183
13. https://idosi.org/ijmr/ijmr8(3)17/1.pdf