Mycobacterium fortuitum : Introduction, Morphology, Pathogenicity

Introduction

Table of Contents

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.

Fig. Mycobacterium fortuitum growth on modified Middlebrook 7H9 broth with PANTA supplement after 3 days of incubation

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.

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