Mycobacterium fortuitum is a type of bacterium belonging to the Non-Tuberculous Mycobacteria (NTM) group, distinct from the species that causes tuberculosis. These NTM are widespread environmental organisms that can cause opportunistic infections in humans. M. fortuitum is classified as a Rapidly Growing Mycobacterium (RGM), a designation that distinguishes it from slower-growing mycobacterial species. This bacterium has become increasingly recognized in clinical settings, especially as a cause of skin and soft tissue infections.
Characteristics of the Organism
Mycobacterium fortuitum is categorized within the genus Mycobacterium and is a member of the Rapidly Growing Mycobacteria group. The term “rapidly growing” is a clinical definition, meaning the organism can form visible colonies in laboratory culture within seven days, which is much faster than other mycobacteria. Structurally, it is a non-motile, rod-shaped bacterium that possesses a distinctive, waxy cell wall rich in mycolic acids. This unique cell wall structure makes the bacterium naturally resistant to many common disinfectants and antibiotics. A particular challenge in treatment is the potential for inducible macrolide resistance, which is mediated by the erm gene. This gene can be dormant but is activated by exposure to certain drugs, leading to treatment failure.
Environmental Sources and Acquisition
The natural reservoir of M. fortuitum is the environment, found ubiquitously in soil, dust, and natural water sources across the globe. This organism can survive in complex water systems, including municipal water supplies, hospital water fixtures, and treated tap water, due to its resistance to common disinfectants like chlorine. Its ability to form biofilms also allows it to persist on surfaces and within plumbing.
Human infection most often occurs when the organism is directly introduced beneath the skin barrier through traumatic inoculation. This can happen through simple puncture wounds, but more commonly through practices like tattooing, acupuncture, or cosmetic procedures such as pedicures involving contaminated footbaths.
A particularly concerning route of transmission is iatrogenic infection, meaning the infection is caused by medical activity. Outbreaks have been linked to contamination during surgical procedures, especially those involving implants or prosthetics. Infections are also seen following the use of contaminated medical devices, such as catheters, or from abscesses developing after injections. The bacterium’s resistance to standard sterilization procedures contributes to its ability to cause these nosocomial, or hospital-acquired, infections.
Types of Infection Caused
M. fortuitum causes a wide spectrum of diseases, with the most frequent presentation being localized skin and soft tissue infections. These typically manifest as persistent, non-healing wounds, abscesses, or nodules at the site of inoculation, such as a surgical incision or a traumatic injury. These localized infections are slow to develop and often present a diagnostic challenge because they do not respond to standard antibiotic therapy.
The bacterium is also known to cause pulmonary disease, though this is less common than skin infections and often occurs in individuals with pre-existing lung conditions. Patients with conditions like chronic obstructive pulmonary disease (COPD) or bronchiectasis are particularly susceptible. Symptoms can mimic those of tuberculosis or other NTM lung diseases, including a persistent cough, sputum production, and sometimes coughing up blood.
Less frequently, M. fortuitum can cause deeper, more severe infections, especially in individuals with compromised immune systems. This can include osteomyelitis (infection of the bone), often following a deep puncture wound or orthopedic surgery. Other serious but rare conditions include keratitis (infection of the cornea) following eye trauma or surgery, and disseminated disease, usually limited to severely immunocompromised patients.
Diagnosis and Management
Suspicion of an M. fortuitum infection often arises when a localized wound or pulmonary condition fails to improve despite weeks of broad-spectrum antibiotic treatment. Definitive diagnosis requires the isolation of the organism in a specialized laboratory setting. Tissue samples or respiratory specimens must be cultured on specific media intended for mycobacteria.
Since M. fortuitum is a rapidly growing species, molecular testing, such as Polymerase Chain Reaction (PCR) or gene sequencing, is often necessary to confirm the exact species and differentiate it from other RGM. Accurate identification is paramount because treatment protocols vary significantly.
A mandatory step in management is Drug Susceptibility Testing (DST), which determines which specific antibiotics are effective. Due to the organism’s intrinsic resistance, treatment nearly always involves a combination of two or more agents to prevent further resistance. Common effective drugs include aminoglycosides like amikacin, certain macrolides, and fluoroquinolones.
Management requires a combined medical and surgical approach, especially for localized infections. Surgical debridement (removal of infected tissue) is frequently necessary to reduce the bacterial load. If the infection is associated with a foreign body, such as an implant or prosthetic joint, its removal is usually necessary for a complete cure. Treatment duration is typically prolonged, often lasting several months.