Mycobacterium chelonae is a bacterium belonging to the large group known as Non-Tuberculous Mycobacteria (NTM). These organisms are distinct from those causing tuberculosis and leprosy, and they commonly inhabit the environment. M. chelonae is categorized as a Rapidly Growing Mycobacterium (RGM) because it forms colonies quickly in laboratory cultures. It is considered an opportunistic pathogen, typically causing disease only in people with compromised immune systems or when introduced through a break in the skin.
Environmental Origin and Transmission
Mycobacterium chelonae is found ubiquitously in the environment, inhabiting soil, dust, and water sources globally. This organism survives in treated water systems, including municipal tap water, because it is resistant to common disinfectants like chlorine. The bacteria thrive by forming biofilms, which are dense collections of microorganisms that adhere to surfaces such as plumbing and water distribution pipes.
Human infection occurs when the bacterium is introduced into the body, primarily through two main routes. The first is traumatic inoculation, which happens when broken skin is exposed to contaminated soil or water. This can occur from minor cuts, pedicures using contaminated footbaths, or tattoos where non-sterile water is used to dilute ink.
The second major route is iatrogenic transmission, referring to healthcare-associated infections. M. chelonae contaminates medical equipment, cleaning water, and injectable solutions, leading to infections following various procedures. Infections have been reported after cosmetic injections, surgery, and from contaminated catheters or dialysis equipment.
Clinical Manifestations of Infection
The skin and soft tissues are the most common sites of M. chelonae infection. For individuals with healthy immune systems, the infection typically appears as localized skin lesions at the site of inoculation. These cutaneous infections can manifest as firm, red nodules, abscesses, or inflammation resembling cellulitis.
The lesions may progress to form pustules or draining sinus tracts and often fail to respond to standard antibiotics. In some cases, the infection presents in a sporotrichoid pattern, involving a chain of inflammatory nodules along the lymphatic vessels of an extremity. Since the organism grows best at lower temperatures, the infection tends to affect the arms and legs.
M. chelonae can also cause pulmonary disease, often mimicking other chronic lung conditions. Patients with existing conditions like chronic obstructive pulmonary disease or cystic fibrosis are more susceptible. In those with weakened immune systems, such as transplant recipients or individuals on long-term steroid therapy, the infection can become disseminated, spreading throughout the body and affecting organs like bones and joints.
Diagnostic Challenges and Treatment Protocols
Diagnosing Mycobacterium chelonae infection is challenging because symptoms can easily be mistaken for those of more common bacterial or fungal infections. The organism grows slowly compared to typical bacteria, often requiring specialized culture media and prolonged incubation periods of several days or weeks. Due to this delay, patients may undergo multiple rounds of ineffective antibiotic treatment before the correct pathogen is suspected.
Once a mycobacterium is isolated, laboratories must use specific molecular testing, such as PCR or gene sequencing, to accurately identify the species. Accurate species identification is important because it dictates the choice of treatment, which differs significantly from that of its close relatives. The next step is performing antibiotic susceptibility testing, which determines which drugs will be effective against the specific strain.
Treatment for M. chelonae involves a prolonged course of combination antibiotic therapy, lasting a minimum of four to six months for skin infections. Macrolides, such as clarithromycin, form the backbone of the drug regimen because M. chelonae is generally susceptible. Macrolide monotherapy is avoided to prevent the rapid development of drug resistance, so a second agent is often added based on susceptibility results. These additional drugs may include aminoglycosides, linezolid, or certain tetracycline derivatives. For localized skin and soft tissue infections, medical management is supplemented with surgical debridement or complete excision of the infected tissue.