Mycobacteroides abscessus is a rapidly growing non-tuberculous mycobacterium (NTM) recognized as an opportunistic pathogen that causes infections in humans. This organism is distantly related to the bacteria that cause tuberculosis, but M. abscessus is prevalent in the environment and typically infects individuals with pre-existing health issues. The bacterium is resistant to many common antibiotics. Treating these infections is a lengthy process, often requiring a specialized therapeutic approach.
Environmental Sources and Routes of Infection
The natural habitat of Mycobacteroides abscessus is the environment, where it is found in soil, dust, and water sources globally, including rivers, lakes, municipal water, and household plumbing. The organism often colonizes water distribution systems and persists within biofilms, making it resilient to standard disinfection methods.
Human exposure typically occurs through the inhalation of aerosolized water droplets from sources like showers or nebulizers. Another route of infection is direct inoculation, which happens when contaminated materials breach the skin barrier. This can occur during surgical procedures, traumatic wounds, cosmetic injections, or via contaminated medical devices. While most infections are acquired environmentally, certain strains have been implicated in patient-to-patient transmission, especially in healthcare settings like cystic fibrosis clinics.
Clinical Disease Manifestations
Infections caused by M. abscessus manifest primarily as pulmonary disease or skin and soft tissue infections. Pulmonary disease is the most common manifestation, affecting individuals with underlying structural lung conditions such as cystic fibrosis, bronchiectasis, or chronic obstructive pulmonary disease (COPD). Symptoms include a persistent cough, fatigue, shortness of breath, and sometimes weight loss.
The organism also causes localized skin and soft tissue infections, often following trauma or a medical procedure. These infections present as abscesses, nodules, or boils that may be red and tender. They can become chronic and typically do not respond to standard antibiotics. In rare cases, especially in immunocompromised individuals, the infection can disseminate to bones, joints, or other internal organs.
Identifying the Infection
Diagnosing M. abscessus infection is challenging because standard tests are often insufficient. Suspicion arises when routine cultures are negative or the infection fails to respond to broad-spectrum antibiotics. Mycobacteria have a waxy cell wall that resists Gram staining, requiring specialized Acid-Fast Bacilli (AFB) staining, such as the Ziehl-Neelsen method, for initial detection.
Once an AFB smear is positive, the sample is cultured. M. abscessus is characterized as a “rapidly growing mycobacterium,” forming colonies within seven days. Definitive identification requires molecular methods, such as Polymerase Chain Reaction (PCR) or gene sequencing, to confirm the exact species and subspecies. This analysis is important because different subspecies have varying natural resistance. A Drug Susceptibility Test (DST) must be performed on the isolated organism to determine effective antibiotics and guide the treatment plan.
The Complexity of Treatment
Treatment for Mycobacteroides abscessus is challenging due to the organism’s intrinsic resistance to many antimicrobials and the need for prolonged, multi-drug regimens. The standard approach requires a combination of multiple antibiotics, often three or four drugs simultaneously, guided by Drug Susceptibility Testing results. A key component of this regimen is a macrolide antibiotic, such as clarithromycin or azithromycin, as it is one of the few active oral agents.
The macrolide’s effectiveness is complicated by “inducible macrolide resistance,” present in certain subspecies of M. abscessus. The bacterium possesses a functional erm(41) gene that activates after a few days of macrolide exposure, rapidly making the drug ineffective. For this reason, the initial phase of treatment involves an intensive course of intravenous (IV) antibiotics for two to four months, often including agents such as amikacin, cefoxitin, or imipenem.
The continuation phase transitions to a combination of oral antibiotics. These must be taken for a long duration, typically a minimum of 12 months after the patient’s cultures have turned negative. This extended treatment carries a high risk of side effects, requiring frequent monitoring for toxicity. If the infection is localized and resistant to medical treatment, such as in severe pulmonary disease or a persistent skin abscess, surgical intervention to remove the infected tissue may be necessary to achieve a cure.