*Mycobacterium abscessus*

*Mycobacterium abscessus*
Name	Mycobacterium abscessus
Domain	Bacteria
Phylum	Actinomycetota
Class	Actinomycetia
Order	Mycobacteriales
Family	Mycobacteriaceae
Genus	Mycobacterium
Species	M. abscessus
Binomial	Mycobacterium abscessus
Binomial authority	(Runyon 1965)

*Mycobacterium abscessus* is a rapidly growing, multidrug-resistant nontuberculous mycobacteria that is a significant cause of human disease, particularly in individuals with underlying lung conditions or compromised immune systems. It is notorious for its intrinsic resistance to many standard antibiotics, making infections difficult to treat and a major concern in healthcare settings. The bacterium is ubiquitous in the environment, found in water, soil, and dust, and is recognized as an emerging pathogen linked to healthcare-associated outbreaks.

Microbiology and taxonomy

Originally classified within the Mycobacterium chelonae group, it was redefined as a distinct species based on genetic sequencing, including analysis of the 16S rRNA gene and the hsp65 gene. The species is further divided into three subspecies: M. abscessus subsp. abscessus, M. abscessus subsp. massiliense, and M. abscessus subsp. bolletii, with distinctions important for predicting antibiotic susceptibility. As a rapidly growing mycobacterium, it forms visible colonies on culture media like Lowenstein-Jensen medium within seven days. Its cell wall, rich in mycolic acid, shares characteristics with other mycobacteria like Mycobacterium tuberculosis but lacks the cord factor associated with virulence in that pathogen.

Pathogenesis and clinical significance

It causes a spectrum of diseases, primarily chronic pulmonary infections resembling those of Mycobacterium avium complex in patients with pre-existing lung diseases such as cystic fibrosis or bronchiectasis. It is also a major cause of soft tissue and skin infections, often following traumatic injury, surgery, or invasive procedures like liposuction or mesotherapy. In healthcare settings, outbreaks have been traced to contaminated medical equipment, including heater-cooler units used in cardiac surgery and inadequately sterilized endoscopic devices. Disseminated disease is rare but can occur in severely immunocompromised hosts, such as those undergoing treatment for hematological malignancy or recipients of organ transplant.

Diagnosis

Diagnosis requires a combination of clinical suspicion, radiographic findings, and microbiological confirmation. Sputum samples or tissue biopsies are cultured on selective media like Middlebrook 7H10 agar. Identification to the species and subspecies level is crucial and is achieved through molecular techniques such as polymerase chain reaction and DNA sequencing of specific genetic targets, as biochemical tests are unreliable. The American Thoracic Society and the Infectious Diseases Society of America provide guidelines for diagnosing pulmonary disease, which require positive cultures from at least two separate expectorated sputum samples or one bronchial wash.

Treatment and antibiotic resistance

Treatment is prolonged, complex, and often poorly tolerated due to the bacterium's extensive intrinsic and acquired resistance. It is inherently resistant to standard anti-tuberculosis drugs like isoniazid and rifampin. Therapeutic regimens typically involve a multidrug combination including a macrolide antibiotic like clarithromycin, provided the isolate is susceptible, along with an intravenous agent such as amikacin and other drugs like tigecycline or imipenem. Resistance is mediated by a variety of mechanisms, including the presence of the erm(41) gene, which can confer inducible macrolide resistance, and mutations in the rrs gene conferring aminoglycoside resistance. The subspecies M. abscessus subsp. massiliense often lacks a functional erm(41) gene, making it more susceptible to macrolides.

Epidemiology

It is found globally in water and soil ecosystems. Pulmonary disease is most commonly reported in the United States, Australia, and Europe, particularly among patients with cystic fibrosis. Healthcare-associated outbreaks have been documented worldwide, including notable incidents linked to Brazilian plastic surgery clinics and hospitals in the United Kingdom and the United States. There is no evidence of person-to-person transmission; infection is acquired from environmental sources or contaminated medical procedures.

Research and public health implications

Current research focuses on understanding its virulence factors, such as biofilm formation and the role of the type VII secretion system, and developing novel therapeutics. The National Institute of Allergy and Infectious Diseases supports research into new drug candidates. Public health efforts are directed at preventing healthcare-associated infections through stringent water quality control in hospitals, proper sterilization of surgical instruments, and adherence to guidelines from bodies like the Centers for Disease Control and Prevention. Its role as an emerging pathogen in cystic fibrosis has led to increased screening and infection control measures in specialized care centers like the Cystic Fibrosis Foundation-accredited network.

Category:Mycobacteria Category:Antibiotic-resistant bacteria