By Gary M. White, MD

Atypical mycobacterial infection--AFB Surfer with biopsy-proven AFB (acid-fast bacilli) infection.

Nontuberculous mycobacteria are called atypical mycobacteria. In the laboratory, once stained, these organisms resist acid and/or ethanol-based decolorization procedures, hence the name acid-fast. Although they are quite prevalent in nature, infection is relatively rare. Immunocompromised hosts may develop systemic infection whereas infection in a normal host is usually localized. The organisms that are most commonly encountered in clinical practice, Mycobacterium avium, M. intracellulare, M. kansasii, M. fortuitum, M. abscessus, and M. chelonae, are frequently found in water sources and soil.

The most common source of infection is a fish tank or swimming pool (thus the term swimming pool granuloma or fish tank granuloma) caused by the organism Mycobacterium marinum. Patients often don't associate hand trauma from an aquarium to their skin lesion. Repeated, direct questioning when faced with an indolent, nodular, inflammatory lesion of the hand is in order.

Some families with a marked tendency to infection with mycobacteria have been identified as having a genetic defect in the receptor for interferon gamma.

See also AFB infection of tattoos.


Perhaps the most common presentation is the inflammatory nodule on the extremity that spreads proximally along the lymphatics. Local trauma is often the source of inoculation and thus, the dorsa of the hand--especially about the knuckles--is a common site. Other presentations are possible including a solitary verrucous or smooth, inflammatory nodule or multiple, draining abscesses.

Hydrotherapy foot baths at nail salons can be a source of mycobacterial infections of the lower leg. Atypical mycobacterium infections should be suspected in patients (especially women) with persistent boils on the lower legs and recent exposure to hydrotherapy foot baths.


A biopsy should be sent for microscopic examination and culture. The pathologist should be alerted to the suspected diagnosis so that special stains may be done. Similarly, the laboratory should be alerted so that the proper culturing media, etc., is used. For culture, it is best to send not a swab, but instead as much tissue as possible submitted in 2-3 ml sterile saline in a sterile leak-proof container.


Standard antibiotics based upon culture should be used. Many times however, a presumptive diagnosis is made and therapy is initiated prior to the culture results being available. Typical antibiotics used include minocycline 100 mg BID, trimethoprim/sulfamethoxazole, isoniazid, rifampin, kanamycin, clarithromycin, and cefoxitin. Therapy takes many months and should be stopped 1-2 months after clinical resolution of the lesions occurs. Six months of therapy is typical, but some have required 1-2 years. An infectious disease specialist can help with therapy.

The combination of linezoid and clarithromycin has been used with great benefit in immunocompromised patients with M. chelonae skin infection [JEADV 2016;30;101].

Additional Pictures

Fish tank granuloma on thumb and sporotrichoid spread of nodules up arm.
Atypical mycobacterial infection--Sporotrichoid spread Atypical mycobacterial infection--Sporotrichoid spread

Atypical mycobacterial infection--AFB Atypical mycobacterial infection--AFB

AFB infection from contact with a crawfish.
Atypical mycobacterial infection--AFB

Primary lesion on the knuckle with secondary lesion on the arm.
Atypical mycobacterial infection--AFB

AFB infection after pedicure. Contaminated water splashes on the skin causing an inflammatory nodule to develop on the shin.
Atypical mycobacterial infection--AFB


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