August 2021 – Clinical Microbiology and Infectious Disease Pathology

A 60-year old veterinarian presented with 4-week history of multiple nodular lesions on the left hand (Image 1). He had no history of travel, gardening, or any immunocompromising condition. He owned a Koi pond and recalled sustaining minor injuries from the spruce needs occasionally while cleaning the pond. Patient underwent biopsy and cultures were obtained. Mycobacterial cultures incubated at 30 degrees Celsius grew yellow pigmented colonies on exposure to light after 14 days of incubation (Image 2). No growth was seen at 37 degrees Celsius. Ziehl-Nielson stain was positive for acid-fast bacillus (Image 3). QuantiFERON was performed and was weakly positive as well. 

Image 1: Nodular lesions on dorsal left hand
Image 2: Yellow colonies on Middlebrook 7H10 media
Image 3: Ziehl-Nielson stain showing acid fast bacilli

Which of the following is the most likely organism identified in this case?

  • Mycobacterium marinum
  • Mycobacterium tuberculosis
  • Mycobacterium leprae
  • Mycobacterium kansasii

The correct answer is ...

The correct answer is: Mycobacterium marinum.

The history of injuries related to koi ponds, preferable growth at 30 degrees Celsius, yellow pigmentation on exposure to light and weakly positive QuantiFERON makes Mycobacterium marinum the most likely underlying pathogen in this case. Even though infection with M. tuberculosis typically results in a positive QuantiFERON, the clinical presentation and growth characteristics are not typical of M. tuberculosis. Growth of M. tuberculosis is typically seen at 37 degrees Celsius with dry, rough non-chromogenic colonies. M. leprae can cause cutaneous findings; however, it cannot be grown in the routine culture media. M. kansasii also produces yellow pigmentation on exposure to light and can cause false positive QuantiFERON; however, growth is preferably seen at 37 degrees Celsius rather than at 30 degrees Celsius, and pulmonary disease is most commonly seen.

M. marinum was first isolated from fish in 1926 and was recognized as a human pathogen in 1951 (1). Infection with M. marinum is relatively uncommon. It is also known as fish tank granuloma and typical exposure leading to infection includes aquarium-related fish or shellfish injuries. Diagnosis requires a high degree of clinical suspicion. AFB smear may be negative in more than two-thirds of patients and hence does not rule out infection, and cultures remain the gold standard for diagnosis (2). It takes about 1-2 weeks to grow in the culture media with optimal growth temperature of 28-32 degrees Celsius, and rare-to-no-growth at 37 degrees Celsius. Colonies produce a yellow pigment after exposure to light (photochromogen).

M. marinum can cause false positive QuantiFERON test. QuantiFERON test is based on the production of interferon gamma in response to early secretory antigen-6 (ESAT-6) and culture filtrate protein (CFP-10) of M. tuberculosis. ESAT-6 and CFP-1 are also found in M. marinum, which may result in false positive QuantiFERON. Other non-tuberculous mycobacteria that may cause false positive QuantiFERON include M. kansasii and M. szulgai.

The treatment for M. marinum is not well defined and is based on case reports. Usually it is susceptible to rifampin, ethambutol, clarithromycin, trimethoprim-sulfamethoxazole, and doxycycline. Isolates are usually resistant to isoniazid and pyrazinamide. General approach is to treat with at least two agents for 4-8 weeks after the resolution of symptoms, with total treatment duration of about 3-4 months (3).


  1. Nordén A, Linell F.1951. A new type of pathogenic mycobacterium. Nature. 168:826.
  2. Castillo NE, Gurram P, Sohail MR, Fida M, Abu Saleh O. Fishing for a Diagnosis, the Impact of Delayed Diagnosis on the Course of Mycobacterium marinum Infection: 21 Years of Experience at a Tertiary Care Hospital. Open Forum Infect Dis. 2020 Jan 4;7(1):ofz550. doi: 10.1093/ofid/ofz550. PMID: 31988976; PMCID: PMC6975249.
  3. Griffith, D. E., et al. (2007). "An official ATS/IDSA statement: diagnosis, treatment, and prevention of nontuberculous mycobacterial diseases." Am J Respir Crit Care Med 175(4): 367-416.

Madiha Fida, M.B.B.S.

Fellow, Clinical Microbiology
Mayo Clinic

Nancy Wengenack, Ph.D.

Consultant, Clinical Microbiology
Mayo Clinic
Assistant Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

MCL Education

This post was developed by our Education and Technical Publications Team.