Mycobacterium xenopi
Cultural characteristics
Biochemical characters
Phylum Actinobacteria, Class Actinobacteria, Order Actinomycetales, Suborder Corynebacterineae, Family Mycobacteriaceae, Genus
Mycobacterium xenopi Schwabacher 1959.
Acid-fast, long (4-15 μm) rods (shorter in lesions); beading and palisade arrangements occuring. No cord formation.
Colonies on Lowenstein-Jensen medium after incubation at 37 ºC for 14 or more
days, are smooth, non-pigmented, most becoming yellow on ageing.
Colonies on Middlebrook 7H10 agar, have compact centers, surrounded by a fringe of
microscopically branching filaments on the agar surface. Colonies become adherent
to the medium by a button-like growth into the agar.
Optimum growth temperature is 40-45 ºC. Can grow well at 50 ºC. No growth at 22 or
25 ºC. Does not grow on MacConkey agar or on media supplemented with 5% (w/v)
According to Jenkins et al.,
M. xenopi presents as a scotochromogen when grown to
maturity as single colonies.  The pigment is enhanced by growth in light and the
organism is therefore occasionally taken to be a photochromogen.
First isolated from skin granulomas of the toad Xenopus laevis. Isolated from human secretions usually without associated disease.
Susceptible to hydroxylamine (500 µg/ml), isoniazid (1 µg/ml), streptomycin (8 µg/ml), and rifampicin. Resistant to ethambutol (2
µg/ml) and tiophene-2-carboxylic acid hydrazide (1 µg/ml).
Rarely associated with chronic pulmonary disease. Rarely found in disease of the genitourinary tract.
Experimental infection: the disease was reproduced by inoculation of a massive dose (0.8 mg) of the culture into another cold-blooded
animal of the same species (
Xenopus laevis).  
Intraperitoneal inoculation in mice produces a variable response with limited numbers of macroscopic lesions appearing in the kidney,
liver, lungs, or spleen. Guinea pigs and rabbits receiving 4 mg intramuscularly develop caseous abscesses at the site of inoculation.
  1. John G. Magee and Alan C. Ward 2012. Family III. Mycobacteriaceae Chester 1897, 63AL in Bergey’s Manual of Systematic
    Bacteriology, Volume Five The Actinobacteria, Part A, Michael Goodfellow & al. (editors), 312-375.
  2. Loredana Gabriela Popa, Mircea Ioan Popa 2009. Identificarea bacililor acido-rezistenti in: Tratat de microbiologie clinica, Dumitru
    Buiuc, Marian Negut, ed. a III-a, Editura Medicala, 881-890, ISBN (13) 978-973-39-0593-6.
  3. Schwabacher H. A strain of Mycobacterium isolated from skin lesions of a cold-blooded animal, Xenopus laevis, and its relation to
    atypical acid-fast bacilli occurring in man. J Hyg (Lond) 1959; 57:57-67.
  4. Jenkins, P. A., Marks, J., Schaefer, W. B. Thin-layer  chromatography of mycobacterial lipids as an aid to  classification: the
    scotochromogenic mycobacteria including Mycobacterium scrofulaceum, M. xenopi, M. aquae, M. gordonae, M. flavescens.
    Tubercle 53:118- 127, 1972.
  5. Stanford JL, Gunthorpe WJ. A study of some fast-growing scotochromogenic mycobacteria including species descriptions of
    Mycobacterium gilvum (new species) and Mycobacterium duvalii (new species). Br J Exp Pathol 1971; 52:627-637.
  6. Attorri S, Dunbar S, Clarridge JE 3rd. Assessment of morphology for rapid presumptive identification of Mycobacterium
    tuberculosis and Mycobacterium kansasii. J Clin Microbiol. 2000;38(4):1426‐1429.
  7. Torkko P, Suomalainen S, Iivanainen E, Suutari M, Tortoli E, Paulin L, Katila ML. Mycobacterium xenopi and related organisms
    isolated from stream waters in Finland and description of Mycobacterium botniense sp. nov. Int J Syst Evol Microbiol 2000; 50:283-
  8. Stanford JL, Gunthorpe WJ. A study of some fast-growing scotochromogenic mycobacteria including species descriptions of
    Mycobacterium gilvum (new species) and Mycobacterium duvalii (new species). Br J Exp Pathol 1971; 52:627-637.
  9. Tsukamura M. Numerical identification of slowly growing mycobacteria. Microbiol Immunol. 1985;29(11):1039‐1050. doi:10.1111/j.
Positive results for arylsulfatase (10 days; variable at 3 days), acid phosphatase, catalase (inactivated at 68 ºC), alpha- and
beta-esterase, nicotinamidase, pyrazinamidase, and tellurite reduction (most strains).

Negative results for citrate utilization, semiquantitative catalase test, beta-galactosidase, nitrate reduction, niacin production, Tween
80 hydrolysis, and urea hydrolysis.
No utilization as sole carbon source of acetate, citrate, succinate, malate, pyruvate, benzoate, fumarate, glucose, fructose, sucrose
ethanol, and propanol.
(c) Costin Stoica
Culture media
Biochemical tests
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