Mycobacteria – Ongoing Interest in an Old Pathogen

By: Jvo Siegrist, AnalytiX Volume 8 Article 5

The genus Mycobacterium is known and dreaded as the causative agent of serious diseases like tuberculosis (M. tuberculosis) and leprosy (M. leprae)

Jvo Siegrist, Product Manager Microbiology ivo.siegrist@sial.com   
Markus Auly Product Management Assistant

Mycobacterium avium complex (MAC) infection has gained notoriety recently as a significant cause of death in AIDS patients. After a period where Mycobacteriumrelated diseases were considered to be eradicated – at least in countries with high medical standards – the occurrence of multiresistent strains and a worrisome number of problematic infections in immunocompromised individuals have generated a new interest in research on this genus.

Mycobacteria are aerobic, often microaerophilic, and generally nonmotile bacteria that are characteristically acid-alcohol fast 1. This is due to their distinctive hydrophobic cell wall, comprised of a thick layer of mycolic acid and outer lipids in addition to the normal peptidoglycan, which gives them considerable protection against acids, alkali and certain antibiotics that attack bacterial cell walls. Mycobacteria are classified acid-fast Gram-positive (because they lack an outer cell membrane), although they do not retain the crystal violet stain as typical Gram-positive bacteria do. Many mycobacteria can survive and grow in nutritionally poor environments such as water puddles and even chlorinated tap water. Other species like M. leprae are difficult to cultivate and seem to be obligate parasites.

Mycobacteria’s exceptional hardiness and low nutritional demands are the principles of their isolation on such media as the Gruft-modified Loewenstein-Jensen medium (see Table 1). The supplemented antibiotics are intended to eliminate all Gram-negative and normal Gram-positive germs and spare only the more resistant Mycobacteria. Appropriate staining methods include the procedures according to Ziel-Neelson or Kinyoun as well as the auramine fluorochrome method, all of which are available from Sigma-Aldrich (see Table 2). The auramine fluorochrome is a specific stain for Acid Fast Bacilli (Mycobacterium sp.) in specimens and in culture. This fluorescent method, which is actually considered the best procedure, stains mycobacteria selectively by binding dye to themycolic acid of the cell wall. The differentiation of the numerous species and subspecies has in the past been based on a variety of physiological tests2, but molecular biological methods are gaining in importance3.

Figure 1 Mycobacterium (Scanning electron microscope image; Photo Mazen T. Saleh, Laurentian University)

Figure 1 Mycobacterium (Scanning electron microscope image; Photo Mazen T. Saleh, Laurentian University)

 

For more details about our products for analytical microbiology, please visit sigma-aldrich.com/microbiology

 

Cat. no. Media & supplements
63237 TB-Medium Base according to Loewenstein-Jensen
51803 Gruft Mycobacterial Supplement
M0178 Middlebrook 7H9 Broth Base
M0303 Middlebrook 7H10 Broth Base
M0428 Middlebrook 7H11 Broth Base

Table 1 Media for detection, isolation, differentiation of mycobacteria

 

Cat. no. Media & supplements
21820 Carbol-Fuchsin solution according to Ziehl-Neelsen
21819 Carbol-Fuchsin solution according to Kinyoun
05151 Fluorescent Stain Kit for Mycobacteria
56694 Acid Alcohol solution
30503 Phenolic auramine solution

Table 2 Products for staining of mycobacteria

Materials

     

References

  1. Ryan, K. J.; Ray, C. G., eds. Sherris Medical Microbiology, 4th ed.; McGraw Hill: New York, 2004.
  2. Koneman, E. W.; Allen, S. D.; Janda, W. M.; Schreckenberger, P. C.;Winn, W. C., Jr. Diagnostic Microbiology, 5th ed., Lipincott Williams & Wilkins: Philadelphia, 1997.
  3. Parish, T. Making Sense of Mycobacteria. In Mycobacteria: Molecular Biology and Virulence; Ratledge, C., Dale, J., Eds.; Trends in Microbiology, 2000, 8 (5), p 245.

 

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