Black Pigmented Anaerobes

Anaerobic bacteria are the special class of bacteria that thrive in environments devoid of oxygen. Anaerobic bacteria, both Gram-positive and Gram-negative cocci and rods, make up a large portion of the natural human flora.

Large populations of anaerobic bacteria thrive in low-oxygen location in humans, such as the human colon, gingival crevices, tonsillar crypts, nasal folds, hair follicles, the urethra, and the vagina. Because of their nature, anaerobes proliferate readily in dead tissue. The infected tissues typically contain a variety of different anaerobes, as well as facultative and often aerobic bacteria. Multiplication of these aerobic or facultative organisms in association with anaerobes in infected tissue lowers oxygen content further and creates an environment that encourages the growth of anaerobic bacteria.

Anaerobic bacteria can cause infections in any area of the body. The indigenous anaerobic flora could become pathogenic if displaced from their normal habitat. Therefore, majority of anaerobic illnesses are acquired endogenously. 

Anaerobes produces black-pigmented colonies on blood agar using the protoheme found in erythrocytes. These black-pigmented Gram-negative anaerobic rods are present as native flora on mucosal surfaces. These microorganisms may enter tissues and cause infection if the mucosa are damaged. They predominate in infections involving organisms that originate in the oropharynx, including infections of the central nervous system, the head and neck, the teeth and the pleuropulmonary tract. Pigmented anaerobes are too associated with life threatening infections such as bacteraemia, endocarditis, intracranial abscess, necrotizing pneumonia and necrotizing fasciitis. Surgery along with antibiotics is the treatment for infections caused by pigmented anaerobes. Furthermore, many of these bacteria produce beta-lactamase, which necessitates even more rigorous therapy.

There is still much to be done to ascertain the relative significance of the many taxa of Gram-negative anaerobes with black pigmentation and the numerous virulence factors that they produce. Scientists can delve extensively into the wide variety of anaerobes to gain a deeper understanding of them.

References:

1.T.J.M. van Steenbergen, A.J. van Winkelhoff, J. de Graaff. Classification and typing methods of black-pigmented Gram-negative anaerobes. FEMS Immunology and Medical Microbiology 1993;6: 83-88.

2.Finegold SM. Anaerobic Gram-Negative Bacilli. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 20. Available from: https://www.ncbi.nlm.nih.gov/books/NBK8438/

3.Finegold SM, Strong CA, McTeague M, Marina M. The importance of black-pigmented gram-negative anaerobes in human infections. FEMS Immunol Med Microbiol. 1993 Mar;6(2-3):77-82. 

4.Duerden, B.I. (1980) The identification of Gram-negative anaerobic bacilli isolated from clinical infections J. Hyg., 84, 301–313.