• Corynebacteria belong in the family Mycobacteriaceae and are part of the CMN group (Corynebacteria, Mycobacteria and Nocardia).
• The family Mycobacteriaceae are Gram-positive, nonmotile, catalase-positive and have a rodlike to filamentous morphology (Corynebacteria are often pleomorphic).
• As a group, they produce characteristic long chain fatty acids termed mycolic acids. In the image to the right, the R-groups represent these chains. For Corynebacteria, chains of 28-40 carbons are common; for Nocardia, chains of 40-56 carbons are produced; for Mycobacteria, the chains are 60-90 carbons in length.

   

  • Corynebacterial cell walls contain thin spots which leads to some Gram variability and "ballooning" that produces a "club-shaped" cell. Old cells store inorganic phosphate, which can appear as metachromatic granules when stained.
  • C. diphtheriae is the etiologic agent of diphtheria.
  • These organisms colonize the mucus membranes of the respiratory tract and produce the enzyme neuraminidase which splits N-acetylneuraminic acid (NAN) from cell surfaces to produce pyruvate which acts as a growth stimulant.
  • C. diphtheriae also produces diphthin, which is a protease that inactivates IgA.
  • Toxigenic strains carry the gene tox, which resides on certain bacteriophages; lysogenization leads to toxigenicity.
  • The toxin that is produced is a single polypeptide of 62,000 daltons and contains a single disulfide cross-link. Digestion with trypsin gives 2 fragments, A and B. The B (binding) fragment attaches to cell surfaces then proteases release the A (active) fragment to enter the cell. In the cell, the toxin acts as an ADP-ribosyltransferase, inactivating translation factor EF2.

  Humoral immunity (antitoxin) is important in preventing disease.

• Diphtheria exists throughout the world and occasional outbreaks occur almost yearly.
• The Schick test can be used to ascertain population risk. This test involves the injection of a minute amount of the diphtheria toxin under the skin. The absence of a reaction indicates immunity.

Clinical: Muscle weakness, edema and a pseudomembranous material in the upper respiratory tract characterizes diphtheria.
Laboratory: Tellurite media is the agar of choice for isolation of Corynebacteria, which produce jet black colonies.

Sanitary: Reduce carrier rate by use of vaccine.
Immunological: A vaccine (DPT) prepared from an alkaline formaldehyde inactivated toxin (i.e. toxoid) is required. Passive immunization with antitoxin can be used for patients.
Chemotherapeutic: Penicillin, erythromycin or gentamicin are drugs of choice.

MAC is an opportunistic infection caused by two similar types of bacteria named Mycobacterium avium and Mycobacterium intracellulare. Because these bacteria are so similar, they are referred to together as "MAC," which stands for "Mycobacterium avium complex" (the disease or infection they cause is also commonly referred to as "MAC"). It has been determined, however, that Mycobacterium avium is the predominant organism seen in most MAC infections in people with AIDS.

The MAC organisms are a specific type of bacteria known as "mycobacteria." You may already be familiar with another type of mycobacteria, called Mycobacterium tuberculosis, which (as the name implies) is responsible for causing tuberculosis.