General Characteristics of the Genus
Members of the genus Corynebacterium are slender, straight, or slightly curved rods. Their structural feature is polymorphism ("irregular shape") and thickened, club-shaped, or pointed ends.
In smears, cells are arranged singly, in V-shaped pairs at an angle, or in "palisade" stacks. The bacteria stain unevenly (resembling beads) due to intracellular accumulations of polymetaphosphate—metachromatic granules known as volutin grains.
Physiologically, they are facultative anaerobes with a fermentative metabolism. They are non-motile, non-acid-fast, and do not form spores. For cultivation, most species require rich nutrient media containing serum or blood.
Ecology and Classification
The genus Corynebacterium includes more than 60 species. Bacteria are ubiquitous in the environment (plants, water, soil) and in humans and animals.
In the human body, they are part of the normal microbiota: colonizing the skin, upper respiratory tract, gastrointestinal tract, and urogenital mucous membranes. Most species act as opportunistic microorganisms (commensals), however, the type species—Corynebacterium diphtheriae—is a strict pathogen and causes diphtheria.
Opportunistic Corynebacteria (Diphtheroids)
Diphtheroids (coryneform bacteria) are morphologically and culturally very similar to true diphtheria agents. Related genera (Arthrobacter, Cellulomonas, etc.) are also included in this group. Their role in human pathology is not fully understood, but many are capable of causing opportunistic infections under conditions of immunosuppression.
Significant species of diphtheroids:
- Corynebacterium pseudodiphtheriticum: A commensal found in the pharynx of 70% of healthy individuals. Characterized by short bacilli arranged in palisades, containing almost no volutin granules. An important diagnostic feature is the presence of urease.
- Corynebacterium xerosis: Normal microbiota of the skin and mucous membranes (eyes, nose). Cells are barrel-shaped. Lipophilic strains exist that require lipids for growth.
- Corynebacterium ulcerans: Zoonosis (often from cattle, transmitted via raw milk). Can cause diphtheria-like lesions. Some strains produce toxin.
- Corynebacterium jeikeium: Part of the skin flora, more common in men due to a requirement for fatty acids. Can cause wound infections, pneumonia, and endocarditis.
- Corynebacterium cystitidis: Inhabits the perineum. By breaking down urea, it raises urine pH, promoting stone formation and cystitis.
- Corynebacterium minutissimum: The causative agent of erythrasma—a skin condition (reddish-brown rash in major folds). Under Wood's lamp illumination, lesions show a coral-red fluorescence.
Arcanobacterium haemolyticum
Previously, this microorganism was classified under corynebacteria (C. haemolyticum) due to its close morphological and tinctorial similarity. It is a facultative anaerobe.
It is transmitted via airborne droplets and causes chronic tonsillitis, skin lesions, septicemia, and osteomyelitis. On blood agar, it forms transparent small colonies with a clear zone of complete hemolysis. It grows slowly.