General characteristics of the genus
Representatives of the genus Corynebacterium are thin, straight, or slightly curved rods. Their structural feature is pleomorphism ("irregular shape") and thickened, club-shaped, or pointed ends.
In smears, cells are arranged singly, in pairs at an angle (V-shape), or in stacks resembling a "picket fence" (palisade). Bacteria stain unevenly (resembling rosary beads) due to the presence of intracellular clusters of polymetaphosphate—metachromatic granules known as volutin granules.
Physiologically, they are facultative anaerobes with a fermentative type of metabolism. They are non-motile, non-acid-fast, and non-spore-forming. For cultivation, most species require rich nutrient media containing serum or blood.
Ecology and classification
The genus Corynebacterium includes over 60 species. Bacteria are ubiquitous in the environment (plants, water, soil) and in human and animal organisms.
In the human body, they are part of the normal microflora: they colonize the skin, upper respiratory tract, gastrointestinal tract, and mucous membranes of the urogenital system. 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 the true diphtheria agent. Related genera (Arthrobacter, Cellulomonas, etc.) are also included in this group. Their role in human pathology is not fully understood, but many of them are capable of causing opportunistic infections in immunosuppression or certain conditions.
Significant species of diphtheroids:
- Corynebacterium pseudodiphtheriticum (false diphtheria bacillus): Commensal, detected in the pharynx in 70% of healthy individuals. Characterized by short rods arranged in a "palisade," contains almost no volutin granules. An important diagnostic sign is the presence of urease.
- Corynebacterium xerosis: Normal microflora 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). Capable of causing diphtheria-like lesions. Some strains secrete 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 decomposing urea, it increases urine pH, which promotes stone formation and cystitis.
- Corynebacterium minutissimum: Causative agent of erythrasma—a skin lesion (reddish-brown rash in major folds). Under Wood's lamp illumination, the lesions emit a coral-red fluorescence.
Arcanobacterium haemolyticum
Previously, this microorganism was classified among corynebacteria (C. haemolyticum), as it has great morphological and staining similarities with them. It is a facultative anaerobe.
It is transmitted by airborne droplets and causes chronic tonsillitis, skin lesions, septicemia, and osteomyelitis. On blood agar, it forms transparent, small colonies with a distinct zone of complete hemolysis. It grows slowly.
Briefly: answers to topical questions
What elective nutrient media are used for the cultivation of Corynebacterium diphtheriae?
For the cultivation of Corynebacterium diphtheriae, special serum and blood media are used, as the pathogen is fastidious regarding nutrient substrates.
- Roux medium is an elective medium consisting of coagulated horse serum.
- Klauber medium II is a differential diagnostic blood-tellurite medium.
- Tinsdale medium — serum agar with potassium tellurite and cystine.
The addition of potassium tellurite inhibits the growth of concomitant microflora, to which the diphtheria pathogen is resistant.
What is the mechanism of action of diphtheria histotoxin on the target cell?
The mechanism of action of diphtheria histotoxin consists of the irreversible disruption of translation at ribosomes, leading to cell death. The toxin consists of two fragments:
- B-fragment — binds to target cell receptors and forms a transport channel for penetration.
- A-fragment — penetrates the cytosol and exhibits enzymatic activity.
The toxin acts as an ADP-ribosyltransferase: the A-fragment transfers ADP-ribose to elongation factor 2 (EF-2), blocking its function and completely halting protein synthesis.
What pathogenicity factors does Corynebacterium diphtheriae have besides exotoxin?
In addition to diphtheria exotoxin, Corynebacterium diphtheriae possesses a number of other pathogenicity factors that promote adhesion, colonization, and invasion.
- Fimbriae — ensure attachment and reproduction of bacteria on the epithelium of mucous membranes.
- Microcapsule — contains cord factor, which is an important virulence factor.
- Aggression enzymes — include lipase, protease, neuraminidase, hyaluronidase, and phosphatase, which facilitate the invasion of the pathogen into host tissues.
What staining methods are used to detect volutin granules in corynebacteria?
To detect metachromatic polyphosphate granules (volutin granules) in corynebacteria, special staining and microscopy methods are used.
- Neisser staining — a specific method in which granules stain red-purple and the cytoplasm stains blue.
- Loeffler staining — a method using methylene blue.
- Fluorescent microscopy — performed using the fluorochrome coriphosphine.
Toluidine blue or methylene blue can also be used to detect the metachromasia phenomenon.
Class and exam questions
What features distinguish C. pseudodiphtheriticum from the diphtheria agent?
The cells of the false diphtheria bacillus are small, uniform in size, arranged in a 'palisade' pattern, and practically devoid of volutin granules. In addition, this species produces urease, which serves as an important biochemical marker.
What disease does C. minutissimum cause?
This species causes erythrasma—a skin disease characterized by reddish-brown rashes in the axillary and inguinal folds that glow coral-red under Wood's lamp rays.
How does C. cystitidis promote kidney stone formation?
This microorganism decomposes urea, leading to alkalinization (an increase in pH) of the urine. The change in acidity creates favorable conditions for the formation of crystals and urinary stones.
What else to study on this topic
- Detailed microbiological and biochemical characteristics of Corynebacterium diphtheriae
- Mechanism of action of diphtheria toxin
- Specific prophylaxis and treatment of diphtheria
- Growth features of lipophilic corynebacterium strains on nutrient media
- The role of metachromatic granules in bacterial metabolism
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