General Characteristics
Basic features
General characteristics of Corynebacterium species include:
- Gram-positive, club-shaped bacilli (coryne = club shaped)
- Non-motile
- Non-spore forming
- Non-encapsulated
- Aerobic or facultatively anaerobic
- Contain metachromatic (volutin) granules
Micrograph of Corynebacterium diphtheriae
Image: “12163” by the CDC/Graham Heid. License: Public domain.Growth and biochemical properties
- Catalase positive (split hydrogen peroxide into water and oxygen)
- Oxidase negative, indole negative
- Ferment glucose, starch, and glycogen
- Grow well at 37°C (98.6°F) on blood or serum-containing media:
- Hoyle’s/Tinsdale’s agar (tellurite medium; C. diphtheriae forms black colonies)
- Loeffler’s medium (enhances development of metachromatic granules)
Corynebacterium ulcerans colonies on a blood agar plate
Image: “Corynebacterium ulcerans 01” by the CDC/Dr. W.A. Clark. License: Public domain.
Corynebacterium diphtheriae on Tinsdale’s agar plate
Image: “16561” by the CDC. License: Public domain.
Clinically relevant species
- C. diphtheriae:
- Major pathogenic species
- Causes diphtheria (pharyngitis and cutaneous forms)
- Non-diphtherial Corynebacterium (diphtheroids):
- Mostly contaminants
- May be pathogenic, especially in immunocompromised hosts
- C. minutissimum:
- Normal skin inhabitant
- Can cause erythrasma: superficial infection of intertriginous areas
- Many species cause diseases in domestic animals.
Corynebacterium diphtheriae
Epidemiology
- In immunized individuals, the rate is low: < 5 per 100,000.
- Remains endemic in certain parts of the world (Brazil, Nigeria, India, certain parts of the former Soviet Union)
- No racial or sex predilection
- Can affect children and adults
Transmission
- Humans are the only known reservoir.
- Airborne droplets
- Direct contact with respiratory secretions
- Contact with skin lesions
- Can be transmitted by immunized and asymptomatic carriers
Virulence factors
- Not an invasive organism
- Pathogenicity is based on diphtheria toxin (exotoxin):
- Only produced if lysogenized by a β-bacteriophage
- Has 2 subunits:
- A—active
- B—binding
- B binds to the cell membrane and A enters the cell.
- Ribosylation of elongation factor (EF-2)
- Inhibits protein synthesis
- Causes cell death
Clinical presentation
Diphtheria (respiratory):
- Sore throat, malaise, cervical lymphadenopathy, low-grade fever
- Pharyngeal erythema with spots of white and gray exudate
- Can also have nasal, laryngeal, and tracheobronchial forms
- Pseudomembranes:
- Form in ⅓ of cases
- Composed of fibrin, necrotic debris, inflammatory cells, and organisms
- Tightly adherent to underlying tissue; bleeds with scraping
- Can spread to any portion of the respiratory tract
- “Malignant diphtheria”:
- Associated with extensive pseudomembranes (“pseudomembranous pharyngitis”)
- Severe “bull’s neck” lymphadenopathy
- Severe swelling of tonsils and uvula
- Can result in airway obstruction, respiratory stridor, and eventual death
- Exam tip: A common question stem may include an immigrant with a sore throat; physical exam reveals tonsils with grayish exudate.
Diphtheria: pediatric patient with diphtheria presenting with the characteristic grayish-white membrane covering the posterior pharyngeal wall
Image: “39015940254_849c8705a0_c” by Russell Watkins/Department for International Development. License: CC BY 2.0.
Patient infected by Corynebacterium: severe lymphadenopathy, known as “bull’s neck”
Image: “5325” by CDC. License: Public domain.
Complications/systemic toxemia:
- Cardiac:
- Myocarditis
- Arrhythmias
- Neurologic:
- Local neuropathies
- Cranial neuropathies
- Peripheral neuritis
- Renal: renal failure from hypertension or direct exotoxin effects
Cutaneous (wound) diphtheria:
- Initial papule that becomes a chronic, non-healing ulcer
- Ulcer is usually covered with a grey membrane.
- May be caused by toxin or non-toxin-producing strains
- In the United States, most common among alcoholic homeless men or drug users
Cutaneous diphtheria: chronic non-healing ulcer
Image: “1941” by the CDC. License: Public domain.
Identification
- Cultures:
- Should be obtained from throat or nose, including a portion of membrane
- Cultured on Loeffler’s or tellurite media
- Toxin detection:
- Elek’s test: isolates toxin-producing bacteria from the pseudomembrane
- Enzyme immunoassay (EIA)
Prevention
- Toxoid vaccine
- Infants are protected by maternal antibodies.
Management
- Passive immunization or anti-toxin: 1st line
- Antibiotics: Erythromycin or penicillin are the 1st choices for 14 days.
- Supportive management:
- Airway protection
- Bedrest
Mnemonic
“ABCDEFGs of Corynebacterium diphtheriae”:
- ADP-ribosylation
- β-bacteriophage
- Corynebacterium
- Diphtheriae
- Elongation Factor 2 (EF-2)
- Granules
References
- Barroso L.F., & Pegram P.S. (2020). Clinical manifestations, diagnosis, and treatment of diphtheria. UpToDate. Retrieved December 28, 2020, from https://www.uptodate.com/contents/clinical-manifestations-diagnosis-and-treatment-of-diphtheria?search=diphtheria&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1#H11
- Barroso L.F., & Pegram P.S. (2019). Epidemiology and pathophysiology of diphtheria. UpToDate. Retrieved December 28, 2020, from https://www.uptodate.com/contents/epidemiology-and-pathophysiology-of-diphtheria?search=diphtheria&source=search_result&selectedTitle=2~150&usage_type=default&display_rank=2
- Frassetto, L.A. (2019). Corynebacterium Infections Treatment & Management. https://emedicine.medscape.com/article/215100-treatment
- Nisha R. (2020). Corynebacterium diphtheriae: Properties, Pathogenesis, and Lab Diagnosis. https://microbeonline.com/corynebacterium-diphtheriae-properties-pathogenesis-diagnosis/
