GENUS CORYNEBACTERIA:
This genus contains many species, is widely distributed in nature, and is part of a spectrum, with Mycobacterium and Nocardia, of similar cell wall structure containing mycolic acids. The species of major importance is C. diphtheriae. This and other pathogens within the genus need to be distinguished from commensal corynebacteria.
Corynebacterium diphtheriae
Characteristic:
Gram-positive, non-capsulate, non-sporing, non-motile rods, 2-6 µm in length. In Gram-stained films cells arranged as “Chinese letters” or palisades and showing irregular staining or granule formation are characteristic. Non-fastidious, but growth enhanced by inspissated serum (Loeffler medium). Capable of aerobic and anaerobic respiration.
Transmission:
Normal habitat: usually nasopharynx, occasionally skin of humans. Infection is usually spread by aerosol. Patients may carry toxigenic organisms for up to 2-3 months after infection.
Pathogenesis:
Diseases is due to production of diphtheria toxin controlled by the tox gene, which is integrated into the bacterial chromosome on a lysogenic phage. When concentration of exogenous inorganic iron (Fe3+) is very low, toxin production is maximal; the selective advantage to the organism is unknown. The mode of action of the toxin is to block protein synthesis of the host cells by inactivating an elongation factor.
Diseases:
Diphtheria caused by toxigenic strains of C. diphtheriae. Focus of infection may be the throat or, increasingly commonly the skin.
DIPHTHEROIDS
Commensal Corynebacteria are normally present in the throat, skin, conjunctiva and other area. These may sometimes be mistaken for C.diphtheriae and are called diphtheroids. In general, diphtheroids possess few or no metachromatic granuals and are arranged in palisade (Parallel rows) rather than in Chinese letter pattern. Some diphtheroids are indistinguishable from diphtheria bacilli microscopically and required to be differentiated by biochemical test and more reliably by virulence test.
LABORATORY DIAGNOSIS
Sample collection:
1. Throat and nasopharyngeal swab
2. skin swab from for diagnosis of cutaneous diphtheria
Methods for identification of orgainism:
Gram-positive, non-capsulate, non-sporing, non-motile rods
Direct methods:
1. Gram staining:
Gram-positive rods are seen arranged as Chinese letters” or palisade( commansal corynebacteria)
2. Albert staining:
C. diphtherae stains green and beaded due to the presence of dark staining granules in rods. These granules,known as volutin granules or metachromatic granules, are energy storing inorgainic poly-phosphate units
3. Methylene blue staining:
C. diphtherae stains unevenly with dark blue staining granules in rods
4. Culture:
Culture media: Loffler’s serum medium, Tellurite blood agar (Black colony)
Grow well aerobically. Temperature range for growth is 20-40ºC
5. Biochemical reactions:
Sugar fermentation test: ferment Glucose and maltose with production of acid.
Catalase test positive
6. Animal inoculation:
Intra-cutaneous injection in to guinea pig leads to death of the animal.
Indirect methods:
Toxigenicity testing of C. diphtheria by Elek gel precipitation test and Schick test.
