This culture is used to detect Corynebacterium diphtheriae in clinical specimens. It should be considered in patients who present with signs and symptoms consistent with diphtheria, which is caused by local infection, most commonly respiratory or cutaneous infection, or by systemic disease caused by the action of diphtheria toxin, primarily on the heart, central or peripheral nervous system, liver, and kidney. Diphtheria is now uncommon in countries that have implemented widespread vaccination programs against this pathogen.
Method:
Specimens must be inoculated onto special media, including selective, enriched and differential media, for isolation of C. diphtheriae.
Respiratory or cutaneous specimens are inoculated onto SBA or CNA agar (to detect other pathogens), and agar-enriched media containing cystine and tellurite, such as cystine " “tellurite blood agar or modified Tinsdale agar for C. diphtheriae detection.
On cystine " “tellurite agar, C. diphtheriae, Corynebacterium ulcerans, and Corynebacterium pseudodiphtheriae produce black colonies surrounded by a dark halo.
The identification of suspect colonies must be confirmed. Toxin-producing and non " “toxin-producing strains of C. diphtheriae may be isolated from clinical specimens. C. diphtheriae isolates should be referred for toxin production testing.
Turnaround time: 48 " “72 hours for initial isolation. Additional time is required for confirmation of the identity, toxin testing, and further characterization of suspicious isolates.
Special Collection and Transport Instructions
The laboratory should be alerted before specimen submission to ensure that appropriate medium is available for culture inoculation.
Swab specimens are collected from multiple inflamed sites of the pharynx or other respiratory mucosal surfaces. Collection of specimens from near or under any diphtheritic membrane is recommended.
Aspirate, swab, or tissue samples for detection of cutaneous diphtheria are collected following general collection recommendations for skin lesions.
Routine transport media can be used for transport.
Interpretation
Expected results: No growth.
Positive results: Isolation of toxigenic strains of C. diphtheriae from upper respiratory or cutaneous lesions is diagnostic of diphtheria.
Negative results: Submission of multiple specimens may be required for C. diphtheriae isolation.