Laboratory diagnosis of Diphtheria

Laboratory diagnosis of Diphtheria:

Collection of specimens Culture and identification:

A swab is employed to gather specimen from the inflamed regions of the membranes which are formed in the throat and nasopharynx, Nasal swabs are carried to detect carriers. Material from wounds must be eliminated by swab or aspiration, taking care to avoid normal skin flora. The specimen must be immediately transported to the laboratory or inoculated onto appropriate media.

Smears can be prepared of the original material for following Neisser or Loffler’s methylene blue stain for surveillance of metachromatic granules and gram stain for coryneform morphology.

Specimens for C. diphtheria must be streaked onto a blood agar plate and onto a tellurite having medium like Hoyle’s medium, cystine-tellurite agar or altered Tinsdale medium. The tellurite consisting media have the benefit of being both selective and differential; tellurite restrains the growth of most nasopharyngeal flora and C. diphtheria has a rare capability to decrease tellurite salt to black-colored tellurium, rendering the colonies black.

A Loeffler’s serum slant can too be inoculated, since C. diphtheria grows most quickly on this lipid-rich medium. When smears are made and gram stained from colonies grown on Loeffler medium, metachromatic granules can be explained clearly.

After isolation, recognition of C. diphtheria is not hard. Tests which are significant to differentiate C. diphtheria from other coryne-bacteria and coryneform organisms are making of catalase, urease and pyrazinamidase, nitrate reduction, sucrose, glucose, mannose, xylose and maltose fermentation and using of starch and glycerol.

Detection of toxigenicity:

Manufacture of toxin is determined by guinea pig lethality testing in which toxigenic strains whenever injected into guinea pigs kill the animals in 24 to 48 hours. The other standard in vitro test is Elek’s immunoprecipitation assay.

A fresh growth in the diagnosis of diphtheria is the employ of polymerase chain reaction (i.e., PCR) to notice the toxin gene. In this PCR test a 0.9 kb part of the toxgene is amplified.

 

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