How to Identify Corynebacterium Diphtheriae from Throat and Mouth Specimens
Corynebacterium diphtheriae is the causative organism for diphtheria. It can be identified and isolated in the medical laboratory by following the procedures described below. Remember that all samples and specimens must be collected aseptically to avoid cross-contamination which may lead to false positive or false negative result. 1. Culture the Specimen Blood agar Innoculate the swab specimen collected from a patient on a plate of blood agar. Use the loop to make also a few stabs in the agar (well area). Colonies of S. pyogenes growing below the surface will show more distinct zones of haemolysis because of the aerobic conditions provided. When a swab is received in silica gel (e.g. from a health centre), moisten it first with sterile nutrient broth and then inoculate the plate. Add a 0.05 unit bacitracin disc (Reagent No. 15) to the plate. This will help in the identification of S. pyogenes. Some workers also add a co-trimoxazole disc (as used for sensitivity testing) which prevents the growth of other bacteria, making it easier to see beta-haemolytic S. pyogenes colonies. Incubate the plate preferably anaerobically or, when this is not possible, in a carbon dioxide enriched atmosphere overnight at 35 – 37o Candle jar incubation will detect most beta-heamolytic streptococci. Note: Beta-haemolytic streptococci produce larger zones of haemolysis when incubated anaerobically. Read the full article