Corynebacterium diphtheriae

Hello, everyone. My name is Dr. Sarah Fouch. Within this short recording, we will be considering Corynebacterium diphtheriae, the clinical condition that this organism is associated with, and also the virulence factors it can produce to cause the clinical symptoms associated with diphtheria, and also the other complications that can be associated with a Corynebacterium diphtheriae infection.

Corynebacterium diphtheriae are gram-positive bacilli. They appear club-shaped when visualising them under microscopy. Sometimes they can also appear in a Chinese letter pattern. Now, older cultures will appear slightly different when gram-stained, and this is due to the formation of metachromatic granules. These granules will also appear bluish-purple if methylene blue is used. Corynebacterium diphtheriae are non-motile, so they do not possess flagella and they are not able to form spores, which means they cannot survive in the environment for longer periods of time. There are three biotypes of Corynebacterium and these are Corynebacterium diphtheriae gravis, Corynebacterium diphtheriae mitis, and Corynebacterium diphtheriae intermedius. They can be differentiated by their colonial appearance, haemolysis patterns, and biochemical reactions. Corynebacterium diphtheriae gravis produces colonies that are non-haemolytic on blood agar. Whereas the biotypes Corynebacterium mitis and Corynebacterium intermedius exhibit small zones of β-haemolysis, and remember, β-haemolysis is the complete breakdown of the red blood cells. Now, Hoyle's tellurite agar is often used for the isolation of Corynebacterium diphtheriae and each of the different biotypes will produce colonies that appear slightly different. These colonies will range from grey and opaque through to small black translucent colonies. You may wonder why I'm talking to you about the three biotypes. However, they do have clinical significance. Because Corynebacterium diphtheriae gravis is associated with severe disease. Corynebacterium diphtheriae intermedius, intermediate disease, and Corynebacterium diphtheriae mitis, mild disease. There is also a fourth biotype, if you do some research around this subject area, and this is called belfanti. However, this is rarely described as being toxigenic so it is not often associated with disease. Diphtheria is not a new disease, and the organism was first observed by Klebs in 1883, and grown within the laboratory by Loeffler in 1884. Interestingly, the non-toxigenic strains were indistinguishable from the toxigenic strains at this time.