Tag Archives: C. diphtheriae

Iditarod and Diphtheria

3 Mar

Today is the ceremonial start of the Iditarod! Which, if you live in Alaska, is kind of a big deal. And if you live elsewhere, it’s just another Friday.
Either way, today is a good day.

The Iditarod is a 1,000ish mile race (though this year it’s only 975 miles) commemorating the infamous diptheria epidemic and celebrated every year with a grand ceremonial start and Alaskans bickering which dog, Balto or Togo, deserves more credit for the original race.

The original run was actually a huge deal. Alaska is huge state with very few roads. Hell, even our capitol city cannot be reached by road. There weren’t, and still aren’t, any roads to Nome. So when the diphtheria epidemic hit Nome in 1925, people panicked. The nearest antitoxin supply was in Anchorage, about 1000 miles away. It was the middle of winter, and winter storms produced strong gales of -85 degrees F. Nearly every Alaskan can recite this story in their sleep.

So today, while most of Anchorage is crammed downtown to celebrate the ceremonial start of the Iditarod, I am curled up with  warm cup of coffee and celebrating the Iditarod in my own way: With good ol’ fashioned science.
(Plus it’s really, really snowy out today, and my poor little car still doesn’t have snow tires.)

Today’s topic: Corynebacterium diphtheriae!


 C. diphtheriae is the adorable little bugger responsible for diphtheria, and has some odd little quirks that are fun to explore.
First, the basics: It’s a Gram-positive, highly pleomorphic bacilli, belonging to a peculiar group of bacterium called Actinobacteria. It is non-motile and aerobic. It often arranges itself in peculiar ways that microbiologists refer to as “Chinese letters.” (Unless you’re a Chinese microbiologist, in which case you would probably refer to them as Korean letters or something.) The less racist/more politically correct microbiologists refer to the morphology as a “picket fence” or “palisades” referring to how they lay next to one another. Its very distinct, instantly recognizable, and always makes me want to build kick-ass purple picket fence around my home.

(Because white picket fences are sooooo 6 decades ago.)

As for agars, C. diphtheriae is typically grown on Loeffler medium or tellurite agar. CTBA agar, which contains postassium tellurite, cystine, bovine serum, and sheep’s blood, is a common one found in labs due to the fact C. diphtheriae will form black colonies with characteristic brown halos, which differentiate it from most other Corynebacterium species.

(Be warned though, that C. ulcerans and C. pseudotuberculosis will also cause brown halos).

C. diphtheriae is also metachromatic, which means that a single stain will result in two or more different colors. This is due to the granules found at the polar ends of the bacterium, known as Babes Ernst Granules, which will stain a different color and are responsible for the notable club-shape of the bacterium. Ponder’s and Alberts’s stains can be used to demonstrate this phenomenon.

 Which looks pretty freaking groovy.

C. diptheriae produces a particularly nasty toxin which is responsible for Diptheria. Medical microbiologists must demonstrate the presence of the toxin, and they do so with the Elek’s Test. Developed in 1949, the Elek’s Test uses immunodiffusion to demonstrate the toxin. The patient’s isolate is placed on the agar in a straight line, alongside a known toxic and non toxic strains (which serve as positive and negative controls). Then a strip of paper containing antitoxin is laid across them in a perpendicular fashion. The antitoxin reacts with the toxin, forcing it to visibly precipitate out into the agar.

If the patient’s isolate contains the diptheria toxin, then peripitin lines will be formed. Iron will inhibit toxin production, so the Elek’s Test must be performed on a media with low iron content and an alkaline pH.

The toxin itself is pretty interesting. A bacteriophage with the Tox gene will infect a C.diptheriae bacterium, which will incorperate the Tox gene into its own genome and thus become a toxin producer. The toxin is secreted and actually nontoxic until it encounters trypsin, which will result and two fragments: A and B. Fragment B binds to receptors on eukaryotic cells and mediates the entry of Fragment A into the cell. Once in the cytoplasm, Fragment A disrupts protein synthesis. This disruption of protein synthesis is what contributes to the toxigenicity of the toxin.

Diptheria itself is characterized by a grey-white pseudomembrane formation on the throat, which honestly looks pretty nasty. And while I am usually all for disgusting photos of medical conditions, today I feel more like posting a photo of sled dogs, who hold the award for the most ridiculous tongues ever.

(All the better to lick your face with, my dear.)

So thank you, C. diphtheria, for giving Alaskans kickass piece of history that displays the extremes we will endure to help fellow Alaskans, a wonderful mid-winter celebration, and hilarious photos of sled dogs.

If you are interested in learning more, especially about the history of Diptheria epidemiology, Todar’s Online Textbook of Bacteriology is a wonderful resource.