Hemophilus influenzae

Hi everyone. My name is Dr. Sarah Fouch, and within this short recording, we will be considering Haemophilus influenzae. We will think about the clinical conditions that this organism is associated with and the virulence factors that it can produce to make it a successful pathogen.

So these are small pleomorphic gram-negative organisms. Now, when I say pleomorphic, I mean different sizes. So some may be a bit longer than others, and some may be a bit wider than others. We call Haemophilus influenzae fastidious, and they're a little bit like a young child that has a particular fancy for their dinner, so they might like tomato sauce on everything. Haemophilus influenzae needs growth factors in order to be able to grow on culture media. Now, the growth factors that Haemophilus influenzae require are the X factor, and this is haemin which is derived from blood, and also the V factor, so NAD. If they have these growth factors, we can achieve nice, large colonies on culture media. When you think about haemin, you automatically think to yourself, we can grow the organism on a blood plate. But growing Haemophilus influenzae on a blood plate is a little bit like giving someone their dinner covered in cling film and saying to them, "Here you are. Here's your dinner. You've got to eat it, but you can't remove the cling film." So what we need to do is we need to do something to the blood in order to release the X and the V factors so that they are available for the Haemophilus influenzae to use and produce nice big colonies. What do we do to the blood plate? What we will do is we will heat it up; and this is called chocolatising the agar. We heat the blood plate, it will release the growth factors, and it will turn the blood plate to a nice chocolatey brown colour, and that's why it's called chocolate agar. Now, by heating the colonies, we are releasing the growth factors, so we are releasing the X and the V factors, and the organism can then utilise those factors and grow nice, large colonies. Now, interestingly, if you have a blood plate, and you have Staph aureus present within that culture, you can see an image here where we have a blood plate and we have a streak of Staph aureus on that blood plate. If we have Staph aureus and Haemophilus influenzae present, Staph aureus is able to break down the red blood cells, and by breaking down the red blood cells, we're having the release of the X and the V factor. Now, if you look at the image, around the Staph aureus, you can see the growth of Haemophilus influenzae, and the Haemophilus influenzae is able to grow close to the Staph aureus because Staph aureus is able to break down those red blood cells. We often call these satellite colonies of Haemophilus influenzae. So they produce nice, large colonies because the growth factors are available to them. Elsewhere in the culture, they will look like little pinprick colonies, because the Haemophilus influenzae cannot access the X and V factor.