Hi everyone and welcome to Biology Professor. Today we're going to be talking about how to use mannitol salt auger. So mannitol salt auger is a kind of bacterial growth media that has a few cool characteristics. The first is that it is a selective medium.
Selective means that it allows the growth of some organisms while inhibiting the growth of others. So there are other types of media that are non-selective, things like LB auger or nutrient auger, that if you expose them to environmental bacteria lots and lots of different things will grow on that kind of auger. But mannitol salt auger is selective. It allows some organisms to grow and not others. So specifically in the case of mannitol salt auger, it allows the growth of Staphylococcus species.
So species that are in the genus Staphylococcus would include things like Staphylococcus aureus or Staphylococcus epidermidis. If you hear someone talking about having a staph infection, it's an infection by some species within this genus. Mannitol salt auger also allows bacteria that are in the microcock let's see micro cock aca a little bit of a different difficult one to spell microcock aca to grow so for example things like microcaucus luteus can grow um I'm Manitou Salt Auger. And the reason that it allows these two to grow is because they can survive the relatively high salt concentration of this auger. So this auger is about 8 to 10% salt.
And so that tends to inhibit other bacteria. So for example, if you were looking under a microscope after having done a gram stain, you would know that Staphylococcus and micrococcus are both gram positive cocci, meaning that they're spheres in shape, but mannitol salt auger can let you know that you're working with either Staphylococcus or micrococcus and definitely not streptococcus, which is another gram positive coccus round shaped bacterium. because it can't live in that high salt concentration. So the high salt concentration inhibits not only the growth of Streptococcus but also the growth of virtually all gram-negative bacteria. Mayonchal salt auger has another interesting characteristic.
In addition to being selective, it is considered a differential medium. So differential media means that it contains chemicals that visually distinguish species based on their biochemical characteristics. So you can see with your eyes that some colonies are a different species than another based on their biochemical characteristics.
In the case of mannitol salt augers specifically, the way that it is differential is it contains mannitol Manitoul is a sugar alcohol, basically a kind of carbohydrate, and it contains the pH indicator phenol red. This is what gives the manitoul salt auger its characteristic kind of bright pink color. And so because it has mannitol and this pH indicator phenol red, the pH indicator will have a color change when it detects acid.
So when acid is produced from bacteria that can ferment the mannitol, so remember the mannitol? calls a carbohydrate, if the bacteria can ferment it, it's going to result in an acid byproduct and that is going to, that acid is going to drop the pH, which is going to change the color of the pH indicator. creating a yellow zone.
Not sure why around has two Rs there. Why have a yellow zone around the colonies that are composed of a bacterial species that can ferment the mannitol. So again, mannitol salt auger is selective. It allows only Staphylococcus and Mycococci bacteria to be able to grow.
And it's differential. If you see a yellow zone, it means that that bacteria can ferment mannitol. So let's take a look at how this works in practice.
Here I've drawn an agar plate with four quadrants and we're going to see what four different kinds of bacteria would look like on a mannitol salt agar plate. So the first One we'll consider is Streptococcus pyogenes. Streptococcus pyogenes is a member of the Streptococcus genus, and that is one of the types of bacteria that is inhibited by this type of auger. That means that even if Streptococcus pyogenes, if a culture was added to this quadrant and the plate was incubated, nothing is going to grow. On the other hand, Staphylococcus epidermidis.
Micrococcus luteus and Staphylococcus aureus are all going to be able to grow on mannitol salt auger because they are members of either the Staphylococcus genus or the micrococciate family. And so Staphylococcus epidermidis would be able to grow, so would Micrococcus luteus, and so would Staphylococcus aureus. So that's the selective portion.
These three can grow. This one is inhibited. Now let's talk about the differential portion.
Remember that if any of these three types of bacteria can ferment the mannitol, that's going to cause the color change with the pH indicator when the acid is produced, resulting in a yellow zone. And it turns out that Staphylococcus epidermidis and Micrococcus luteus do not ferment mannitol. There are other sources of nutrition in the mannitol salt agar that they can use, and so they'll grow, but they will not change the color of the mannitol salt agar.
On the other hand, Staphylococcus aureus is going to be able to grow and ferment the mannitol, resulting in a yellow zone. And unfortunately, I don't have a yellow dry erase marker, but there would be sort of a yellow zone. Basically, the portion of the plate where Staphylococcus aureus was present would be a really distinctive yellow color.
I encourage you to Google mannitol salt auger plate and do a Google image search and you can see some really interesting images that show that distinctive yellow zone. Now if you're interested in learning more about gram positive versus gram negative bacteria, I use those terminology when talking about the gram positive bacteria that can grow here and then the ones that are inhibited like gram positive streptococcus and all gram negative bacteria. If you want to know more about what those words mean, gram positive or gram negative...
gram-negative, you can check out my video on gram-positive versus gram-negative bacteria. But this will be it for today, so thank you for watching Biology Professor!