Hi everyone and welcome to Biology Professor. Today we're going to be talking about Eosin methylene blue agar, also known as EMB agar, which is a bacterial growth medium. So it's a growth medium that has a couple of cool characteristics.
It is selective and it is differential. Let's talk about what that means. So it is selective, selective for gram-negative bacteria.
The reason for this is actually that it contains dyes that are toxic to gram-positive bacteria. So it's selected for gram-negative bacteria, meaning it allows them to grow. It contains these dyes that are called ESNY and methylene blue. That's where it takes its name. And those are toxic to gram-positive bacteria.
It is also differential. So it's a differential specifically for fecal coliform bacteria. So for coliform bacteria, differential meaning that it allows you just by the way they grow on the plate to distinguish coliform bacteria from non-coliform bacteria. Now you might be asking questions about what coliform means. Well, coliform bacteria are rod-shaped, gram-negative, non-spore forming bacteria.
that can be modal or non-modal that can ferment the sugar lactose which is also found in this media they ferment the sugar lactose producing acid and gas as byproducts coliforms many of them are not actually harmful to people but a lot of them are found in the gut of humans and other animals and so if you have coliform bacteria Showing up in water or food samples, it typically means those samples have been contaminated by fecal matter and could contain other dangerous pathogens. So that's why you might want to be looking for the presence of coliform bacteria through a kind of growth medium like this. So let's talk about how it does its differential characteristic.
It uses those dyes, the same dyes I mentioned before, the same ones that are toxic to the gram-positive bacteria, that's the EOSYNY and methylene blue, and it uses these as pH indicators. So these dyes form a dark purple at low pH, and this allows you to distinguish bacteria that can ferment the lactose. Remember that coliforms ferment lactose. producing not only gas but an acid so an acid will lower the pH and that will make the the dark purple color show up so by plating these kinds of bacteria on EMV auger you can determine you can distinguish those that ferment lactose like E.coli from those that do not like salmonella excuse me like salmonella and shigella and that's because The colonies that do ferment lactose, like E.
coli and other types of coliform bacteria, they will actually be nucleated. Being nucleated just means that the colony has a dark center in it from that dye. And that's because the dye absorption increases when the pH decreases.
So when the lactose is fermented, that acid product comes about that lowers the pH, which increases the dye absorption, which means that the sort of dark purple pH indicator dye that gets absorbed by the colony and gives it a dark center. And so that means that coliform bacteria will have those dark nucleated centers, whereas non-coliform bacteria, the ones that do not ferment the lactose and so do not decrease decrease the pH, they're not going to absorb that dye. The dye also isn't going to be changing color in the same way. And so they will have colorless colonies.
So coliforms will have dark centers, non-coliforms will be colorless. Another cool thing with EMB auger is that some colonies will actually develop a metallic green sheen. So it's this very sort of shiny reflective type of green that's very distinctive and that's due to the interaction with those guys that we've been talking about and particular strong acid end products of fermentation in specific types of bacteria. If those bacteria are modal and they have flagella, the movement of the flagella can also kind of contribute to that sheen.
The most well-known one is E. coli. But also some species of Citrobacter and some species of Enterobacter can also have that metallic green sheen.
Something else to keep in mind is that if you have some bacteria that are weak fermenters, meaning that they can ferment the lactose, but it's kind of slow and it's not necessarily a lot, the pH isn't going to change as much because while acid will be produced, it won't be very much. That is going to produce pinkish colonies. So maybe like a faint pink color that's you know noticeable but not very strong. And so from all of this you can see the EMB auger has a lot of ways to distinguish different kinds of bacteria.
If it's a weak fermenter you'll see the pink sheen. If it is that metallic green you know it's going to be one of these types of bacteria. If it's got the the dark colony, then you know it is a coliform.
And so these are all different ways that you can use EMB auger to distinguish these different types of bacteria. If you're interested in learning about other kinds of selective and differential media, things like mannitol salt auger or mcconkey auger, then check out my playlist on bacterial growth media. That's it for today, and thanks for watching Biology Professor.