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  • Alex Cloherty

Bacteria wear winter coats, too.

Updated: Aug 26, 2020

Today's post will be a short one, but on an important topic inspired by a question from Aniek. She asked about the different immune evasion strategies that two main types of bacteria have. But before we get into how these bacteria hide away from the immune system, first we have to begin to understand the difference between these two main types of bacteria.

Since the 1800s, microbiologists have used one particular quick, easy, and effective way to identify two different types of bacteria: the Gram stain. This staining method is named after the man who invented it, Hans Christian Gram. I won't go into the details of the procedure for staining bacteria, but the end result is that some bacteria end up stained purple, while in other types of bacteria the purple stain gets washed out, leaving them red. My 2nd year Microbiology professor gave us an easy way to remember the background for this: "P-Purple-Positive-Peptidoglycan".

Peptidoglycan?! What's that? Basically, it's a thick fuzzy coat of sugar and protein on the surface of "Gram-positive" bacteria. You can think of them as wearing a winter parka, which helps protect them from their dangerous outside world. This parka helps keep in moisture, for instance, which the cells need to survive. However, the downside is that the human immune system is pretty good at recognizing this peptidoglycan parka. This means that Gram-positive bacteria have had to develop a whole host of other ways to sneak away from the human immune system – like avoiding parts of the innate immune system, like the complement pathway, or parts of the adaptive immune system, like antibodies (to put it simply).

Gram-negative bacteria, which don't have this thick parka, have had to develop other strategies for avoiding destruction by antibiotics and the human immune system. Although these bacteria also have a peptidoglycan coat, it's more like a thin fleece, over which they wear a "windbreaker": a second cell membrane. This "windbreaker" helps keep out antibiotics. At the moment, Gram-negative bacteria have been classified as having a greater general risk for antimicrobial resistance than Gram-positive, although both Gram-negative and Gram-positive bacteria are able to share antibiotic resistance genes between each other, kind of like get-out-of-jail-free cards.

For this post, the take-home point is that because these different types of bacteria wear different coats, they have to use different strategies to stay alive. In the future, I'll be going into more detail about the life cycles of these different bacterial groups!

For now, stay warm!


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