Please, I’m very confused. Context is medical.
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When looking at cells in a microscope, you usually need to use some sort of a colored stain, because cells are mostly transparent and hard to tell apart. One common stain is a *Gram stain* (no relation to the unit of mass, the “Gram” here is a guy’s name), which is a dark purple dye washed out with a light pink dye.
A bacteria whose cell wall holds on to the Gram stain shows up as purple afterward, and we call these bacteria *Gram positive*. A bacteria whose cell wall doesn’t hold onto it is stained a light pink by the second bit of the Gram stain, and we call these bacteria *Gram negative*.
It turns out that this corresponds to a piece of the bacteria’s anatomy: Gram positive bacteria have a thick layer of [peptidoglycan](https://en.wikipedia.org/wiki/Peptidoglycan) in their cell walls, while Gram negative bacteria have a thin layer. It’s also medically relevant, because in general Gram positive bacteria are more vulnerable to antibiotics.
More broadly, Gram positive/negativity is a very common and very easy method of narrowing down what kind of bacteria you’re looking at. For example, *Y. pestis* (plague) is a Gram-negative species, so if you’re looking at a bacteria and it takes the Gram stain, you know it’s *not* plague.
It is a way of separating bacteria in to two groups based on certain properties, primarily related to the type of cell wall they have. The term comes from a technique called Gram staining, i.e. applying certain chemicals that will change the color of the bacteria. Gram negative bacteria will show up as pink/red while gram positive bacteria will show up as purple/indigo. This can be done very quickly and can help doctors narrow down the type of infection a person might have. Identification of the specific type of bacteria using cell cultures takes longer, so using Gram staining you can start treatment based on the probable type of bacteria using the negative/positive result and other clues (such as bacteria shape, patient symptoms, etc.).
The term Gram comes from the name of the scientist, Hans Christian Gram, who first came up with the technique in the late 1800’s and is not related to the physical property for mass, the gram.
When bacteria are stained using a method called Gram staining, they can be classified into two groups: gram positive and gram negative. Bacteria that are gram positive will appear purple under a microscope, while bacteria that are gram negative will appear pink. The difference between these two groups has to do with the structure of the cell wall. Gram positive bacteria have a cell wall made up of a single layer of peptidoglycan, while gram negative bacteria have a cell wall made up of two layers of peptidoglycan. This difference in cell wall structure affects the way that the bacteria interact with the environment around them.
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