DaveC426913 said:
Aychamo: a question:
Why are bacteria labelled as gram- and gram+? Is there a fundamental property that it is relevant to group them like this?
Well, with bacteria, for the most part, if you just looked at them without doing anything to them, under the microscope most of them bascailly look the same (either as spheres or rods.) Gram staining is just a technique to help classify bacteria.
The http://en.wikipedia.org/wiki/Gram_stain" helps microbioligsts distinguish between different bacterial species. Gram positive bacteria have a thick wall of peptidoglycan, which retains the violet color stain in the gram stain procedure (read the link above), and gram negatives don't have the thick outer wall of petidoglycan, so they don't retain the violet stain, and they instead pickup the counter stain, which is red.
So yes, it's the composition of their outer membrane (the amount of peptidoglycan) that allows them to be classified as gram positive or gram negatives.
I'm sure you've heard of "Strep throat" or a "Staph infection". Those are both (medically important) gram positives (and there are many others, like Clostridium botulinum (which causes botulism, the food poisoning, etc). And there are a ton of medically important gram negatives (ie, E. Coli, Haemophilus influenza [what you have the Hib vaccine for], Psuedomonas, N. ghonorrhea, etc.)
Gram negatives are especially dangerous because they have what's called an "endotoxin." It doesn't actually hurt you until the bacteria is dead. When a gram negative dies, it releases, from its LPS layer (lipopolysaccharide), a Lipid A component (which is the endotoxin) that wreaks havoc on your body. That's what I was talking about earlier when I said if you have a patient with gram negative bacteremia (baceria in the blood) and you kill all the bacteria, they will release the lipid A, and the patient can die in 5 hours! I'm not sure what you do in terms of treating the patient in that case.
I'm only a 3rd sem medical student, so I'm sure some of the microbiologists on the board will murder my explanations and explain things better :)
Aychamo