Question:
Although Nesseriae are gram negative they are sensitive
to penicillin (which is usually used against gram
positive bacteria). Why is this so?
Replies:
The bacterial cell wall is made primarily of a substance known as
peptidoglycan. It is protein based (peptido-) and sugar based (-glycan).
The sugar parts are actually two sugars called n-acetyl-muramic acid and
n-acetyl-glucosamine. They alternate in long rows. They are joined by
tetrapeptides that hook them together kind of like a chain link fence. This
makes for a very strong cell wall. Penicillin acts by preventing the
tetrapeptides from cross-linking and the cell wall loses its strength. The
organisms then become sensitive to changes in osmotic pressure and burst.
Gram positive organisms have a very thick peptidoglycan layer and therefore
are most sensitive to penicillin based antibiotics. But gram negative
bacteria also have peptidoglycan in their wall, even though it is only about
20%. So penicillin so far has been able to kill gonorrhea organisms.
However, many are becoming resistant to this antibiotic.
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