Pen G is the original penicillin isolated by Flemming. This antibiotic is
not effective against most gram negative bacteria, contrary to Adam's post.
The reason is that gram negative bacteria possess an outer membrane which in
most cases is impermeable to penicillin G. Some Gram negative cocci such as
N. gonorrheae possess outer membranes which are permeable to penicillin G,
and for many years this was the drug of choice for treating gonorrhea.
However, the gonococcus has developed other ways to counteract penicillin,
so this drug is no longer useful.
Later penicillin derivatives such as a ampicillin and methicillin are active
against gram negs, again because they can get through the outer membrane to
reach the cell wall.
Jay Mone'
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