Why is Streptomycin less effective against gram-positive bacteria?

I'm an A-Level Biology student and am in the middle of a Microbiology
coursework.

I had to research and compare the effectiveness of Streptomycin against a
gram positive (Staphylococcus albus) and a gram-negative (E.coli) bacteria.
I found that Streptomycin was significantly more effective against the
gram-negative strain, a pattern supported by background research. I know
that Streptomycin works by inhibiting protein synthesis but can't see why
this would be more effective against gram negatives than gram positives,
particularly given the fact that gram-negatives have the second outer
membrane to inhibit the entry of chemicals into a cell.
Can anyone help please?
I will make due reference to any help in the Bibliography section of the
coursework.
Many thanks.
Jenny

This exact question has just been addressed in sci.bio.microbiology.

Jenny Gibbs wrote: