Why is Streptomycin less effective against gram-positive bacteria?

[Jenny Gibbs]

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