Ames test excision repair system question

[TytoAlba95]

Hi there.
I have some questions from the Ames test. I am following Wikipedia and a book to understand this concept.
My understanding :
Ames test is a fast and convenient test that uses auxotrophic bacteria (those that cannot grow in histidine lacking medium) to find out if some chemical is mutagenic.
When the bacteria are plated with the mutagen and some his, with the passage of time as his depletes, only prototrophic bacteria survives (those that have undergone reversal mutation due to the mutagen in his gene). This indicated that the agent is mutagenic.

1. 'Also the tester strains carry mutations in the genes responsible for lipopolysaccharide synthesis, making the cell wall of the bacteria more permeable, and the excision repair system to make the test more sensitive.'(from the book)

I don't get this point, ... the excision repair system to make the test more sensitive, at all.
For the first part, I think the mutation is to make the cell more permeable to the mutagen.

2.'Using appropriate strains, compounds causing base substitutions and frameshift mutations can be distinguished' (book)

How's this made possible? Is it through the reversal mechanism only?

3. Also at the beginning of the article in both my references, it has been mentioned that Ames test assumes, that the mutagen also acts as a carcinogen. I couldn't find any application of this assumption in both my references.

 

[WooKi]

1.There are mutations in cell wall synthesis system AND in excision repair system to make test more sensitive (Wikipedia). The point is that mutation occurs on one strand only. For mutation to become heritable DNA must be replicated before the mutation is eliminated by reparation system. To increase chances that mutations become heritable excision repair is reduced in test strains.
2.This is explained in Wikipedia article. Some compounds may preferentailly cause certain type of mutation, frameshifts or base substitutuons. Strains with different mutations exist to increase chances to detect a mutagen.
3.It is written (Wikipedia) 'may act as a carcinogen'.