Why do following antibiotics inhibit the bacterial growth of Rhodospirillium Rubrum?

Question

Ampicillin and penicillin weakly inhibit bacterial growth of R. rubrum, and neomycin and chloramphenicol strongly inhibit growth. Why?

Answer 1

Since ampicillin and amoxicillin only weakly inhibit the growth of this bacteria, it can be assumed that the bacteria is Gram negative and contains at least some type of outer envelope covering a very thin layer of peptidoglycan. Since chloramphenicol and neomycin can inhibit the growth, it would suggest that the protein-synthesis inhibiting drugs would be beneficial antibiotics to this N-fixing bacteria.