Some bacteria are able to grow in the streptomycin containing medium due to:

Bacteria are able to grow in a Streptomycin containing medium due to development of resistance to the bacteria by a process called natural selection. Normally bacteria cannot survive in antibiotic containing medium but if it does so it must have acquired resistance against that antibiotic. These are well adapted to grow in streptomycin containing medium and thus are more evolved. So due to natural selection only, the more evolved and better-adapted species is able to survive.