This may have changed but I don't think it has but the CDC (Centres for Disease Control), in America issue guidelines about treatments for infectious diseases. As GBS is one of the most dangerous infections a newborn can get they advise that once a pregnant woman has tested positive for GBS she is always considered positive for GBS. This may explain why some doctors do not retest at the time of the waters breaking. That and it can take at least 24 hours to get the result back and by then bub may have already been born. Regarding antibiotics the hospital where I used to work and gave birth in liked to start antibiotics as soon as the waters broke for those who knew they where GBS positive or for those whose status was unknown. I know in the antenantal clinic they tested everyone. As a bare minimum, it is great if Mum could have had a dose of penicillin or a substitute via and IV at least 4 hours prior to birth. If this can't happen some peadiatricians may want bub to have IM injections of penicillin as a cover.

You can find this document on the CDC's website and they give rationales for their guidelines as well, as well as references to other articles on this.