You don't necessarily need to culture the uterus, the guarded culture is routinely done for culturing the vaginal tract and cervix. MY repro vet recommends doing it about 1-2 months before a heat cycle so that there is enough time to get the culture results back and treat. And as an FYI, just throwing your bitch on Keflex or Baytril is a reason why there tends to be an overgrowth of other bacteria and leads to the resistance of others.
I had a bitch cultured after she missed a second breeding (dead semen and chill 10 the first time), when her culture results came back it was Ecoli, but NOT susceptible to either KEFLEX or BAYTRIL. Had I just treated her with those it wouldn't have cleared the Ecoli and likely would have created an even bigger problem.
I don't just "TRY" antibiotics. I am a nurse and I have to deal with RESISTANCE to them all the time. Do you know WHY there is resistance??? Because people use antibiotics incorrectly,ie. using them when there is no indication,using non susceptible antibiotics when a culture is warranted, using the incorrect dose, incomplete dose,etc. Antibiotics are powerful drugs ,their indiscriminate use does have effects far and wide, you just may not see them right away or haven't come up against them yet.
And yes she did have symptoms, 2 misses and a discharge. All bitches have E.Coli in their vaginal tract,as well as Mycoplasma and quite a few others, it's when there is an OVERGROWTH of bacteria there is a problem that can prevent conception,the most common according to my repro vet. In the time it takes to get culture results returned and the bitch treated correctly it would be right on target for her heat cycle, and by treating correctly there isn't going to be return of overgrowth.