Hi. I'm Dr. Gotthelf. I have been a vet for 35 years and I would like to use my experience to help you with your pet's medical problem.
At her age, there may be some type of problem in her immune system causing this recurrent skin infection. Sometimes this is caused from a food sensitivity. Ask your vet if a hypoallergenic food trial for 6 weeks would be helpful.
There are some dogs that just have a bacterial infection of the skin without us knowing why. In many of these dogs, we will use long term antibiotics (for months) as well as pulsing the antibiotic therapy (giving it on the weekends only or on a week on-week off basis). There are many veterinary dermatologists that are using topical disinfectants more and more. Chlorhexidine sprays and even bleach solutions (1:30 dilution) really help to disinfect the skin without having to use so many antibiotics.
SOme dogs that are on steroids will keep a chronic infection of the skin. If that is the case, an alternative drug, like cyclosporine, may be needed.
Sometimes, the vet will want to do a bacterial culture and antibiotic susceptibility test. This will tell them if the antibiotic that they are using is killing the bacteria or if it is just slowing the growth. If there are more than one bacteria causing the skin infection, one antibiotic may not be as good as using 2 antibiotics specifically to kill each of the bacteria.