The same is true for many other classes of antibiotics, they are now synthetically produced. In veterinary medicine, the newest group of commonly used antibiotics are the fluoroquinolones such as Baytril. In my opinion, much of the problem of antibiotic resistant bacteria can be traced back to misuse of antibiotics by physicians and veterinarians alike. Remember, antibiotics are very limited in their activity. They kill microorganisms, usually bacteria.
Misuse of antibiotics can occur in several scenarios in the veterinary profession.
The first is using antibiotics in the abscence of a susceptible infection. Cats are commonly affected by urinary bladder inflammation, or cystitis. The vast majority of cats with cystitis have no actual bladder infection. Yet, many of these cat are prescribed antibiotics, often amoxicillin, that have no benefit to the animal. Most cases of cystitis in the cat are self-limiting, that is they get better on their own.