Many people will point out that the antibiotics given to animals are mostly different from those used on humans. However, as your example points out there are a

Many people will point out that the antibiotics given to animals are mostly different from those used on humans. However, as your example points out there are actually only a small number of families of antibiotics each of which share a specific method of attack on bacteria. And bacteria that become resistant to a cheap antibiotic, like virginiamycin, will do so in a way that also renders them resistant to their more expensive cousins that have been reserved for human use.