There are a couple dozen classes of antibiotics, of which penicillins and glycopeptides (vancomycin) are just two. Personally, I have just been through 4 classes while trying to fight cellulitis (a penicillin, 2 lincosamides, a sulfonamide, and a tetracycline). I was probably a day or two away from being hospitalized for IV glycopeptide treatment. In my case, the problem was the speed with which the infection was progressing; I had daily appointments with my orthopedist until we found the right combination. Still, there are a variety of antibiotics that can be tried, alone or in combination. There is only a fuzzy concept of "tiers". Its more about strength and side effects.