cephalosporins

gram- dont develop resistence eaisly

1st generation ( cephalexin , cefazolin ) 
mainly gram+, strepto and staph so important for skin infection 
can be used to treat some cases of UTI 

2nd generation ( cefuroxime , cefoxitin ) 
more gram- than 1st and also cover gram+ , so mainly used to treat UTI gram- and some gram+ strepto infection like tosillitis otitis media ...
cefoxitin also cover anaerobics ( GI flora ) best choice of GI surgery prophylaxis ...

3rd generations (ceftrixone , cefotaxime ceftazidim )
excellent gram- so used to treat UTI 
encapsulated bacteria such as pnemo ,meningo , Hib 
cross blood brain barrier so its important for meningitis 
gonorrhea 
dont cover staph dont cover anaerobics 
ceftazidine the only that covers pseudomonas aerogenosa ...