If you want to find information pertaining to antibiotics and other antimicrobials used in veterinary medicine I think that the best guide out there is:

The Antimicrobial Index

Most importantly it is regularly updated with thousands of microorganisms. Its packed with antimicrobial susceptibility info in the form of minimum inhibitory concentration data and can be used to discover a vast variety of new antimicrobial agents.

Hope you will also find it useful!