The present invention comprises crystalline forms of 3R,4R)-4-[3-(S)-hydroxy-3-(6-methoxyquinolin-4-yl)propyl]-1-[2-(2-thienylthio)ethyl]piperidine-3-carboxylic acid, represented by the structure: and as characterized herein by powder X-ray diffraction patterns as form A and form B, processes for preparing form A from the purified amorphous form of (3R,4R)-4-[3-(S)-hydroxy-3-(6-methoxyquinolin-4-yl)propyl]-1-[2-(2-thienylthio)ethyl]piperidine-3-carboxylic acid, a processes for preparing form A by heating monohydrated form C of (3R,4R)-4-[3-(S)-hydroxy-3-(6-methoxyquinolin-4-yl)propyl]-1-[2-(2-thienylthio)ethyl]piperidine-3-carboxylic acid, a pharmaceutical composition comprising form A, a pharmaceutical composition comprising form A and form B or monohydrated form C, a pharmaceutical composition comprising form A, form B and monohydrated form C, a method for treating a bacterial infection with form A, a method for treating a bacterial infection with form A and form B or monohydrated form C, and a method for treating a bacterial infection with form A and form B and monohydrated form C.