the present invention comprises monohydrated form c of - 4 -- hydroxy - 3 - propyl ]- 1 - ethyl ] piperidine - 3 - carboxylic acid , represented by the structure and as herein defined by powder x - ray diffraction , processes for preparing monohydrated form c from amorphorous - 4 -- hydroxy - 3 - propyl ]- 1 - ethyl ] piperidine - 3 - carboxylic acid or form a of - 4 -- hydroxy - 3 - propyl ]- 1 - ethyl ] piperidine - 3 - carboxylic acid , pharmaceutical compositions comprising monohydrated form c , pharmaceutical compositions comprising monohydrated form c and form a or form b of - 4 -- hydroxy - 3 - propyl ]- 1 - ethyl ] piperidine - 3 - carboxylic acid , pharmaceutical compositions comprising monohydrated form c , form a and form b , methods for treating bacterial infection with monohydrated form c , methods for treating bacterial infection with monohydrated form c and form a or form b , methods for treating bacterial infection with monohydrated form c , form a and form b , and a process for the preparation of form a from monohydrated form c .