The present invention relates to optically pure S-(−)-benzoquinolizine carboxylic acids, their derivatives, salts, pseudopolymorphs, polymorphs and hydrates thereof, substantially free of their R-(+)-isomers, to processes for preparation of the optically pure S-(−)-benzoquinolizine carboxylic acids, their derivatives, salts, pseudopolymorphs, polymorphs and hydrates thereof substantially free of their R-(+)-isomers, and to pharmaceutical compositions comprising the S(−)-benzoquinolizine carboxylic acids, their derivatives, salts, pseudopolymorphs, polymorphs and hydrates thereof. These compounds and compositions can be used to systemically and topically treat bacterial Gram-positive, Gram-negative and anaerobic infections, specially resistant Gram-positive organism infections, Gram-negative organism infections, mycobacterial infections and emerging nosocomial pathogen infections, while avoiding toxic effects associated with the administration of the racemic mixture of RS-(±)-benzoquinolizine carboxylic acid. The compounds and compositions of this invention can also be used to treat diseases and disorders caused by Gram-positive, Gram-negative and anaerobic bacteria, and diseases and disorders caused by resistant Gram-positive organisms, Gram-negative organisms, mycobacteria and nosocomial pathogens.