The present invention relates to S-(-)-optically pure benzoquinolizine carboxylic acid of formula I, and their pharmaceutically acceptable salts, substantially free of their R-(+)-isomers, to processes for their preparation and to pharmaceutical compositions comprising an active compound of the invention. The compounds have excellent antibacterial activity. The compounds and compositions can be used for the treatment of local and systemic bacterial infections, diseases and disorders caused by antibiotic-sensitive and antibiotic-resistant microbial strains, without the concomitant liability of adverse effects associated with the racemic mixture of the active compound of the invention.