This invention provides novel 4-oxoquinolizine compounds and their uses for a series of broad-spectrum antibiotics having no cross-resistance to existing or emerging classes of antibiotics. In addition the novel 4-oxoquinolizine compounds are useful against CDC Category A and B pathogens The invention also provides pharmaceutical compositions comprising certain 4-oxoquinolizines in combination with subinhibitory concentrations of polymyxin B against clinical isolates which are resistant to quinolones, carbapenems and other antimicrobial agents.