Approximately 500 subjects will be enrolled into this study and randomized 1:1 to receive CXA-201 or comparator (levofloxacin) resulting in 250 subjects per treatment arm. Subject participation will require a minimum commitment of 35 days and a maximum of 42 days. Subjects will be hospitalized for the administration of all doses of IV study therapy. A test of cure visit will occur at 7 days after the last dose of study drug and a late follow-up evaluation or contact will occur a minimum of 28 days and a maximum of 35 days after the last dose of study drug.