Inclusion Criteria:  Aged 19 years or older Diagnosed with hospital-acquired or ventilator-associated pneumonia and being treated in an intensive care unit Patient or his/her legal proxy agrees to participate and is able to provide informed consent  Exclusion Criteria:  Has been treated with antibiotic for HAP/VAP for 24 hr or longer Requires antibiotic treatment for indications other than HAP/VAP Bacteria has been isolated from respiratory specimens within 7 days prior to screening Immunocompromised host whose major differential diagnosis includes Pneumocystis jirovecii or cytomegalovirus pneumonia Expected to die within 2 days since screening due to underlying disease Has an advance directive against mechanical ventilation or cardiopulmonary resuscitation Does not want to participate or unable to provide consent Determined to be unfit by the study investigator