Are bacteria always present in sputum?

See below: Sputum specimens are generally obtained when a patient coughs up a specimen. This sputum is briefly in the mouth which is full of various usual flora bacteria. The sputum specimen usually grows some of these oral flora on culture. When patients are receiving antibiotics or if sputum is obtained directly from the bronchi via transtracheal aspirate or bronchoscopy the sputum can be sterile.