sputum culture

When a lung infection brews, the phlegm coughed up usually carries the germ responsible. A sputum culture is the lab's way of catching that germ: a sample of phlegm is smeared onto nutrient jelly and kept warm, so any bacteria present grow into visible colonies that can be identified — like planting seeds and seeing what sprouts.

Technically, sputum culture is the laboratory growth of microorganisms from a sputum sample to identify a respiratory pathogen and, crucially, test which antibiotics will kill it. A good sample comes from deep in the chest rather than from saliva; the lab examines it under the microscope first, and a sample full of squamous cells from the mouth is rejected as contaminated. Bacteria typically grow within one to two days, and antibiotic sensitivity testing follows.

The result lets doctors switch from broad empirical antibiotics, started before the germ is known, to a narrower drug aimed precisely at the identified organism and its sensitivities — better for the patient and for limiting resistance. It is especially valuable in severe or hospital-acquired pneumonia and when standard treatment is failing.

Its limits are real: many patients cannot produce a good deep sample, prior antibiotics can suppress growth, and some important pathogens such as Mycoplasma or many viruses do not grow on ordinary cultures and need other tests. So a negative culture does not rule out infection.