A sputum culture is done to find and identify the microorganism causing an infection of the lower respiratory tract such as pneumonia (an infection of the lung). If a microorganism is found, more testing is done to determine which antibiotics will be effective in treating the infection. Sputum is material coughed up from the lungs and expectorated (spit out) through the mouth. A person with a fever and a continuing cough that produces pus-like material and/or blood may have an infection of the lower respiratory tract. Infections of the lungs and bronchial tubes are caused by several types of microorganisms, including bacteria, fungi (molds and yeast), and viruses. A chest x ray provides visual evidence of an infection; a culture can grow the microorganism causing the infection. The microorganism is grown in the laboratory so it can be identified, and tested for its response to medications, such as antifungals and antibiotics.