Sputum culture

Sputum is material coughed up from the lungs and spit out through the mouth.A sputum culture is done to find and identify the germ causing an infection such as pneumonia (an infection of the lung). If a specific germ is found, more testing is done to determine which antibiotic will best treat the infection.

A person with a fever and a continuing cough that produces pus-like materialor blood may have an infection of the lungs and bronchial tubes. These infections are caused by several types of germs, including bacteria, fungi (molds and yeast), and viruses. A chest x ray helps a doctor see the infection; a culture can grow the germ causing the infection so it can be identified.

Based on the symptoms, the doctor decides what group of germ is probably causing the infection, and then orders one or more specific types of cultures: bacterial, viral, or fungal (for yeast and molds). For all culture types, the sputum must be carefully collected into a sterile container so that germs normally in the mouth don't contaminate the sample. Once in the laboratory, eachculture type is handled differently.

A portion of the sputum is smeared on a microscope slide for a Gram stain. Another portion is spread over the surface of several different types of culture plates, and placed in an incubator at body temperature for one to two days.

To do a Gram stain test, the lab technician stains the slide with purple andred stain and examines it under a microscope. Gram staining makes sure the specimen doesn't contain saliva or material from the mouth. If there are many skin cells and few white blood cells, the specimen is not pure sputum and can't be cultured. The specimen may be rejected and a new specimen requested. Ifthere are lots of white blood cells and bacteria of one type, this means there is an infection. The color of stain picked up by the bacteria (purple or red), the shape (such as round or rectangular), and the size provide valuable clues as to the bacteria's identity and helps the doctor predict what antibiotics might work best. Bacteria that stain purple are called gram-positive; those that stain red are called gram-negative.

During incubation, bacteria present in the sputum sample multiply and will appear on the plates as visible colonies. The bacteria are identified by theirappearance, by the results of biochemical tests, and through a Gram stain ofpart of a colony.

A sensitivity test is also done to test which antibiotics will treat the infection by killing the bacteria.

The initial result of the Gram stain is available the same day, or in less than an hour if requested by the physician. An early report is usually available after one day, which will reveal if any bacteria have been found yet, and if so, their Gram stain appearance. The final report is usually available in one to three days, and includes complete identification and an estimate of thequantity of the bacteria and a list of the antibiotics to which they are sensitive.

A fungal culture is done to look for mold or yeast. The sputum sample is spread on special culture plates that will encourage the growth of mold and yeast. Different biochemical tests and stains are used to identify molds and yeast; cultures for fungi may take several weeks.

For a viral culture, sputum is mixed with commercially-prepared animal cellsin a test tube. Characteristic changes to the cells caused by the growing virus help identify it. The time to complete a viral culture varies with the type of virus, but it may take from several days to several weeks.

Tuberculosis is caused by a slow-growing bacteria called Mycobacterium tuberculosis. Because it doesn't easily grow using routine culture methods,special procedures are used to grow and identify this bacteria. A small portion of the sputum is smeared on a microscope slide and stained with a specialacid-fast stain. The stained sputum is examined under a microscope for tuberculosis organisms, which pick up the stain, making them visible. This smear can identify the bacteria within 24 hours.

To culture for tuberculosis, portions of the sputum are placed in tubes of broth that promote the growth of the organism; growth and identification may take two to four weeks.

Other microorganisms that cause various types of lower respiratory tract infections also require special culture procedures to grow and identify. Mycoplasma pneumonia causes a mild to moderate form of pneumonia, commonly called walking pneumonia; Bordetella pertussis causes whooping cough; Legionella pneumophila, Legionnaire's disease; Chlamydia pneumoniae, an atypical pneumonia; and Chlamydia psittaci, parrot fever.

Pneumocystis carniicauses pneumonia in people with weakened immune systems, such as people with AIDS. This organism does not grow in culture. Special stains are done on sputum for pneumonia caused by this organism. The diagnosis is based on the results of these stains, the patient's symptoms, and medical history.

The specimen for culture should be collected before antibiotics are started,since antibiotics may prevent germs in the sputum from growing in culture. The best time to collect a sputum sample is early in the morning, before the patient has anything to eat or drink. After rinsing the mouth with water to decrease mouth bacteria and dilute saliva, the patient must cough up sputum fromwithin the chest. Taking deep breaths and lowering the head helps bring up the sputum. Sputum must not be held in the mouth but immediately spat into a sterile container. For tuberculosis, the doctor may want the patient to collect sputum samples on three consecutive mornings.

If coughing up sputum is difficult, a healthcare worker can have the patientbreathe in sterile saline produced by a nebulizer. This nebulized saline coats the respiratory tract, loosening the sputum, and making it easier to coughup. Sputum may also be collected by a doctor.

If tuberculosis is suspected, collection of sputum should be carried out in an isolation room.

Sputum from a healthy person would have no growth on culture. A mixture of microorganisms, however, normally found in a person's mouth and saliva often contaminate the culture. If these microorganisms grow in the culture, they maybe reported as normal flora contamination.

The presence of bacteria and white blood cells on the Gram stain and the isolation of a microorganism from culture is evidence of a lower respiratory tract infection.

Microorganisms commonly isolated from sputum include: Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, Legionella pneumophila, Mycoplasma pneumonia, Klebsiella pneumoniae, Pseudomonas aeruginosa, Bordetella pertussis, and Escherichia coli.

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