Patient Test Information

Bacterial Sputum Culture

Also known as:

Respiratory Culture; Bacterial Culture, respiratory; Sputum Culture, routine bacterial

Formal name:

Culture, sputum

Related tests:

Gram Stain, Susceptibility Testing, AFB Testing, Blood Culture, Fungal Tests, Complete Blood Count, Basic Metabolic Panel, Blood Gases

Were you looking instead for tests performed on sputum to detect fungi [see Fungal Tests] or mycobacteria (tuberculosis) [see AFB Smear and Culture]?

Why Get Tested?

To detect and identify the cause of bacterial pneumonia or some other lower respiratory tract infections; to monitor the efficacy of treatment

When to Get Tested?

When you have symptoms associated with a lower respiratory tract infection; when you have been treated for bacterial pneumonia or any other lower respiratory tract infection

Sample Required?

A sputum sample (deep respiratory secretions, not saliva), usually collected first thing in the morning; sometimes, depending on the infection, up to 3 sputum samples might be collected over consecutive days.

Test Preparation Needed?

You may be instructed to rinse your mouth out with water prior to collection and to avoid food for 1-2 hours before the sample is collected.

How is it used?

A bacterial sputum culture is used to detect and diagnose bacterial lower respiratory tract infections such as bacterial pneumonia or bronchitis. It is typically performed with a Gram stain to identify the bacteria causing a person's infection.

Sometimes lower respiratory tract infections are caused by pathogens that cannot be detected with routine bacterial sputum cultures. This is either because the pathogens require very specific nutrients to grow in culture or because they grow very slowly. When this is suspected to be the case, specialized tests may be done in addition to or instead of a routine culture to help identify the cause of infection. These additional tests include, for example, an AFB smear and culture to detect tuberculosis and non-tuberculous mycobacteria infections, a fungal culture, or a Legionella culture.

A bacterial sputum culture may be ordered by itself, along with a complete blood count (CBC) to evaluate the type and number of white blood cells as an indication of infection, and/or along with a blood culture to test for septicemia.

Sputum is not sterile, so when a person has an infection, there will typically be both normal flora and pathogenic bacteria present. If pathogenic bacteria are identified during a sputum culture, then antimicrobial susceptibility testing is usually performed so that the appropriate antibiotics can be prescribed.

When is it ordered?

A bacterial sputum culture is ordered when a health practitioner suspects that someone has a bacterial infection of the lungs or airways, such as bacterial pneumonia. This may show as changes in the lungs as seen on a chest x-ray. Symptoms may include:

  • Cough
  • Fever, chills
  • Muscle aches
  • Fatigue
  • Trouble breathing
  • Chest pain
  • Confusion

Sometimes a sputum culture may be ordered after treatment of an infection, to verify its efficacy.

What does the test result mean?

If pathogenic bacteria are detected in a person with signs and symptoms of a lower respiratory tract infection, then it is likely that the person's symptoms are due to a bacterial infection.

The most common cause of bacterial pneumonia in adults in the U.S. is Streptococcus pneumoniae (pneumococcus). Other common bacteria include:

  • Staphylococcus aureus (staph)
  • Haemophilus influenzae
  • Moraxella catarrhalis
  • Klebsiella pneumoniae
  • Streptococcus pyogenes

If pathogenic bacteria are not detected with a culture, then it may be that the person's symptoms are due to a viral infection, or that the pathogen was not present in sufficient quantity in the sample collected. It may also be due to the fact that the microorganism responsible is not detectable with a routine bacterial culture.

Examples of organisms NOT detected with a routine bacterial culture of the sputum include:

  • Mycoplasma pneumoniae and Chlamydia pneumonia–common in young adults
  • Legionella species
  • Mycobacterium tuberculosis (tuberculosis; see AFB Smear and Culture)
  • Pneumocystis jiroveci–a fungus that can be seen in those who have had organ transplants or who have HIV/AIDS
  • Other fungi (see Fungal Tests)
  • Viruses
  • Rarely, a parasitic infection may also cause symptoms of a lower respiratory infection.

Is there anything else I should know?

People whose lungs have become damaged through disease, exposure to toxins or chronic exposure to irritants, or from previous infections have an increased risk of recurrent infections.

With bacterial respiratory infections, the sputum may have a thick consistency (viscous), appear discolored - yellowish, greenish, grayish, or (rarely) rusty or bloody - and may have an unpleasant odor.

What is being tested?

Sputum is the thick mucus or phlegm that is expelled from the lower respiratory tract (bronchi and lungs) through coughing; it is not saliva or spit. Care must be taken in the sample collection process to ensure that the sample is from the lower airways and not from the upper respiratory tract. Sputum samples may be expectorated or induced (See the section below on sample collection.)

Bacterial sputum cultures detect the presence of disease-causing bacteria (pathogens) in people who are suspected of having bacterial pneumonia or other lower respiratory tract infections. Bacteria in the sample are identified and susceptibility testing is performed to guide antibiotic treatment.

Sometimes a respiratory infection is caused by a pathogen that cannot be grown and identified with a routine bacterial sputum culture. Other tests, such as an AFB smear and culture, fungal culture, or viral culture, may be ordered in addition to or instead of a routine culture.

Typically, the first step in the routine analysis of a sputum sample is a Gram stain to identify the general type of bacteria that may be present. The sample is then placed on or in appropriate nutrient media and incubated. The media encourages the growth of bacteria that are present, allowing for further testing and identification.

Sputum is not sterile. That means that when a person has a bacterial respiratory infection, there will typically be harmless bacteria that are normally present in the mouth, throat, etc. (normal flora) as well as disease-causing (pathogenic) bacteria present.

A trained laboratorian differentiates normal flora from pathogenic bacteria and identifies the various types of bacteria present in the culture. Identification is a step-by-step process that may involve several biochemical, immunological, and/or molecular tests tests and observations of the organism's growth characteristics.

Antimicrobial susceptibility testing is frequently required to guide the treatment and to determine whether the bacteria present are likely to respond to specific antibiotics.

The sputum culture, Gram stain(s), and susceptibility testing all contribute to a report that informs the health practitioner which pathogen(s) are present and which antibiotic therapies are likely to inhibit their growth. 

How is the sample collected for testing?

Sputum samples may be coughed up or induced. Samples that are coughed up are expelled into a sterile cup provided by the laboratory. Deep coughing is generally required, and the person should be informed that it is phlegm/mucus from the lungs that is necessary, not saliva. If someone cannot produce a sputum sample, then it can often be induced by following instructions provided and inhaling a sterile saline or glycerin aerosol for several minutes to loosen phlegm in the lungs. Steam inhalation or a hot shower can also be useful in loosening the phlegm. Sometimes, induction of sputum might be assisted by a respiratory therapist technician.

All samples collected should be taken to the laboratory promptly for processing while they are fresh. Sputum samples must be evaluated and accepted by the laboratory before they are processed.

Useful sputum culture results rely heavily on good sample collection. If examination of a Gram stain of the sample reveals that it contains a significant number of normal cells that line the mouth (squamous epithelial cells), then the sample is not generally considered adequate for culture and a re-collection of the sample may be required. If the sample contains a majority of white blood cells that indicate a body's response to an infection, then it is considered to be an adequate sample for culturing.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

You may be instructed to brush your teeth and rinse your mouth with water prior to sample collection. You may also be instructed to avoid food for at least 1-2 hours before the sample is collected, which is usually first thing in the morning.

  1. Can bacterial pneumonia be prevented?

    There is a pneumococcal vaccine that helps protect against invasive Streptococcus pneumoniae (pneumococcus) infections, the most common cause of bacterial pneumonia in adults. There is also a different pneumococcal vaccine for young children that protects them against serious infections with these bacteria, and there is a vaccine for infants to protect against serious infections by Haemophilus influenza type b.

  2. Why would my doctor collect more than one sputum sample?

    This may be done if the first sample is not considered adequate or if the doctor wants to order additional tests on sputum samples.

  3. Why would a separate test be ordered for fungus or an AFB smear and culture?

    These tests detect organisms that do not grow on nutrient media typically used for a routine bacterial sputum culture. Fungus or mycobacterium species may take several weeks to grow in the laboratory, and they require the use of special stains to be detected under the microscope.

  4. I have been sick for more than a week. Why would my doctor order a sputum culture now?

    It may be that your doctor suspects that you have developed a bacterial infection secondary to an initial viral infection or that your infection has not responded as expected to treatment.

  5. Once I have been treated for a bacterial lower respiratory tract infection, can the infection return?

    For most people, once the infection has been successfully treated, it will not return. If the treatment was not successful, then the infection may persist or re-emerge. If someone has an underlying condition that increases their risk of lung infection, such as a lung disease, then they may experience recurrent (new) infections. In some cases, these recurrent infections can become increasingly challenging to treat.