Sputum Culture, Bacterial

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 healthcare 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. Signs and 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. A positive culture report typically identifies the pathogen that was detected.

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

A culture that is reported as "no growth in 24 or 48 hours" is usually considered negative. 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 Testing)
• 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.

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.

Why would my healthcare practitioner collect more than one sputum sample?

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

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.

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

It may be that your healthcare practitioner 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.

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.