Aerobic Culture

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Subject: Aerobic Culture

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Definition and Use

  • Aerobic cultures are indicated for the detection of common aerobic bacterial pathogens in patient specimens taken from sites with signs and symptoms of bacterial infection (e.g., swelling, redness, heat, pus, or exudate). Site-specific bacterial cultures (e.g., sputum culture, genital culture) are recommended, if available. Specimens may be inoculated on several types of aerobic culture plates and broth media and may include selective and enriched media. Typical media for aerobic cultures include

    • Supportive media to isolate nonfastidious organisms, like sheep blood agar (SBA).

    • Enriched media to isolate organisms with special nutritional requirements, like chocolate agar.

    • Selective media to suppress the growth of specific types of bacteria. Selective media are often formulated so that colonies of different types of organisms that are able to grow on the media have different appearances. MacConkey is an example: Selective: nonfastidious gram-negative bacilli are able to grow. Differential: lactose fermenters are distinguished from lactose nonfermenters.

    • Solid versus Broth Media

      • Culture media may be prepared in a solid or broth phase.

      • Solid media (culture plates) are inoculated with a small amount of specimen. Mixed cultures are recognized by differences in colony morphology. The amount of each type of organism (and relative proportions in mixed cultures) can be estimated (e.g., rare, light, moderate, or heavy).

      • Pyogenic infections are usually associated with growth of a single (or predominant) pathogen in moderate or heavy amounts.

      • Broth media can be inoculated with a larger volume of specimen than agar plates, which may improve detection of infections with low concentrations of pathogens, but the amount of bacteria in the specimen cannot be estimated from broth cultures.

      • Broth media may allow detection of some relatively aerotolerant anaerobic pathogens. Broth cultures have been associated with an increased rate of contamination.

  • Expected results: No pathogen isolated

  • Turnaround time: 48–72 hours

    • In positive cultures, additional time is required for isolation, identification, susceptibility testing, and further characterization, as appropriate.

Special Collection and Transport Instructions

  • Standard precautions apply. Ensure that material from the site of infection is collected.

  • Decontaminate skin or mucous membranes that must be crossed to obtain the specimen.

  • Use appropriate sterile supplies to collect the specimen. Place the specimen in a sterile, leak-proof container for transport. Ensure that the lid is firmly tightened, but avoid overtightening. Use specific transport medium and/or procedures as required for suspected pathogens (described below) or if transport to the lab will be prolonged (>2 hours). Apply a label to the specimen with information to identify the patient and type of specimen, as described below. Transport the specimen to the lab as quickly as possible, avoiding extremes of temperature. Note that collection protocols for some types of specimens require specific training and/or certification of the health care professional performing the collection. Examples include collection of bone marrow and CSF specimens.

Limitations

  • Anaerobic culture is recommended for infections at sites likely to be infected by anaerobic pathogens. Examples include pelvic infections, intra-abdominal infections, abscesses, and traumatic and surgical wounds. Certain aerobic pathogens, such as Legionella species, require special processing or culture techniques for detection.

  • Common pitfalls:

    • Specimens may be collected from sites that are not the primary site of active infection (even though there may be signs of inflammation at the site).

    • Inadequate site preparation may result in false-positive cultures due to specimen contamination with endogenous flora. Contaminated specimens may also mask the recognition of slow-growing or fastidious pathogens in the culture.

 
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