Approach to Gram stain and culture results in the microbiology laboratory
- Joshua Freeman, MBChB, FRCPA
Joshua Freeman, MBChB, FRCPA
- Clinical Microbiologist
- Auckland Hospital, Auckland, New Zealand
- Sally Roberts, MBChB, FRACP, FRCPA
Sally Roberts, MBChB, FRACP, FRCPA
- Infectious Diseases Physician
- Auckland Hospital
Clinical decisions regarding the management of infections are frequently based on the results of Gram stain and culture. Therefore, it is important that such studies are performed and interpreted correctly.
The quality of the clinical specimen can impact the value of the Gram stain performed. The choice of the specimen sent for Gram stain and culture depends on the site of the infection and the likely pathogens. Good specimen collection should avoid contamination with surrounding colonizing bacteria, provide an adequate volume of material for culture and Gram stain, and be collected as soon as possible after the onset of the infection and prior to initiation of antimicrobial therapy.
Issues relating to the interpretation of Gram stain and culture results are discussed here. Issues relating to the management of specific infections are discussed in detail separately. (See related topics.)
The Gram stain is used to differentiate between different types of bacteria based on the biochemical properties of their cell walls. The method is named after Danish scientist Hans Christian Gram (1853 to 1938), who developed the technique in order to distinguish between two different bacterial causes of pneumonia (Streptococcus pneumoniae and Klebsiella pneumoniae).
Gram staining of clinical specimens (including sterile and nonsterile body fluid specimens, biopsy specimens, and positive culture specimens) is useful for guiding empiric clinical management for bacterial infections pending definitive culture data and/or molecular data. Gram staining also allows visualization of yeast.
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- GRAM STAIN
- - Sterile sites
- - Nonsterile sites
- INTERPRETING GRAM STAIN RESULTS
- Gram-positive organisms
- - Gram-positive cocci
- - Gram-positive bacilli (rods)
- Gram-negative organisms
- - Gram-negative cocci
- - Gram-negative bacilli (rods)
- Medium to long, plump
- Medium to long, thin
- Short to long, pleomorphic
- Growth conditions
- Culture media
- - Blood agar
- Types of hemolysis
- - Chocolate agar
- - MacConkey agar
- - Selective media
- Colony morphology
- Bench tests
- - Coagulase test
- - Catalase test
- - Oxidase test
- - Lancefield grouping
- - Bile solubility
- - PYR hydrolysis
- - MALDI-TOF
- CORRELATING RESULTS