Correlation between a direct Gram-stained smear of the specimen and what grows in culture should always be attempted. However, some bacterial organisms may appear differently when viewed in direct clinical specimens versus isolates growing on solid or in liquid culture media. For example, suppose large amounts of cellular material surround gram-negative organisms in the direct smear. In that case, the cellular material may totally or partially prevent decolorization of the bacteria so that the gram-negative bacteria growing in culture may appear gram-positive or gram-variable on the direct smear.
Correlation helps the microbiologist:
- Select proper (selective) culture media, especially in the presence of mixed flora. Also, inform the microbiologist that an anaerobe may be present in the culture (when indicated by the body site and morphology of the microorganisms on the Gram stain). The microbiologist would be prompted to add appropriate anaerobic media during the culture setup.
- Interpret the smear correctly, using "internal" quality control when direct smear results are compared to culture results.
Correlation with Culture Results - A Note on Safety: Trigger Points
Trigger points are indicators of possible high-risk pathogens that require manipulation in a biosafety cabinet (BSC). Gram stain morphology can be a trigger point whenever a direct smear from a sterile body fluid/body site contains:
- Gram-negative diplococci (rule out Neisseria meningitidis)
- Small gram-negative coccobacilli which stain faintly, and subsequently fail to grow on media (rule out Burkholderia pseudomallei or Burkholderia mallei, Francisella tularensis, Brucella)
- Gram-negative plump rod (rule out Yersinia pestis)
- Large gram-positive rods (rule out Bacillus anthracis)
- Beaded gram-positive bacilli, faint (rule out Mycobacterium)
Remain vigilant. If performing a Gram stain on culture isolates, be wary of the cultures that grow poorly or slowly.
