Procedure: Examining the Smear

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The page below is a sample from the LabCE course Reading Gram-stained Smears From Cultures. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Procedure: Examining the Smear

Examination
  • Gram-stained culture smears are examined using the oil immersion objective of the microscope (total magnification =1000X).
  • Use the lower power objective to focus on the field, then switch to the 1000X oil immersion objective to examine the appearance of the bacteria present.
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.