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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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Gram-Negative Organisms Recovered From Culture

Oxygen Requirement/Gram Stain CharacteristicsCommon SpeciesCharacteristic Gram Stain Morphology/Comments
Aerobic, gram-negative cocci (GNC)
Moraxella catarrhalis
  • Gram-negative diplococci (pairs) with adjacent sides flattened (coffee bean shaped)
Aerobic, gram-negative rods (GNRs) and coccobacilli
  • Pseudomonas: Straight, slender, gram-negative rods (GNRs).
  • H. influenzae: Small , pale staining, coccobacilli or short GNRs.
Anaerobic, GNRs
  • B. fragilis: Pale-staining, pleomorphic rods with rounded ends.
  • F. nucleatum subspecies nucleatum: Pleomorphic GNRs with round to tapered ends.
Facultative anaerobic GNRsEnterics (eg, Escherichia, Klebsiella, Proteus, Enterobacter, Salmonella, Shigella)
  • Typically, the enterics are plump GNRs.
  • There is not a unique morphology that can be used to distinguish between the enteric organisms.