The page below is a sample from the LabCE course Case Studies in Clinical Microbiology. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Listeria Gram stain

The image demonstrates a Gram stain prepared from an 18 hour old broth culture. The organism is a small, non-spore-forming bacillus measuring between 0.4 to 0.5 um by 1.0 to 3.5 um. Coccobacillary forms, diplobacilli, and bacilli in short chains or in diphtheroidal clusters (upper right in the image) may all be observed in Gram stain preparations. When found in the CSF, the organism may be found both intracellularly and extracellulary.