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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Enterococcus Identification

The top image shows the Gram stain of Enterococcus species, demonstrating gram-positive cocci (GPC) arranged in pairs and short chains. The viridans streptococci generally form GPC in long chains; the gram-positive cells of S. pneumoniae, on the other hand, may appear similar in Gram stain.
In the lower image (on the right side of the frame) is shown a 6.5% NaCl tube. The turbidity indicates growth, a unique characteristic for Enterococcus species. Also shown in the lower image (in the left frame) is a bile esculin hydrolysis slant, showing a positive reaction (black pigment) characteristic of Enterococcus species. The streptococci are uniformly negative. Some strains of Leuconostoc species and Pediococcus species can grow in 6.5% NaCl and are bile-esculin positive; however, they should be ruled out because of the negative PYR reaction.