The ability for a Gram-positive coccus to grow in 6.5% sodium chloride can help in the identification of Enterococci.
Media with 6.5% NaCl
Indicates
able to grow
Enterococcus
not able to grow
Group D Streptococcus (S. bovis, S. equinus)
Incubation is at 35°C for 18-24 hours in ambient air.
Growth is indicated by turbidity in broth or growth on agar. The addition of a color indicator can detect acid from the metabolism of glucose during growth.
Enterococcus is also bile-esculin positive, while Group D Streptococcus is bile-esculin negative.
False positive:
(1) mixed isolate
(2) a heavy inoculum may be misread as growth
(3) Other organisms that can grow media 6.5% salt: Aerococcus, group B streptococcus, few group A streptococci
False negative:
(1) organisms settling to bottom of tube (broth culture)
(2) reading the culture too soon
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