Acvtivity 3.2: Physiological
The following tests may be performed on gamma-hemolytic streptococci for identification. All results should be recorded on the lab report. The student should draw a flowchart to indicate the identification scheme.
Bile esculin incorporates two tests. Growth in the presence of 40% bile and the ability to hydrolyze esculin that combines with Fe +3 in the medium to form a black precipitate. Group D streptococci and Enterococcus are tolerant to bile and can hydrolyze esculin.
Streak a loopful of streptococcal colony back and forth on the slanted surface of the bile esculin agar tube. Incubate at 35°C overnight.
Growth on the slant = positive
No growth = negative
Black precipitate = positive
Growth on slant without black precipitate = negative
Enterococcus are able to grow in the presence of 6.5% NaCl but group D streptococci cannot. This key trait distinguishes them from one another.
Inoculate a tube of bain-heart infusion broth containing 6.5% NaCl lightly. Do not make turbidity evident. Incubate at 35°C overnight.
Positive = growth/turbidity = group D Enterococcus
Negative = no growth/clear broth = group D streptococci