Question

Throat swab from red inflamed throat – Streptococcus pyogenes, Streptococcus pneumonia and Enterococcus facecalis Which plates are you going to use to culture the material? Choose two and explain use What is the ingredient in the selective plate? What is the ingredient in the differential plate? What would you see if this plate has growing on it? What antibiotic would you suggest (Use the figure from Ch12 in the Cowan book)

Answer 1

Blood agar and bile esculin agar are two differential media used commonly for Streptococcal identification and culture.

Blood agar

The colonies on blood agar differs among all the three bacteria i.e., S pneumoniae shows partail hemolysis called alpha hemolysis. S pyogenes shows clearing zone called as beta hemolysis. E faecalis shows gamma hemolytic colonies.

Blood agar - it is the nutrient agar supplemented or enriched with red blood cells. The different colonies helps us to identify if the bacteria can lyse RBC i.e., presence or absence of hemolysin enzyme could be tested.

Bile esculin agar

Bile esculin agar can help in differentiating Enterococcus from the other Streptococcal group. It is done to test the ability of bacteria to hydrolyse esculin and form esculetin. It is indicated by black colour formation which is due to the reaction between esculetin and iron complex.

Enterococcus faecalis turns the tube black after incubation whereas S pneumoniae and S pyogenes do not.

Antibiotics

Since the given bacterial isolates are Gram positive bacteria , beta lactam antibiotics should be usrd. Penicillin, amoxicillin, etc are antibiotics to which these bacteria show sensitivity.