Question

Know the names of the following agars, the components of these tests (dyes, indicators, sugars, etc), what organisms they test for, what biochemical reactions they test for, and be able to interpret a result.

Blood agar (5-25)

Bismuth sulfite agar (BSA)

Answer 1

Blood agar

It is an enriched medium which is used to grow fastidious organisms that have complex nutritional requirements. Examples include Streptococcus sp., Neisseria sp., Haemophilus sp. etc. It is composed of components present in nutrient agar in addition to 5% sheep blood with pH of 7.4. It is generally used in detection of hemolytic bacteria having hemolysins. They are differentiated on the basis of their alpha- and beta- hemolytic activity showing changes in color of the medium. If no change is detected, the organism (non-hemolytic bacteria) is said to be possessing gamma- hemolysis activity.

Blood agar can also act as selective medium by addition of dyes and antibiotics. For e.g. blood agar containing crystal violet (dye) or kanamycin (antiobiotic) are used for detection of bacteria present in throat infection and pus, respectively.

Bismuth sulfite agar

It is a selective medium which is used for the isolation of Salmonella sp. and other enteric pathogens that causes gastrointestinal infections. Glucose is used as the primary source of carbon by this medium. It is composed of 1% beef extract, 1% peptone, 1% glucose along with appropriate quantities of sodium phosphate, ferrous sulphate etc. These components help in inhibition of gram-positive organisms and supression of lactose-sucrose fermenting gram-negative organisms, thereby acting in diagnosis of enteric pathogens.