Question

You have just isolated bacteria from the back of your throat. How can you most quickly determine if you have streptococci or staphylococci?

	A.	Hemolysis on blood agar
	B.	Gram stain
	C.	Coagulase test
	D.	Catalase test

A positive phenylalanine test results in a ______color in the tube.

		red
		yellow
		green
		purple

What type of medium is used in the Kirby-Bauer method?

	A.	Chocolate agar
	B.	Mueller-Hinton
	C.	Tryptic soy agar
	D.	Eosin methylene blue agar

What two sugars must be present in Kligler's iron agar to yield the expected results?

	A.	Lactose and glucose
	B.	Starch and glucose
	C.	Sucrose and glucose
	D.	Lactose and sucrose

Answer 1

1). Ans D. Catalase test.

Hemolysis on blood agar- Takes 24 to 48 hours.

Gram stain- Both Staphylococcus and Streptococcus are gram positive. Hence cannot be differentiated.

Coagulase test- Cannot differentiate between Staphylococcus and Streptococcus. This is employed to identify different species of Staphylococcus.

Catalase test- Staphylococcus is catalase positive and Streptococcus is catalase negative. Also it can be done in shorter time.

2). Ans. Green

Some organisms produce the enzyme deaminase which converts phenylalanine into phenylpyruvic acid. In a positive phenylalanine deaminase test, the phenylpyruvic acid thus formed forms a compound with iron of ferric chloride that gives green colour.

3). Mueller-Hinton agar

Kirby-Bauer method, otherwise known as disk diffusion method is used to find antibiotic susceptibility of organism and thus administer suitable antibiotics. Mueller-Hinton agar is used in the procedure.

4). Lactose and Glucose

In Kligler's iron agar test Lactose and Glucose are present. It is used to identify enteric bacilli which can ferment both the sugars. This process also releases hydrogen sulphide that gives black colour.