Bacteria Lab Techniques

Question 1

What do liquid mediums look like/contain?

Answer 1

Straw coloured. Nutrients, containg meat infusions and Peptone (peptic enzymatic digestion of meat)

Question 2

What is traditional name for liquid medium?

Answer 2

Broth

Question 3

Solid medium is made from…

Answer 3

Liquid medium + Agar

Question 4

What biological substance is commonly added to Solid medium?

Answer 4

Blood - which makes Blood Agar.

Question 5

Would you inocculate a specimen onto MacConkey or Blood agar first? Why?

Answer 5

Blood Agar, because MacConkey may transfer over inhibitor.

Question 6

What important properties do transport mediums have? (4)

Answer 6

Moisture, Neutral pH, Reducing environment, Stops diffusion of oxygen.

Question 7

What is selective in MacConkey medium?

Answer 7

Contains bile salts, which some bacteria can survive in but others cannot.

Question 8

Which bacteria can readily grow in MacConkey agar?

Answer 8

Bacteria normally in GIT.

Question 9

What is the sugar in MacConkey’s medium?

Answer 9

Lactose

Question 10

What colour would E.coli ferment on MacConkey’s agar? Why?

Answer 10

Pink/Magenta. The E.coli ferment lactose and produce acid - turning indicator (Neutral Red) pink.

Question 11

What colour would non-lactose fermenters be on MacConkey’s agar?

Answer 11

Yellow

Question 12

What is XLD?

Answer 12

A selective medium used to differentiate between E.coli and Salmonella. E coli are yellow, Salmonella show up black because of H2S.

Question 13

What enzyme can used to be distinguish between Streptococcus and Staphylococcus?

Answer 13

Catalase - on hydrogen peroxide medium.

Question 14

What is the FIRST step when identifying bacteria on a culture plate?

Answer 14

Gram stain.

Question 15

What enzyme is used to distinguish pathogenic Staphylococci from non-pathogenic? Why use plasma not serum?

Answer 15

Coagulase. Plasma has to be used because it contains the clotting factors, serum doesn’t.

Question 16

Why do you use one colony when testing for Antibacterial sensitivity?

Answer 16

So you don’t overwhelm antibiotics

Question 17

What culture mediums are used for sensitivity tests?

Answer 17

Isosentitest or Diagnostic Sensitivity Test (DST) Medium

Question 18

What is put onto sensitivity culture first: bacteria or antimicrobials?

Answer 18

Bacteria.

Question 19

How can you tell if a bacteria is resistant to a antibacterial?

Answer 19

Zones of bacterial death around discs

Question 20

What two enterobacteria can be found in faeces culture?

Answer 20

Campylobacter, Salmonella

Question 21

If there are black colonies on XLD medium, why do you need to do a urease test?

Answer 21

Proteus also make black colonies, but are urease positive.