Bacterial culture media

Question 1

Growth liquid media

Answer 1

○Allows growth of most organisms.

○Luria broth: tryptone, yeast extract, NaCl.

○Cooked meat broth: cooked meat, dextrose, NaCl, yeast extract, iron filings, hemin, vitamin K.

Question 2

Differential liquid media

Answer 2

○Distinguishes between different media.

○Phenol red lactose fermentation broth: peptone, beef extract, NaCl, phenol red, lactose.
○Colour change observed from red to yellow if organism ferments carbohydrate.

Question 3

Enriched agar

Answer 3

○Contains extra nutrients.

○Blood agar: sheep/horse blood, peptone, beef/yeast extract, agar, NaCl.
Haemolysis may be produced which lyse erythrocytes & degrade haemoglobin.

○Alpha haemolytic: some erythrocyte lysis & reduction of haemoglobin to methaemoglobin.
○Beta haemolytic: complete erythrocyte lysis.
○Gamma haemolytic: no haemolysis.

Question 4

Nutrient agar

Answer 4

○Allows for growth of most organisms.

○Peptone, beef/yeast extract, agar, NaCl, distilled water.

Question 5

XLD agar

Answer 5

○Distinguishes between different organisms.

○Lactose, sucrose, NaCl, sodium thiosulphate, L-lysine, xylose, yeast extract, sodium deoxycholate, agar, ferric ammonium citrate, phenol red.

○Lactose & L-lysine fermenters appear yellow (for ex. E. coli).
○Sodium thiosulphate prevents growth of gram positive bacteria.
○Xylose & L-lysine fermenters appear black (for ex. S. typhimurium).

Question 6

CHROMagar MRSA

Answer 6

○Isolation and differentiation of MRSA.
○MRSA turns rose or mauve.
○MSSA growth is inhibited.
○Other bacteria turn blue, colourless or are inhibited.

Question 7

Mannitol salt agar

Answer 7

○Selective and differential media.
○Digest of casein, enzymatic digest of animal tissue, beef extract, D-mannitol, NaCl, agar, phenol red, oxacillin.
○Differentiates pathogenic Staphylococci from non-pathogenic Staphylococci.
○Pathogenic Staphylococci ferment mannitol and so change colour from red to yellow.
○Non-pathogenic staphylococci won’t ferment mannitol so remain red.

Question 8

Sorbitol MacConkey agar

Answer 8

○Peptone, NaCl, bile salts, sorbitol, crystal violet, neutral red, agar.
○Differentiates non-pathogenic E. coli from E. coli0157.
○Most E. coli ferment sorbitol and produce pink colonies.
○E. coli0157 ferments lactose, not sorbitol, so produces colourless colonies.

Question 9

Chromogenic agar

Answer 9

○Colourful plates for specific organisms.
○UTI brilliance agar:
○Rose gal - detects B-galactosidase activity.
□E. coli form dark pink colonies.
□S.aures form light pink colonies.

○X-Glu - detects B-glucosidase activity.
□K. aerogenes form dark blue colonies.
□E. faecalis form blue-green colonies.

○Tryptophan - detects tryptophan deaminase activity.
□P. aeruginosa form brown colonies.
□P. vulgaris form straw coloured or brown halo.

Question 10

Describe the preparation of culture media

Answer 10

1) First, the right volume of agar powder & distilled water are mixed together in a flask.
2) Autoclave tape is put on top.
3) Flask is placed in autoclave for sterilisation.
4) After autoclaving, agar is cooled to roughly 50 degrees.
5) Extra antibiotics or enrichments are added.
6) Agar is poured into petri dishes aseptically using a Bunsen burner to provide a sterile environment.