Lab 8: Selective and Differential Media

Question 1

Name the 7 types of selective and differential media used in this lab

Answer 1

1) BA: Blood agar
2) MSA: Mannitol salt agar
3) EMB: Eosin methylene blue
4) MAC: MacConkey’s agar
5) SS: Salmonella-shigella agar
6) LM: Litmus milk
7) TSI: Triple sugar iron

Question 2

Which selective or differential media involves stabbing agar?

Answer 2

Triple sugar iron (TSI)

Question 3

Which selective/ differential media select for G+ coccus?

Answer 3

1) Blood agar
2) MSA (mannitol salt agar)

Question 4

Which selective/differential media select for G+ rods?

Answer 4

1) LM (litmus milk)

Question 5

Which selective/ differential media select for G- rods?

Answer 5

1) SS: Salmonella-shigella agar
2) MAC: MacConkey’s agar
3) TSI: Triple sugar iron

Question 6

What are the 3 potential results of a BA plate, and what does each result indicate?

Answer 6

1) Nothing: Gamma hemolysis
2) Blood cells are lysed (agar becomes browner and greener around colonies): Alpha hemolysis
3) Blood cells are lysed and degraded (clearing): Beta hemolysis

Question 7

1) What makes up MSA (mannitol salt agar) media?
2) What inhibits most G+ rods from growing on this media? What genus is the exception?
3) Which species are found to ferment with this test and which don’t?
4) Give an example of a positive result and negative result and the color of each

Answer 7

1) Mannitol, NaCl, and phenol red
2) High NaCI inhibits most G+ rods; Bacillus is the exception
3) S.aureus ferments, S. epi does not
4) B. subtilis ferments and turns acid yellow(+), B. cereus doesn’t and stays red(-).

Question 8

1) What makes up EMB (eosin methylene blue) media?
2) Which ingredients inhibit gram positive cells?
3) What do coliforms produce on this media?
4) What species is dark blue/ green and metallic? Which has pink brown centers?

Answer 8

1) Protein, lactose, sucrose, eosin Y, and methylene blue
2) Eosin Y and methylene blue
3) Coliforms produce dark centered colonies with clear, transparent edges
4) EC is dark blue/ green, EA is larger with pinkish brown centers

Question 9

1) What does E.coli look like on EMB (eosin methylene blue) media?
2) What does E. aero look like on EMB?

Answer 9

1) Dark blue green metallic sheen
2) Larger colonies with pinkish brown centers

Question 10

1) What makes up MAC (MacConkey’s agar)?
2) What two ingredients inhibit G+ cells?
3) What is the indicator in MAC and when does it change color?
4) Which species grow well on this media and what do their colonies look like?

Answer 10

1) Protein, lactose, bile salts, NaCl, neutral red, and CV
2) CV and bile salts inhibit G+
3) Neutral red is the indicator, it turns shades of red based on the fermentation of lactose (turns acidic)
4) Salmonella and shigella grow well; their colonies are transparent/ tannish

Question 11

1) What ingredients make up SS (salmonella-shigella) media?
2) What two species grow well on this media?
3) What color are the species that grow well on this media and why?

Answer 11

1) Protein, lac, bile salts, NaCl, sodium thiosulfate, neutral red
2) Salmonella and shigella grow well
3) They’re transparent/ tannish colonies with dark to black centers due to the generation of H2S from thiosulfate reacting with Fe^2+ and producing FeS

Question 12

1) What are the ingredients in LM (litmus milk) media? Which is the pH indicator?
2) What is produced in this media and what color does it turn?
3) What causes clarifying at the surface and clotting at the bottom of the LM tube?

Answer 12

1) Skim milk and litmus (pH indicator).
2) Acid is produced by the fermentation of lac, turns from pink to white
3) The breakdown/ modification of casein protein causes clarifying at the surface and clotting at the bottom

Question 13

1) What things make up TSI (triple sugar iron) media?
2) How are TSI plates scored for fermentation and which combination is not possible?
3) What else produced and what color is it?

Answer 13

1) Protein, sodium thiosulfate, phenol red, and 3 sugars
2) Scoring: slant/ butt (A (acidic) or K (alkaline). A/A: all yellow, 2+ sugars. K/A: only glucose. It will never be A/K. Also looking for gas (bubbles and cracks).
3) The production of H2S, Fe = FeS (black precipitate)

Question 14

1) What are the 3 sugars in TSI media?
2) What is the indicator in TSI media?

Answer 14

1) LAC (1%), SUC (1%), GLU (0.1%)
2) Phenol red

Question 15

1) What do selective media typically contain?
2) Selection by selective media often occurs based on the _______ ______ type the bacteria has (G+ or G-)
3) Give 4 examples of additives to selective media

Answer 15

1) An additive that tends to select for a broad group of bacterial genera or species, while inhibiting the growth of other broad groups.
2) Cell wall
3) High salt content, metals, stains, and antibiotics

Question 16

1) What does differential media focus on the separation of?
2) What is the differentiation between species most often made due to?
3) Give 2 examples of additives found in differential media

Answer 16

1) More narrowly defined groups of bacteria; often may be between species within one genus.
2) Differences in metabolism of unique nutritional additives
3) Sugars, amino acids, or other nutrients