Question

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Part 3: MacConkey agar and EMB agar Watch the following video (LINK) in this folder. You will be answering questions on this video, so please go over the questions first. Go over exercises 4-4 and 4-5. Answer the following questions 1. Define Enterobacteriaceae and coliforms (may have to do some research online). 2. What Enterobacteriacae is EMB and MacConkey agar selective for? 3. What carbohydrate is contained in both EMB and MacConkey agar? 4. What is the inhibitor in MacConkey agar? 5. What is the indicator in MacConkey agar? 6. How do you tell the difference in a fermenter and nonfermenter of lactose on MacConkey agar? 7. In EMB agar, what inhibits the growth of gram positive organisms? 8. What is the indicator in EMB agar? 9. What color on EMB agar is indicative of lactose? 10. What color on EMB agar is indicative of nonfermenters of lactose? 11. Is EMB agar selective, differential or both? 12. Is MacConkey agar selective, differential or both?

Answer 1

Ans 1- enterobacteria - taxonomical family of gram negative bacteria , which are facultative anaerobs and ferment glucose to produce acid . Eg, salmonella , shigella , klebsiella etc

Coliforms - are family of gram negative rods which can ferment lactose sugar to produce acid . Eg E Coli.

2- EMb is used to different E.coli ( coliforms )and enterobacter aerogenes. Gram negative enterobacteria such as Klebsiella , E.coli are selective for mc conkey .

3-- Carbohydrate in mc cokey and EMB agar is lactose sugar, both 10 gm by weight.

4-- Inhibitor of mc conkey agar is bile salts and crystal violet .

5- Indicator of mc conkey agar is neutral red  

( Composition of mc conkey agar is

Peptone ,proteose peptone , lactose sugar , bile salts , sofium chloride, neutral red dye , crystal violet dye ,agar and distilled water )

6- On mc conkey agar a very strong lactose fermenter produces red colored colonies which will be surrounded by bile .A weak lactose fermenter will be either light pink or colorless .while a non lactose fermenter will produce colorless colonies .

7-- Eosin dye and methylene blue act as inhibitors to gram positive bacteria.

8- Differential Indicator in emb agar are also eosin dye and methylene blue .

(Composition of EMB agar is -

Pancreatic digest of gelatin , lactose , eosin Y ,methylene blue, dipottassium phosphate ,fistilled water , agar )

9 & 10 - Strong lactose fermenters produce metallic green colored colonies ,weak lactose fermenters produce brownish pink colored colonies while non lactose fermenters produce either coloreless or transparent colonies or pink colonies.

11- EMB agar is both selective and differential as it is selective for gram negative bacteria and inhibits the growth of gram positive bacteria . And differential as it differentiates between foliforms.

12- Mc conkey agar is also both differential and selective ,fifferentiates between klebsiella and pseudomonas and selective against gram positive bacteria .

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