You are given three stock cultures – Escherichia at a concentration of 1.5 x 10

Question

You are given three stock cultures – Escherichia at a concentration of 1.5 x 103 cfu/ml, Bacillus at a concentration of 4.2 x 103 cfu/ml, and Staphylococcus at a concentration of 2.4 x 104 cfu/ml. You have been asked to make 30 ml of a single mixed culture with a final concentration of 500 cfu/ml of Escherichia, 150 cfu/ml of Bacillus, and 80 cfu/ml of Staphylococcus. Show how you would make this mixture.

a. If you plate 0.4 ml of the mixture in part d on a TSA plate, how many colonies each of Escherichia, Staphylococcus, and Bacillus would you expect to get?

4 b. If you plate 0.4 ml of the mixed culture above on a MacConkey plate, how many total colonies would you expect to get?                                                                                                                                       

Explanation / Answer

To get a required cell concentration, take 10 ml of E. coli culture, 1.071 ml of Bacillus culture, 0.1 ml of Staphylococcus culture, add 18.829 ml of culture or broth. The calculations are as follows:

* E. coli concentration in the stock = 1500 cfu/ml

* E.coli cells required in the mixed culture = 500 cfu/ml in a 30 ml culture = 15000 cfu shall be present in the mixed sample

* Volume of E.coli stock culture that contains 15000 cfu = 10 ml

* Each ml of Bacillus stock contain 4200 cfu. Total cfu of Bacillus required in 30 ml mixed culture = 4500. Volume of stock required is 1.071 ml

* Each ml of Staphyloccus contain 24000cfu. Total cfu required in 30 ml of mixed culture = 2400. Amount of stock required = 0.1 ml

a. In 0.4 ml of the mixture, no of E.coli = 200 cfu, Bacillus = 60 cfu, and Staphylococcus = 32 cfu

b. All the three genera ferment lactose with production of acid, in 0.4 ml, 292 cfu are expected

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