Question

1. I need to know how many cells/mL are in a culture, and I decide to do a plate count. I make dilutions and plate 0.1 mL of each dilution on to TSA plates. I count 20 colonies on a 10^-4 dilution plate. How many cells (CFU’s) per mL are likely in my original culture?

2.

If someone prepares and plates a 10^-6 dilution of an E. coli culture, this is a _____ dilution.

		1:100
		1:10,000
		1:1,000,000
		1:1,000

3.

You are looking at a slide with a sample that you Gram stained. You’re looking at Staphylococcus epidermidis, which your lab manual says should be Gram positive. Your sample looks like pinkish-red clumps of cocci. Your instructor confirmed with the lab prep staff that your culture was S. epidermidis.

Explain whether this sounds correct to you, and if it does not, what could be an explanation for why not.

4.

Why do we call the standard plate count method a viable count?

What is one drawback to using a viable count instead of a direct counting method such as a microscope count to estimate microbial population number?

Answer 1

1- CFU/ml is given by - number of colonies per plate *1/ volume plated * 1/ dilution factor

20 * 1/0.1 * 1/ 10^-4

20 * 10 * 10⁴

2 * 10⁶ CFU/ml

2- 10^-6 dilution means - 1:1,000,000

3- THis is not sound correct to me as it might be possible that we didn't use the grams' iodine properly such hath the dye was released from the cell during the decolourising process. So when the cells were treated with safranin the cells appear pink in colour and gave us the false negative result

4- In case of viable count method we need to plate different dilution so that we get the colony in 30-300 range. Also in case of viable count method we need to wait overnight for the result whereas in case of microscopic count we can result within some time