Homework Sourse1 Your first task will be to enumerate the number of viable
1. Your first task will be to enumerate the number of viable cells in your stock culture flask so you may use them for your experiment. To do so, you make a serial dilution (10 fold for each step) of the stock culture, and from each dilution tube in the series you plate 0.1 ml on a separate nutrient agar plate via the spread plate method, in duplicate. Here are your counts from the dilution series, as colony forming units (cfu, the name for individual colonies in a plate count): Tube Count (cfu) 1 10-1 TNTC; TNTC 2 10-2 322; 310 3 10-3 71; 52 4 10-4 9; 13 5 10-5 1; 3 Using this information, calculate the viable concentration of cells in the stock flask. Remember to think about which plate counts are accurate and use the replicates appropriately.
Solution
Answer:
Colony forming units (cfu) are defined as the number of microorganisms that able to form colonies when cultured in a growth medium using spread plates or pour plate method. It is an indication of the number of viable microorganisms in a sample. And calculated as cfu/ml for liquid sample or cfu/g for a solid sample.
To enumerate the number of viable cells in the stock culture flask (liquid medium), we have to find out the countable colonies (i.e. between 30-300) from the obsereved results of different dilution series.
As per the given conditions in the question, the different counts from the 5 respective dilution series are given below:
TNTC 1: 10-1
TNTC 2: 322 and 310 at 10-2
TNTC 3: 71 and 52 at 10-3
TNTC 4: 9 and 13 at 10-4
TNTC 5: 1 and 3 at 10-5
The countable colonies are found at 10-3 dilution (i.e. 71 and 52). And the average of the two value is approximately 62.
The original number of viable cells in the sample would be calculated from the number of colonies formed and the sample dilution. However, from each dilution tube in the series, we are plating 0.1 ml on a separate nutrient agar plate via the spread plate method. Therefore, the number of viable cells in the stock culture flask will be calculated from the given formula:
cfu/ml = (no. of countable colonies x dilution factor) / volume of culture plate
No. of countable colonies= 62
Dilution factor = 1/dilution = 103
Volume of culture plated = 0.1 ml
Thus, cfu/ml = (62 x 103)/0.1
cfu/ml = 6.2 x 105
