what is the population density/cfu for the last three dilutions? I transferred 1

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what is the population density/cfu for the last three dilutions? I transferred 1mL for dilutions to each tube. thank you in advance!

6. Then I took the same pipette, transferred 1 ml of the 10?.1 solution to the 10^-2 tube, inverted it 10 times to mix and did the same transfer and mixing from 10^-2 to 10A3 , 10?-3 to 10^4, and 10A-4 to 10A-5 7. With a new pipette for each tube, from the stock soil solution tube to the 10-1 plate, I had to transfer 0.1 mL (4 drops), ultimately undergoing a 10 fold dilution 8. I had to repeat step 7, transferring 0.1 mL from the stock soil solution tube to the next lower dilution plate until all are diluted 9. I allowed room for the plates to air dry, sealed them with parafim, incubated in a warm area for 2-3 days, observed the plates and recorded which ones were under the category of TNTC or TFTC, as well as determining the population density using the formula below Pop. Density CFU/(dilution x volume plated) 10. Record in table 1, then pour 5mL of 10% bleach into each surface of the plates for 20 minute, rinse with water, parafilm seal them and discard ata Tables/Observations: Table 1: Experiment 1 Growth Results Plate 10-2 10-3 10-4 10-6 Classificati on TNTC TNTC TNTC 40 38 20 CFU/plate

Explanation / Answer

there is a difference between dilution and volume plated on the plate. you have mentioned that you transferred 1 ml for dilution but did not tell in how much volume (means 1ml in 4 ml buffer or 9 ml of buffer). also, you did not mention how much volume form the dilution you spread on the plate.

so I assumed that you did a dilution - 1 ml in 9 ml of buffer so it makes 10 fold dilution. from that and subsequent dilution you took 0.1 ml and spread on the plate. after 3 days you counted the cfu on the plates (I think you wrote the cfu/plate at classification).

if the cfu/plate are 40, 38 and 20 for 10-4 , 10-5 and 10-6 respectively and you have plated 0.1 ml sample on the plate than,

cfu/ml = no of colonies observed / (dilution * volume plated).

for 10-4 dilution- 40/ 10-4 * 0.1 = 400 * 104 = 4*106 cfu/ml

for 10-5 dilution- 38/ 10-5 * 0.1 = 380 * 105 = 3.8*107 cfu/ml

for 10-6 dilution- 20/ 10-6 * 0.1 = 200 * 106 = 2*108 cfu/ml.

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