Question

Please help me solve these problems. I don't understand

1. Placing a 0.5 ml sample of bacteria into a 4.5 ml sterile water, then transferring a 0.5 ml sample from that dilution to another 4.5 ml sterile water produce a dilution. 2. A technologist in a hospital lab who is trying to determine the concentration of Staphylococcus aureus in a urine sample completed a total plate count. After plating 0.1mL in duplicate, and incubating the plates at 37°C for 24 hours the following data was obtained. What is the CFU/mL of the urine sample? (show work and write your answer in scientific notation.) Dilution Tube Plate A Plate B 10^-6 TNTC TNTC 10^-7 130 150 10^-8 8 12

Answer 1

1. The dilution of the tube can be determined by dividing the amount of sample by the total volume of the solution = 0.5/5 = 0.1. Again 0.5 ml if this sample was added to the total volume was 5ml. So the dilution would be 0.1 * 0.1 = 0.01. So the dilution is 1:100.

2. The number of bacteria in the sample is expressed as colony forming units per ml (CFU/ml). Here, two.plates are given so the number of bacteria would be the mean of the two = 130+150/2 = 140

The CFU/ml can be calculated using the formula:
cfu/ml = (no. of colonies x dilution factor) / volume of culture plate.

So for at 10-7 dilution, Bacteria/ml = (140)*(10^7) / 0.1 = 1.4 * 10¹⁰

The other dilutions show sparce growth so they are not considered.