Label the sterile nutrient agar slant with the source of the culture, your initials, and the date.
Sterilize the loop using the bacticinerator.
Using appropriate aseptic technique, remove a loopful of broth from the mixed culture tube.
Insert the loop into the sterile agar slant tube and starting at the base of the slant (closest to the bottom of the tube), very lightly draw the loop in a zig-zag motion up the slant. Do not dig into the agar. Sterilize the loop in the bacticinerator.
Incubate the slant at 37 C for 24 – 48 hours.
In the following lab observe the slant for growth. Record the results in the table on page 30 in the lab book.
How does a count on a plates get converted to CFUs per gram or ml of sample? Let's illustrate the procedure with an example. Imagine that we perform the following experiment:
Five ml of milk are added to 45 ml of sterile broth. From this suspension, two serial, 1/100 dilutions are made, and 0.1 ml is plated onto Plate Count Agar from the last dilution. After incubation, 137 colonies are counted on the plate.
This problem may be illustrated as follows:
The initial dilution is calculated As follows: So the initial dilution is 1/10 or 0.1 or 10 -1 .