How many bacteria were on the soap? How do they get there?

Question

Assume that after washing your hands, you leave ten bacterial cells on a new bar of soap. You then decide to do a plate count of the soap after it was left in the soap dish for 24 hours. You dilute 1 g of the soap 1:10 to the 6th power and plate it on a standard plate count agar. After 24 hours of incubation, there are 168 colonies. How many bacteria were on the soap? How did they get there?

Answer 1

if there were 168 colonies, that means that there were 168 bacterial cells that went onto the agar. if you diluted the original 1g sample 1x10^6, then there were 168,000,000 bacterial cells in the 1g sample. assuming that the bacteria were equally spread out on the bar of soap (and assuming that they could survive there), then the number of bacterial cells on the soap at the time the sample was taken would be equal to 168,000,000*(total grams of soap). they got there from the person touching the soap

Answer 2

Since your plate represents 1x10-6 gram of soap, and a colony is considered derived from a bacterium, you have 1.68x10^8 bacteria/gram.

NOTE: Since contamination is a surface feature, unless you have bacteria that eat soap, a 1g sample is not appropriate. You should base it on area. Since bacteria can double in a hour, if not less, 2^24 is about 1x10^7. So a count of 1.7x10^8 just from reproduction of the surviving 10 bacteria is possible.

Answer 3

The soap isn't bacteriocidal, man!