lab 19: microorganisms in food: plate count method and serial dilutions

reservoir

a place where infectious agents can multiply (food)

plate count method

approximate the number of microorganisms in a reservoir

CFU

colony forming units (number of colonies)

The four reservoirs for infectious microbes that cause diseases in humans are:

The respiratory tract

The gastrointestinal tract

The urogenital tract

The skin and mucous membranes

To calculate the number of CFU per gram:

Plate count = 89 colonies

Dilution factor = 1:1,000

Number of CFUs per gram = 89 colonies × 1,000 = 89,000 CFUs per gram

Plates show 32 and 9 colonies respectively.

Total colonies = 32 + 9 = 41

Total plates = 2

Average colonies per plate = 41 / 2 = 20.5

Since the plates are on a 1:10,000 dilution, the average bacterial count per gram = 20.5 × 10,000 = 205,000 CFUs per gram.