exercise 8: Quantification of Bacterial numbers in Milk

a method used to estimate the number of bacteria in a sample (ie. soil, water or food)

outlined by the American Public Health Association

all food contains bacteria
- if numbers are low, food is good
- if numbers are high, food is spoiled

< 20,000 cfu (bacteria)/ml in pasteurized milk

< 300,000 cfu (bacteria)/ml in raw milk

- diseased cow
- poor sanitation of machines
- poor sanitation of milk handlers
- contamination of milk in transport
- pasteurization done incorrectly

- cholera (vibrio cholera): diarrhea, dehydration, lethargy

- typhoid fever (salmonella typhi): high fever, stomach pain, loss of appetite

- dysentery (entamoeba histolytica): nausea, vomiting, bloody stool

- diphtheria (clostridium diptheriae): sore throat, low fever, swollen lymph nodes

- scarlet fever (streptococcus pyogenes): sore throat, fever, rash on neck and face

- cows are given vaccinations to prevent disease
- milk is pasteurized: heated to kill organisms
- milk must also be refrigerated during transport: cold temp slows microbial growth

we add milk to petri dish with agar and count the number of bacterial colonies that grow

pour plate method

colony forming units

A stepwise dilution (occurring from tube to tube) to reduce the number of organisms in an environment

number of CFU/volume plated (ml) x total dilution used ----> number of CFU/ml