Both beneficial and harmful microorganisms are present in food. Certain microorganisms are required for the preparation of various dairy and meat products, whereas the presence of harmful microorganisms is responsible for serious outbreak of food poisoning, etc. Similar to the analysis of milk and water, bacterial analysis of food samples and other enteric organisms are carried out for the presence of bacterial pathogens.
- Petri dishes
- Nutrient agar medium
- Sterile water
- Mechanical blender
- EMB agar
- Weigh 10 g of food sample and transfer in 90 ml sterile distilled water (1:10 i.e. 10-1 dilution).
- Keep this mixture in sterile mechanical blender and blend for 5 minutes.
- Transfer 1 ml of water sample from dilution 10−1 into a test tube containing 9 ml sterilised distilled water so as to get another dilution 1:100 (10−2 ). Shake gently for 2 minutes.
- Similarly, prepare 1:1000 (10−3 ) dilution by transferring 1 ml mixture from 10−2 dilution to a test tube containing 9 ml distilled water.
- Transfer aliquots of 1 ml from 10−2 and 10−3 dilution separately into sterile Petri dishes in triplicate and pour 15 ml of sterilised nutrient agar medium in each plate.
- Mean while, prepare a plate containing EMB agar and inoculate it with any of the sample.
- Incubate all the plates at 35ºC for 24 hours.
Examine the plates for the presence of 30 to 300 colonies. Count them and calculate the accurate bacterial population by multiplying the number of colonies with dilution factor. If there is metallic sheen on the EMB agar, it indicates the presence of E. coli and it demonstrates the possibility of faecal contamination in food.
Dr. R. C. Dubey – Practical Microbiology