Explain the Most Probable Number (MPN) Techniques?
Different procedures and media may be used depending on the food tested and the information sought. Media which can be used include Brilliant green lactose bile broth, Eosin methylene blue agar, lauryl sulfate tryptose broth, violet red bile agar or lactose broth etc. Presumptive count of coliforms in food is determined using Lauryl Sulphate Tryptose (LST) broth and Violet Red Bile (VRB) agar.
Former helps in detection of microbes that produces gas from lactose (gas in Durham tube) while acid production is detected by VRB agar (purple red colonies surrounded with a precipitate). Additional testing is done by inoculating from the positive presumptive tests into Brilliant Green Lactose Bile (BGLB) broth for confirmation. Gas in Durham tube confirms the test. Samples from the positive confirmed test are then streaked on to Eosin Methylene Blue (EMB) agar for completing the test. Lactose fermenters produces the black or dark center colonies (Positive result). Black colonies with green sheen are probably of E. coli.