Agar plate count, also known as standard plate count, is a microbiological technique used to estimate the number of viable bacteria in a sample by culturing them on nutrient agar. It's a widely used method, particularly in food and water testing, to determine the overall bacterial load. The process involves diluting the sample, spreading it on the agar, incubating, and then counting the resulting colonies. Preparation and use Dissolve the medium: Suspend the appropriate amount of PCA powder in distilled water according to the manufacturer's instructions (e.g., 23.5 grams per 1000 ml). Heat to boiling: Ensure the medium dissolves completely. Sterilize: Autoclave at 121°C (15 lbs pressure) for 15 minutes. Cool: Allow the medium to cool to 45-50°C. Inoculate: Pour plate technique (recommended by APHA): Serial dilutions of the sample are prepared, and appropriate dilutions are added to sterile Petri plates. Sterile molten agar is then poured into the plates, and the plates are rotated gently to ensure uniform mixing. Spread plate technique: A smaller volume of diluted sample is spread on the surface of solidified agar plates. Incubate: Invert the plates and incubate at the appropriate temperature and duration, typically 30°C for 72 hours for mesophilic bacteria. Count colonies: After incubation, count the colonies on plates containing 25-250 colonies. Plates with counts outside this range are considered less reliable. Calculate CFU: Multiply the colony count by the dilution factor to determine the CFU per gram or milliliter of the original sample.