Introduction Microorganism is an organism that is microscopic or submicroscopic, which is too small to be seen under naked eyes. However, the numbers of microorganisms in a given sample are required to know in certain aspect such as dairy industries, diseases investigation, and so on. Because of this, a variety of methods have been developed for the enumeration of microorganisms like direct microscopic counts, filtration and viable plate counts (Jacquelyn G. Black 1999). Among the methods of enumeration, viable plate counts are being used most frequently to measure bacterial populations. In viable plate counts, we are measuring the number of viable cells, unlike the microscopic counts which cannot distinguish live from dead cells. However, it takes some time for the visible colonies to grow (Gerard J. Tortora, 2003).Before doing plate counts, serial dilutions are required. This is because it is hard to count more than 300 colonies on an agar plate if we inoculated directly from the original bacterial suspension or sample without serial dilutions (Kathleen Talaro, 1993). To complete plate counts, we could either use pour plate method or spread plate method and each of them have their advantages and limitations. All the visible colonies are calculated and represented as colony forming units (CFU). Then, the CFU is multiplied with the corresponding dilution factor. As a result, the population of original sample is known. The objectives of this experiment are to learn the process of enumeration to determine the number of microorganisms in a sample and utilize two methods in enumeration which are pour plate method and spread plate method. Pour Plate Method The pour plate, like other viable plate count methods, involves adding a sample to a solid medium that will support microbial growth incubating the plates so that each bacterial cell multiplies to form a colony, and counting the number of colonies that develop. Generally we have no idea of the number of bacteria in a sample, so it is almost always necessary to prepare a dilution series to ensure that you will obtain a dilution containing a reasonable number of bacteria to count. Materials Tap water or pond water sample Nutrient agar pours (15 mL/tube) Sterile dilution water blanks (99 mL) Sterile Petri plates Pipettes (1 mL, sterile) Pipette bulb or mechanical pump Marking pen Water bath at 50°C Methods 1. Six nutrient agar pours are melted in a boiling water bath. After they liquefy, they are mixed and placed in a 50°C water bath until they are ready to use. 2. Three 99 mL dilution blanks are labeled as 10 -2 , 10 -4 , and 10 -6 respectively. Six Petri plates are labeled 10 -2 through 10 -7 . 3. The unknown sample is shaken to ensure an even distribution of microorganisms (generally shaking side to side for 25 times). 1 mL of sample is removed aseptically with a sterile pipette and transfer it to the 10 -2 dilution blank.