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The Miles and Misra Method (or surface viable count) is a technique used in Microbiology to determine the number of colony forming units in a bacterial suspension or homogenate. The technique was first described in 1938 by Miles, Misra and Irwin who at the time were working at the LSHTM. [1] The Miles and Misra method has been shown to be ...
The pour plate technique is the typical technique used to prepare plate count agars. Here, the inoculum is added to the molten agar before pouring the plate. The molten agar is cooled to about 45 degrees Celsius and is poured using a sterile method into a petri dish containing a specific diluted sample.
The spread plate method wherein the sample (in a small volume) is spread across the surface of a nutrient agar plate and allowed to dry before incubation for counting. [ 11 ] The membrane filter method wherein the sample is filtered through a membrane filter, then the filter placed on the surface of a nutrient agar plate.
Spiral plating is used extensively for microbiological testing of food, milk and milk products and cosmetics. It is an approved method by the FDA. [2] The advantage of spiral plating is less plates used versus plating manually because different concentrations are present on each plate.
The plate reader is set to read optical density at 650 nm every 5 minutes for 12 hours, shaking before each reading. Raw data is imported into Microsoft Excel, where the macro VCC Calculate is run to determine the time required for each growth curve to reach a threshold optical density of 0.02.
Each time the loop gathers fewer and fewer bacteria until it gathers just single bacterial cells that can grow into a colony. The plate should show the heaviest growth in the first section. The second section will have less growth and a few isolated colonies, while the final section will have the least amount of growth and many isolated colonies.
A bacterial reference chart is used to determine the number of bacteria in the sample. Appropriate treatment is applied to the water source once abnormal levels of bacterial activity are noticed. Once water treatment is effective the bacterial count produced by the dip slide test should be low, approximately <10 4. [2]
Indicator bacteria are types of bacteria used to detect and estimate the level of fecal contamination of water. They are not dangerous to human health but are used to indicate the presence of a health risk. Each gram of human feces contains approximately ~100 billion (1 × 10 11) bacteria. [1]