The spiral system allows microbial concentration estimates over a 3 magnitude order. It is also more accurate than manual colony counters. The Spiral plater, which is extensively used in the Food Safety Testing and Pharmaceutical industries saves time, resources, and is cost effective. It is no wonder then, that for a long time, microbiologists have researched colony counters seeking an instrument that offers faster, more accurate counting techniques. The chief disadvantage is the tedious, repetitive, exacting nature of this task. When the three most conventional colony counter methodologies, pour plate, surface spread plate, and the drop count method, are juxtaposed against the spiral plate method, the disadvantages of the three methods become apparent. N" = CFU count in the opposite 1/8th ring sector = CFUĬonventional Plate Methods vs Spiral Plate/Eddy Jet N' = CFU count in the first 1/8th ring sector = CFU Refer to the table provided with the Spiral Plater to note the dispensed volume in the ring where the count has been performed and the method used.įollow this formula to quickly determine the CFU/ml: Continue by counting the colonies in the same rings or the opposite sector. When a sufficient colony count is achieved, counting can be resumed until the entire number of colonies within this same ring sector have been counted. It is necessary to begin counting at the outermost grid ring inwards towards the center. The colony counting is performed within these ⅛ sectors of the counting grid’s rings. Each ring is again divided into three rings (3c, 3b, 3a, and 4c, 4b, and 4a). In 150 mm plates only 1, 2, 3, and 4 ring sectors are counted. With 100 mm Petri plates,counting is restricted to the 3rd and 4th counting grid ring sections only. When counting a plate which has been spiral plated, the spiral plating grid is divided into 8 equal sections, which are then divided into 4 concentric rings. How would a colony counter work in conjunction with a spiral plate? Use a marker to point each counted colony on the back of the Petri dish. If the spread, pour, or drop methodologies are employed, the colony counting will be self-evident: each colony dot is to be counted just once. Next, the plate will be incubated under conditions deemed appropriate for the microorganism after which time the colonies may be counted. NOTE: This range will vary based on the method employed. To obtain optimal spread plating results, prior to inoculating the plate sample, serial dilution of a minimum of three plates must be performed. Plates that exceed 250 CFUs are too numerous to count or may additionally inhibit some bacterial growth.Ĭonversely, plates with fewer than 25 colonies are not considered sufficient or statistically representational of the sample analyzed. The number of microbes/ml = number of colonies x dilution of the sample.Ĭommon Manual Colony Counter Methodology IssuesĪs noted in the FDA BAM (Chapter 3), the appropriate colony counting range is 25-250 colonies per plate. The general formula to calculate CFU/ml is well known: From a given known volume sample count, the number of CFUs per plate to CFU per ml of bacteria culture or grams of sample, is extrapolated. Efficient automatic colony counters offer an ideal solution.Ĭurrently, there are different methods available for determining the number of colonies (CFU) in a sample of food, drug, or water. A more expeditious process is necessary to optimize these daily routines to save time, increase accuracy, as well as productivity. ![]() Some reasons for inaccuracies are because the colonies themselves can often be very small, or may be disguised by the culture medium’s specified color. Since the manual colony counters’ enumeration process is extensive it can therefore also be very slow. Counting colonies is an essential task, however it is very tedious and manual counts can easily be inaccurate. Increasingly, colony counting and rapid plating methods are being utilized by a greater number of microbiologists in their daily work routines. This makes bacterial colony counting procedures, part of microbiology laboratory workflow, requisites. In a food microbiology laboratory, daily counting of microbial populations in food is a standard procedure.
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