McFarland Standard: Assessing Microbial Density & Turbidity

McFARLAND STANDARD The number of microbes in a material can be determined in various ways, depending on the material and the type of microbes. There are 2 ways to calculate the number of microbes, namely direct (direct method) and indirect (indirect method).Direct microbial counts are used to show the total number of microbes, both dead and alive. There are several calculation methods, including: usingcounting chamber(hemocytometer); using painting and observation methods; and use a membrane filter. Meanwhile, calculating the number of microbes is indirectly used to determine the number of live or dead microbes or only to determine the number of live ones depending on the method used. The method most often used for indirect calculations is based on the number of colonies (plate count). In calculating this method, several conditions must be met, including: 1. The number of colonies per petridish is between 30-300 colonies, if there are none that meet the requirements, choose a number closer to 300. 2. No colony covers greater than half the area of the petridish, such colonies are known as spreaders. 3. Comparison of the number of bacteria and the results of successive dilutionsbetween the greater dilution and the previous dilution ifequal or smaller than 2 results are averaged; but if it is greater than 2, the microbial count from the previous dilution results is used. 4. If the results are repeated after fulfilling the requirements, the results are averaged.There are 6 types of counting the number of cells, namely: - Microscopic counts - Cup count (TPC) - MPN (Most Probable Number) - McFarland Count - Spectrometry counts - Haemocytometer count A. McFarland Turbidity Standard The McFarland standard is a form of scale that has a size from one to ten, this scale shows the concentration of bacteria per milli liter. This standard was created to estimate the concentration of Gram negative bacteria. McFarland's standard is equalizing microbial concentrations using a BaCl solution21% and H2SO41%. The McFarland turbidity standard is intended to replace individual bacterial counts and to estimate cell density for use in antimicrobial testing procedures.The McFarland standard can be used to determine the approximate concentration of cells in a suspension or solution. The estimated concentration results are in CFU/mL units, and this standard is used to measure the concentration of Gram-negative bacteria such asE. coli. However, its use becomes less accurate when used on other types of bacteria that differ in size and weight, as well as when used on fungi and yeast. Calibration and validation need to be carried out again to obtain correct results.In general, the McFarland standard is placed in a tube labeled number 1 to 10 which is filled with Barium salt solution. Each tube estimated the bacterial density by comparing the McFarland scale number. So, tube number 7 shows a bacterial density at a concentration of 2.1 x 109/ml. To show estimates of bacterial populations, their densities can be compared visually using the McFarland standard. If the density is between scale numbers seven and eight, then the density of bacteria per milliliter is between 2.1 and 2.4 billion/ml. The advantage of using the McFarland standard is that it does not require sufficient incubation time to obtain the desired bacterial density. Meanwhile, the disadvantage is that there will be different views on assessing the level of turbidity of bacterial cellsMcFarland standards are generally labeled 0.5–10 and contain Barium salt solutions. The McFarland standard can be used to visually determine the concentration of cells in a suspension. The McFarland scale was designed to estimate the concentration of Gramnegative bacteria such asE. coli.The disadvantage of the McFarland standard is that the estimation results become inaccurate in the presence of other organisms such as yeast and mold.