There are many different ways to measure the growth of microbial populations. Most methods of counting are based on indirect or direct counts of tiny samples. After, calculations are used to detirmine the size.Usually the procedure is done indirectly with a series of dilutions, making it possible to estimate the number of bacteria in the original sample.
Plate Counts: this is the most frequently used method. Measures the number of variable cells, although it takes a large portion of time for colonies to form. This is a problem in cases in which it is impossible for the lot to be held for long times. It is assumed, with this method, that each grows and divides to produce a single colony, but is not always the case. Often plate counts are known as CFU or colony-forming units. It is important that only a select amount of colonies develop in the plate or there becomes a risk that the over crowding will stop development. They use serial dilution to help with this problem.
Serial Dilutions: continued dilution of a substance in solution. The factor is always constant in the process so that it is a geometric progression and is logarithmic
Pour Plates and Spread Plates: Pour plate method introduces the bacterial suspension into the petri dish, nutrient agar is poured over it and then mixed in, then finally it is incubated. In this method, colonies grow in the agar as well on the surface. Often the Spread plate method is used though. This process includes adding inoculum to the surface of agar and spreading it around uniformly on the surface with a special rod. This helps to avoid contact with the cells with melted agar.
Filtration: Bacteria can be counted this way when it is small enough in lakes or pure streams. This occurs through the bacteria being filtered out and retained on the filter surface, which then gets transfered to a petri dish of nutrient medium where colonies will rise off the surface.
Most Probable number method: The idea of this method is, the greater the number of bacteria in a sample the more dilution is needed to reduce density so that no bacteria is left to grow in the series tubes. Useful in cases in which the microbes cannot grow on solid media or when they do grow in a liquid differential medium
Direct Microscopic Count: a measured volume of bacterial suspension is placed in a defined area on a slide. Motile bacteria are hard to count using this method and also it is very common, when being counted, for the dead cels to be just as likely to be counted as the living, and finally a large number is needed in order to be able to be counted. Benefis, however, include, no incubation time needed, and instruments can often be used to help count in this method.
Plate Counts: this is the most frequently used method. Measures the number of variable cells, although it takes a large portion of time for colonies to form. This is a problem in cases in which it is impossible for the lot to be held for long times. It is assumed, with this method, that each grows and divides to produce a single colony, but is not always the case. Often plate counts are known as CFU or colony-forming units. It is important that only a select amount of colonies develop in the plate or there becomes a risk that the over crowding will stop development. They use serial dilution to help with this problem.
Serial Dilutions: continued dilution of a substance in solution. The factor is always constant in the process so that it is a geometric progression and is logarithmic
Pour Plates and Spread Plates: Pour plate method introduces the bacterial suspension into the petri dish, nutrient agar is poured over it and then mixed in, then finally it is incubated. In this method, colonies grow in the agar as well on the surface. Often the Spread plate method is used though. This process includes adding inoculum to the surface of agar and spreading it around uniformly on the surface with a special rod. This helps to avoid contact with the cells with melted agar.
Filtration: Bacteria can be counted this way when it is small enough in lakes or pure streams. This occurs through the bacteria being filtered out and retained on the filter surface, which then gets transfered to a petri dish of nutrient medium where colonies will rise off the surface.
Most Probable number method: The idea of this method is, the greater the number of bacteria in a sample the more dilution is needed to reduce density so that no bacteria is left to grow in the series tubes. Useful in cases in which the microbes cannot grow on solid media or when they do grow in a liquid differential medium
Direct Microscopic Count: a measured volume of bacterial suspension is placed in a defined area on a slide. Motile bacteria are hard to count using this method and also it is very common, when being counted, for the dead cels to be just as likely to be counted as the living, and finally a large number is needed in order to be able to be counted. Benefis, however, include, no incubation time needed, and instruments can often be used to help count in this method.