- Side view of the chamber showing the cover glass and the space beneath it that holds a bacterial suspension.
- A top view of the chamber. The grid is located in the center of the slide.
- An enlarged view of the grid. The bacteria in several of the central squares are counted, usually at X400 to X500 magnification.
- Concentration of the cells can be calculated by using the average no. of bacteria the avg. number of bacteria in these squares.
- There are 25 squares covering a part of area of 1 mm 2 , then the entire number of bacteria in 1 mm2 of the chamber is (number/square) (25 squares). The chamber is 0.02 mm deep and thus, bacteria/mm 3 = (bacteria/square) (25 squares) (50).
- The amount of bacteria per cm 3 is 10 3 times this value. For example, imagine the average count per square is 28 bacteria: bacteria/cm 3 = (28 bacteria) (25 squares) (50) (10 3 ) = 3.5X 10 7 .
Electronic enumeration of cell numbers
- In this method of microbial growth measurement, bacterial suspension is kept inside an electronic particle counter, within which the bacteria are passed through tiny orifice 10 to 30 μm in diameter.
- This orifice is then connected to the two compartments of the counter which contains an electrically conductive solution.
- The electrical resistance between two compartments will increases momentarily, when bacterium passes through the orifice. This generates an electrical signal which is automatically counted.
- The main disadvantage of this method is that there is no way to determine whether the cell counted is viable or not.
The plate count method
- This method allows the determination of the number of cells that will multiply under certain defined conditions.
- Plate count method can be done in two ways either by spread plate method or by pour plate method.
- This method of bacterial counting is most commonly used with satisfactory results for the estimation of bacterial populations in milk, water, foods and many other materials.
- This technique has some drawbacks because some relatively heat-sensitive microorganisms may be damaged by the melted agar and will therefore be unable to form colonies
Turbidity estimation of bacterial numbers
- For some experimental work, turbidity is a only practical way of monitoring bacterial growth.
- actical way of monitoring bacterial growth. As bacteria multiply in a liquid medium, the medium becomes turbid, or cloudy with cells.
- Turbidity is the Cloudiness or haziness of a media or fluid caused by large no. of individual particles.
- The instrument used to measure turbidity is a spectrophotometer (or colorimeter).
- Microbial mass can be determined by determination of absorption of light.
- In the spectrophotometer, a beam of light is transmitted through a bacterial suspension to a light-sensitive detector, as the bacterial numbers increase, less light will reach the detector.
- As the population increases, absorbance of the light increases by the cells, so the turbidity also increases. Turbidity can be measured by using an instrument spectrophotometer.
- The absorbance is used to plot bacterial growth.
Determination of nitrogen content
- The major constituents of cell material are protein, and since nitrogen is characteristics part of proteins.
- Bacterial population or cell crop can measure in terms of bacterial nitrogen.
- In this growth can be measured by first harvesting the cells and wash them free of medium and then perform a quantitative chemical analysis of nitrogen.
Determination of dry weight of cells
- For filamentous bacteria and molds, the usual measuring methods are less satisfactory. A plate count would not measure this
increase in filamentous mass. - In plate counts of actinomycetes and molds, it is mostly the number of asexual spores that is counted instead.
- This is not a good measure of growth. One of the better ways to measure the growth of filamentous organisms is by dry weight.
- In this procedure, the fungus is removed from the growth medium,
filtered to remove extraneous material, and dried in a desiccator, it is then weighed. - Growth measurement by measuring cell mass is one of the easiest ways, a known volume of culture sample from the ferment or withdrawn and centrifuged.
- It is the most direct approach for quantitative measurement of a mass of cells.
Counting bacteria by filtration method
- When the number of bacteria is extremely few, as in lakes or relatively pure streams, bacteria are often counted by filtration methods.
- During this technique, a minimum of 100 ml of water are passed through a thin membrane filter whose pores are too tiny to permit bacteria to pass.
- After filtration bacteria are filtered out and present on the surface of the filter. Then filter is transferred to a Petri plate containing a in liquid nutrient medium, where colonies grow from the bacteria on the filter’s surface.
- This method is applied frequently to detection and enumeration of coliform bacteria, which are indicators of fecal contamination of food or water.
Most Probable Number (MPN) Method
- Another method for determining the number of bacteria in a sample is the most probable number (MPN) method.
- This statistical estimating technique is based on the fact that the greater the number of bacteria in a sample, the more dilution is needed to reduce the density to the point at which no bacteria are left to grow in the tubes in a dilution series.
- The MPN method is most useful when the microbes being counted will not grow on solid media (such as the chemoautotrophic nitrifying bacteria).
- It is also useful when the growth of bacteria in a liquid differential medium is used to identify the microbes (such as coliform bacteria, which selectively ferment lactose to acid, in water testing).
- The MPN is only a statement that there is a 95% chance that the bacterial population falls within a certain range and that the MPN is statistically the most probable number.
Reference and sources
- https://www.slideshare.net/TanzirAhmed7/reproduction-and-growth-of-bacteria-by-tanzir
- https://sacmicro.files.wordpress.com/2016/02/bacterial-growth-and-measurement-2017.doc
- https://www.scribd.com/presentation/88673536/Lecture-10-MIC310
- https://quizlet.com/230936447/chapter-6-microbial-growth-flash-cards/
- https://www.biologydiscussion.com/microorganisms/measurement-of-cell-numbers-and-cell-mass-ofmicroorganisms/55160
- https://microbeonline.com/techniques-of-isolation-and-enumeration-of-bacteria/
- http://bs.kaist.ac.kr/~jhkim/Chapter_7(pdf).pdf
- https://fahad-hussain.blogspot.com/
- https://www.preservearticles.com/biology/what-are-the-methods-of-measuring-microbial-growth/28473
- http://ecoursesonline.iasri.res.in/mod/page/view.php?id=5146
- https://www.slideshare.net/SaajidaSultaana/different-phases-of-growth-growth-curve
Measurements of microbial growth
Also Read
- Plasmid: Properties, Types, Replication and Organization
- Types of microscopes
- Water as a Microbial Habitat
- Amino acids: physical, chemical properties and peptide bond
- Vector: properties, types and characteristics
- Overview of lac operon an inducible operon
- Deoxyribonucleic acid (DNA)
- DNA replication in prokaryotes
- Techniques for Identifying Mutants
- Citric acid: Introduction, Fermentation, Recovery and Uses
- Viruses Arise from Genetic Recombination and Mutation
- Omicron: Overview, Transmissible, Vaccination and Severity of disease
