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Glossary of Methods Used to Measure Bacterial Growth

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Direct Cell Counts
Used to determine total number of cells; can be used for those bacteria that cannot be cultured.
Direct Microscopic Count
Rapid, but at least 10^7 cells must be present to be effectively counted. Counts include living and dead cells.
Cell-counting instruments
Coulter counters and flow cytometers count total cells in dilute solutions. Flow cytometers can also be used to count organisms to which fluorescent dyes or tags have been attached.
Viable Cell Counts
Used to determine the number of viable bacteria in a sample, but only includes those that can grow in given conditions. Requires an incubation period of aprrox. 24 hours or longer. Selective and differential media can be used to enumerate specific species of bacteria.
Plate Count
Time-consuming but technically simple method that does not require sophisticated equipment. Generally used only if the sample has at least 10^2 cells/mL.
Membrane Filtration
Concentrates bacteria by filtration before they are plated; thus can be used to count cells in dilute environments.
Most Probable Number
Statistical estimation of likely cell number; it is not a precise measurement. Can be used to estimate numbers of bacteria in relatively dilute solutions.
Measuring Biomass
Biomass can be correlated to cell number.
Turbidity
Very rapid method; used routinely. A one-time correlation with plate counts is required in order to determine cell number.
Total Weight
Tedious and time-consuming; however, it is one of the best methods for measuring the growth of filamentous microorganisms.
Chemical constituents
Use chemical means to determine the amount of a given element, usually nitrogen. Not routinely used.
Measuring Cell Products
Methods are rapid but must be correlated to cell number. Frequently used to detect growth, but not routinely used for quantitation.
Acid
Titration can be used to quantify acid production. A pH indicator is often used to detect growth.
Gases
Carbon dioxide can be detected using a molecule that fluoresces when the medium becomes slightly more acidic. Gases can be trapped in an inverted Durham tube in a tube of broth.
ATP
Firefly luciferase catalyzes light-emitting reaction when ATP is present.
What are the 4 main types of measuring bacterial growth?
1. Direct Cell Counts
2. Viable Cell Counts
3. Measuring Biomass
4. Measuring Cell Products
What are the two ways of performing Direct Cell Counts?
1. Direct microscopic counts
2. Cell-counting instruments
What are the three ways of performing Viable Cell Counts?
1. Plate Count
2. Membrane filtration
3. Most Probable Number
What are the three ways of measuring biomass?
1. Turbidity
2. Total Weight
3. Chemical constituents
What are the three indicators used to measure cell products?
1. Acid
2. Gases
3. ATP

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