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Methods to Detect and Measure Bacterial Growth

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angel martin

on 28 January 2015

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Transcript of Methods to Detect and Measure Bacterial Growth

Direct cell count is useful for observing microorganisms.

Direct Microscopic Count
A liquid specimen is added to a glass slide.

Contains a chamber atop a microscopic grid

The specimen is then viewed under a light microscope
Direct Microscopic Count
Direct microscopic count is one of the most rapid cell count methods.

However, 10 million bacteria are needed to be seen
Direct Cell Count
cloudiness of the substance
is proportional to the number of cells

Direct Cell Count
Viable Cell Count

Measuring Biomass
Viable Cell Count
Plate Counts
Counts the number of viable cells since an isolated microbial cell on a nutrient agar plate will give rise to 1 colony
Ideal number on a plate is 30-300
Generally used only if the sample contains more than 100 cells/ml
Two Techniques:
1. Spread-plate
2. Pour-plate
Determines the number of viable cells in a sample, but that number includes only those that can grow in certain conditions
Requires an incubation period of about 24 hours or longer
Ultimately, establishes the number of cells capable of multiplying
When too many cells produce too many colonies to count, you want to decrease the concentration of cells in a sample by using 10-fold dilutions.
Methods to Detect and Measure Bacterial Growth
BY: The A Team

Angel Aguilar
Angelica Beltran
Alchyle Navarro
Allison Monje
Ashley Gregg
A limitation of using turbidity to measure biomass is that the medium must contain a relatively high concentration of cells to be cloudy.

A turbid culture indicates that microbes are present however a clear solution does not guarantee their absence.

Acid and Gas Production
Once a microorganism goes through
bacterial growth, it can sometimes
create acids and gases as a result.
Is the total mass of organisms in a given area or volume
The plates are incubated in both methods and then the number of colonies is counted.
Total weight of a culture can be measured but is tedious and time consuming

Due to this the biomass is usually used to study only filamentous organisms

Filamentous organisms
Are long visible chains, threads, or filaments that sometimes intertwine
- Detected by a pH indicator added to the
culture medium
- Weak acids can act as natural dyes to indicate
ions in a solution
- The pH value range of the microbe is specific
to the color shown on the pH indicator chart
or key.
- Most bacteria prefer the neutral range from 6 to 8.
- Acidity and alkalinity inhibit most microbial growth
From that number, the concentration of
colony-forming units (CFUs)
in the sample can be determined because of the plate count.
When calculating CFUs, consider:
1. number of colonies
2. amount the sample was diluted before being plated
3. the volume plated
In summary, the measure of viable cells account for the fact that microbial cells often attach to one another and then grow to form a single colony.

There are a variety of probes available to measure turbidity
like the spectrometer and turbidimeter.
The measured turbidity, however, depends on the wavelength of light and the angle at which the detector is positioned.
Pro: Allows us to observe cells that cannot be grown in culture
Con: Difficult to distinguish between living and dead cells
the scale with which turbidity is measured in
which measures the intensity of light scattered as a beam of light passes through a water sample.
- Detected by the use of the Durham tube method, which traps gas bubbles that are produced from the growing microbe.
Measuring Biomass
To measure the wet weight, cells in a liquid culture are centrifuged and the liquid supernate is removed.

The weight of the resulting packed cell mass is proportional to the number of cells in the culture

Counts cells electronically in a liquid specimen.

Specimen pass a narrow channel

A machine is used to count the changes in resistance as bacteria passes through.

Specimen must be electrically conducting.

Bacteria are the nonconducting particles that cause the change.
The dry weight can be determined by heating the centrifuged cells in an oven before weighing them.

Flow Cytometer
Supernate is the clear fluid above a sediment or perticipate
Counts cells as by measuring light

Light scatters as cells pass a laser.

Can count total cells or subsets.

Specific subsets can be measured by staining with a florescent dye and tag.
Membrane Filtration
Most Probable Number (MPN)
Method for estimating the concentration of cells in a specimen.
To determine MPN, 3 sets of 5 tubes are prepared.
They are incubated , the presence or absence of any indication of growth is noted.
A table is used that gives a statistical estimate of cell concentration.
Used for liquid samples
Filtrates concentrated microbes.
Incubate filtered organisms on agar medium, the number of colonies that are formed indicate the number of cells in the volume filtered.
Nester, E. (2012). Dynamics of Prokaryotic Growth. In Microbiology: A
human perspective (7th ed.). New York: McGraw-Hill.

Lesson 8: Filamentous Bacteria. (n.d.). Retrieved January 18, 2015, from

Turbidity -- Units of Measurement. (n.d.). Retrieved January 16, 2015, from

Google Images. (n.d.). Retrieved January 17, 2015, from https:

- To detect CO2 gases,
some clinical labs use a
fluorescent sensor method
to indicate a change in pH
which comes along with the
production of CO2.
NTU stands for nephelometric turbidity units
Detecting Cell Products
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