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A diagram of serial dilution. A large beaker on the left contains a dark solution. 1 ml is moved from this beaker to a tube containing 9 ml of broth. This tube has a dilution of 1:10 and is lighter in color than the original beaker. A sample of 0.1 ml from this tube is put on an agar plate; the colonies are too numerous to count. 1 ml is taken out of this tube and placed in a tube containing 9 ml of broth. This tube now has a dilution of 1:100 from the original beaker and is even lighter in color. 0.1 ml is plated on an a agar plate and the colonies are still too numerous to count. 1 ml is taken from this tube and placed in another tube containing 9 ml broth. This is now a dilution of 1:1000 from the original beaker and the tube is lighter than the last. 0.1 ml is taken out of this tube and placed on an agar plate; there are 389 colonies. 1 ml is taken out of this tube and placed in another tube containing 9 ml broth. This is now a dilution of 1:10,000 from the original beaker and this tube is even lighter than the last. 0.1 ml is taken out of this tube and placed on an agar plate; there are 50 colonies. 1 ml is taken out of this tube and placed in a tube containing 9 ml of broth. This is a dilution of 1:100,000 from the original beaker and this is the lightest tube of all. 0.1 ml is taken from this tube and placed on an agar plate; there are 2 colonies.
Serial dilution involves diluting a fixed volume of cells mixed with dilution solution using the previous dilution as an inoculum. The result is dilution of the original culture by an exponentially growing factor. (credit: modification of work by “Leberechtc”/Wikimedia Commons)

The dilution factor is used to calculate the number of cells in the original cell culture. In our example, an average of 50 colonies was counted on the plates obtained from the 1:10,000 dilution. Because only 0.1 mL of suspension was pipetted on the plate, the multiplier required to reconstitute the original concentration is 10 × 10,000. The number of CFU per mL is equal to 50 × 100 × 10,000 = 5,000,000. The number of bacteria in the culture is estimated as 5 million cells/mL. The colony count obtained from the 1:1000 dilution was 389, well below the expected 500 for a 10-fold difference in dilutions. This highlights the issue of inaccuracy when colony counts are greater than 300 and more than one bacterial cell grows into a single colony.

A diagram of the pour plate method. Step 1 – the bacterial sample is mixed with warm agar (45-50° C). Step 2 – the sample is poured onto a sterile plate. Step 3 – the sample is swirled to mix and allowed to solidify. Step 4 – the plate is incubated until bacterial colonies grow.
In the pour plate method of cell counting, the sample is mixed in liquid warm agar (45–50 °C) poured into a sterile Petri dish and further mixed by swirling. This process is repeated for each serial dilution prepared. The resulting colonies are counted and provide an estimate of the number of cells in the original volume sampled.
A diagram of the spread plate method. Step 1 – a sample (0.1 ml) from a bacterial dilution is poured onto a solid medium. Step 2 – the sample is spread evenly over the surface. Step 3 – the plate is incubated until bacterial colonies grow on the surface of the medium.
In the spread plate method of cell counting, the sample is poured onto solid agar and then spread using a sterile spreader. This process is repeated for each serial dilution prepared. The resulting colonies are counted and provide an estimate of the number of cells in the original volume samples.

A very dilute sample—drinking water, for example—may not contain enough organisms to use either of the plate count methods described. In such cases, the original sample must be concentrated rather than diluted before plating. This can be accomplished using a modification of the plate count technique called the membrane filtration technique . Known volumes are vacuum-filtered aseptically through a membrane with a pore size small enough to trap microorganisms. The membrane is transferred to a Petri plate containing an appropriate growth medium. Colonies are counted after incubation. Calculation of the cell density is made by dividing the cell count by the volume of filtered liquid.

The most probable number

The number of microorganisms in dilute samples is usually too low to be detected by the plate count methods described thus far. For these specimens, microbiologists routinely use the most probable number (MPN) method , a statistical procedure for estimating of the number of viable microorganisms in a sample. Often used for water and food samples, the MPN method evaluates detectable growth by observing changes in turbidity or color due to metabolic activity.

Questions & Answers

describe the process of platelet formation
Joy Reply
what is parasitic helminths
Sadiya Reply
list three categories of symbiotic relationships.
mary Reply
what's the difference between microbial intoxication and infectious diseases
dranimva Reply
microbial intoxication results when a person ingests a toxin or a poisonous substance that has been produced by a microbe while infectious disease results when a pathogen colonize the body and subsequently cause disease.
mary
weighing balance is needed in lab of microbiology for weighing?
amna Reply
It is mainly used for media preparation and product testing purpose.
Nakaweesi
thank you for your joining
nuur
what is microbiology
Shamsuddeen Reply
study of living organisms that are too small to be visible with naked eye
Marah
yes
Ahmed
microbiology is the study of microscopic organisms, either unicellular(single cell), multicellular( cell colony), or acellular (lacking cell).
munachimso
is the word Atypical or a typical bacteria. .am confused pliz help
MUWANGUZI Reply
typical bacteria
Zulpha
okay thank you what does that mean
MUWANGUZI
atypical means that it has some characters from bacteria not all characters ...but tybical means that it has all the characters that bacteria have
Reham
thank you Zulpha
MUWANGUZI
some examples please
MUWANGUZI
thank you Reham
MUWANGUZI
1) typical bacteria contain a cell wall whereas atypical bacteria usually do not contain a cell . 2) typical bacteria can be either Gram-positive or Gram-negative while atypical bacteria remain colorless with Gram staining. 3) cells of typical bacteria are large ,while cells of the atypical small
Marah
Example of atypical : Mycoplasma pneumoniae , chlamydophila pneumoniae , legionella
Marah
what is micro biology
Jauharah Reply
is the study of organisms which can't be viewed by our necked eyes
Egumat
Because it preexisting causing secondary infection after collateral damage of normal microbota
Rafaa Reply
combinations of drugs that can't be taken together and why
Grace Reply
Antagonism: the combined action is less than that of the more effective agent when used alone). All these effects may be observed in vitro (particularly in terms of bactericidal rate) and in vivo والله اعلم
Lenovo
....fermentros have 1-15litre capacity
AMAR Reply
which of the following microorganisms are classified as eukaryotic?
Semugab Reply
how do I see the list?
Melissa
what are the choices?
Melissa
dear tell us the choices
MUWANGUZI
mr semugab give us the list please
Nambi
Fungi
munachimso
what are the new discoveries of microorganisms
Ezibon Reply
bacteriology,viriology,micrology
Egumat
understanding of contributed to attempts to treat contain disease
james Reply
what is a bacteria
ROSE Reply
yes tell us
Judith
a bacteria is the largest living organisms on the planet Earth
Egumat
oh, sure. we have also one big here nearby!
Maxim
Bacteria is a large group of single-cell microorganisms. some causes infections and disease in animals and humans..
munachimso
do you mean procariots?
Maxim

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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