![Microbial cultures fridge](https://upload.wikimedia.org/wikipedia/commons/0/0f/Microbial_cultures_fridge.JPG)
A microbiological culture, or microbial culture, is a method of multiplying
microbial organisms by letting them reproduce in predetermined
culture medium
A growth medium or culture medium is a solid, liquid, or semi-solid designed to support the growth of a population of microorganisms or cells via the process of cell proliferation or small plants like the moss ''Physcomitrella patens''. Different ...
under controlled laboratory conditions. Microbial cultures are foundational and basic diagnostic methods used as a research tool in
molecular biology
Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including biomolecular synthesis, modification, mechanisms, and interactions. The study of chemical and physi ...
.
The term ''culture'' can also refer to the microorganisms being grown.
Microbial cultures are used to determine the type of organism, its abundance in the sample being tested, or both. It is one of the primary
diagnostic
Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines, with variations in the use of logic, analytics, and experience, to determine " cause and effect". In systems engine ...
methods of
microbiology
Microbiology () is the scientific study of microorganisms, those being unicellular (single cell), multicellular (cell colony), or acellular (lacking cells). Microbiology encompasses numerous sub-disciplines including virology, bacteriology, prot ...
and used as a tool to determine the cause of
infectious disease
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...
by letting the agent multiply in a predetermined medium. For example, a
throat culture is taken by scraping the lining of tissue in the back of the throat and blotting the sample into a medium to be able to screen for harmful microorganisms, such as ''
Streptococcus pyogenes'', the causative agent of strep throat. Furthermore, the term culture is more generally used informally to refer to "selectively growing" a specific kind of microorganism in the lab.
It is often essential to isolate a pure culture of microorganisms. A pure (or ''axenic'') culture is a population of
cells or
multicellular organisms growing in the absence of other
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
or types. A pure culture may originate from a single cell or single organism, in which case the cells are genetic
clones of one another. For the purpose of gelling the microbial culture, the medium of agarose gel (
agar
Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from ogonori (''Gracilaria'') and "tengusa" (''Gelidiaceae''). As found in nature, agar is ...
) is used. Agar is a gelatinous substance derived from
seaweed
Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such as ...
. A cheap substitute for agar is
guar gum, which can be used for the isolation and maintenance of
thermophiles.
Bacterial culture
![Anthrax culture](https://upload.wikimedia.org/wikipedia/commons/4/40/Anthrax_culture.jpg)
There are several types of bacterial culture methods that are selected based on the agent being cultured and the downstream use.
Broth cultures
One method of bacterial culture is liquid culture, in which the desired bacteria are suspended in a liquid nutrient medium, such as
Luria Broth, in an upright flask. This allows a scientist to grow up large amounts of bacteria for a variety of downstream applications.
Liquid cultures are ideal for preparation of an antimicrobial assay in which the experimenter inoculates liquid broth with bacteria and lets it grow overnight (they may use a shaker for uniform growth). Then they would take aliquots of the sample to test for the antimicrobial activity of a specific drug or protein (
antimicrobial peptides
Antimicrobial peptides (AMPs), also called host defence peptides (HDPs) are part of the innate immune response found among all classes of life. Fundamental differences exist between prokaryotic and eukaryotic cells that may represent targets for a ...
).
![Synechococcus cyanobacteria-cultures](https://upload.wikimedia.org/wikipedia/commons/3/3b/Synechococcus_cyanobacteria-cultures.jpg)
As an alternative, the microbiologist may decide to use static liquid cultures. These cultures are not shaken and they provide the microbes with an oxygen gradient.
Agar plates
Microbiological cultures can be grown in
petri dish
A Petri dish (alternatively known as a Petri plate or cell-culture dish) is a shallow transparent lidded dish that biologists use to hold growth medium in which cells can be cultured,R. C. Dubey (2014): ''A Textbook Of Biotechnology For Class- ...
es of differing sizes that have a thin layer of agar-based growth medium. Once the growth medium in the petri dish is inoculated with the desired bacteria, the plates are incubated at the optimal temperature for the growing of the selected bacteria (for example, usually at 37 degrees Celsius, or the
human body temperature
Normal human body-temperature (normothermia, euthermia) is the typical temperature range found in humans. The normal human body temperature range is typically stated as .
Human body temperature varies. It depends on sex, age, time of day, exert ...
, for cultures from humans or animals, or lower for environmental cultures). After the desired level of growth is achieved, agar plates can be stored upside down in a refrigerator for an extended period of time to keep bacteria for future experiments.
There are a variety of
additives
Additive may refer to:
Mathematics
* Additive function, a function in number theory
* Additive map, a function that preserves the addition operation
* Additive set-functionn see Sigma additivity
* Additive category, a preadditive category with fi ...
that can be added to agar before it is poured into a plate and allowed to solidify. Some types of bacteria can only grow in the presence of certain additives. This can also be used when creating engineered strains of bacteria that contain an
antibiotic-resistance gene. When the selected antibiotic is added to the agar, only bacterial cells containing the gene insert conferring resistance will be able to grow. This allows the researcher to select only the colonies that were successfully transformed.
Agar based dipsticks
Miniaturised version of agar plates implemented to dipstick formats, eg.
Dip Slide, Digital Dipstick
show potential to be used at the
point-of-care
Clinical point of care (POC) is the point in time when clinicians deliver healthcare products and services to patients at the time of care.
Clinical documentation
Clinical documentation is a record of the critical thinking and judgment of a health ...
for
diagnosis
Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines, with variations in the use of logic, analytics, and experience, to determine " cause and effect". In systems engin ...
purposes. They have advantages over agar plates since they are cost effective and their operation does not require expertise or laboratory environment, which enable them to be used at the point-of-care.
Stab cultures
![Stab culture](https://upload.wikimedia.org/wikipedia/commons/e/e3/Stab_culture.png)
Stab cultures are similar to agar plates, but are formed by solid agar in a test tube. Bacteria is introduced via an
inoculation needle or a pipette tip being stabbed into the center of the agar. Bacteria grow in the punctured area. Stab cultures are most commonly used for short-term storage or shipment of cultures.
Culture collections
Microbial culture collections focus on the acquisition, authentication, production, preservation, catalogueing and distribution of viable cultures of standard reference
microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s, cell lines and other materials for research in
microbial systematics.
Culture collection are also repositories of
type strains.
Solid plate culture of thermophilic microorganisms
For solid plate cultures of thermophilic microorganisms such as ''Bacillus acidocaldarius, Bacillus stearothermophilus, Thermus aquaticus'' and ''Thermus thermophilus'' etc. growing at temperatures of 50 to 70 degrees C, low acyl clarified gellan gum has been proven to be the preferred gelling agent comparing to agar for the counting or isolation or both of the above thermophilic bacteria.
[Lin, Chi Chung and Casida, L. E. (1984) GELRITE as a Gelling Agent in Media for the Growth of Thermophilic Microorganisms. Applied and Environmental Microbiology 47, 427-429.]
Viral culture
Virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's 1 ...
and
phage cultures require host cells in which the virus or phage multiply. For bacteriophages, cultures are grown by infecting bacterial cells. The phage can then be isolated from the resulting plaques in a lawn of bacteria on a plate. Viral cultures are obtained from their appropriate eukaryotic host cells. The
streak plate method
In microbiology, streaking is a technique used to isolate a pure strain from a single species of microorganism, often bacteria. Samples can then be taken from the resulting colonies and a microbiological culture can be grown on a new plate so th ...
is a way to physically separate the microbial population, and is done by spreading the inoculate back and forth with an
inoculating loop over the solid agar plate. Upon
incubation, colonies will arise and single cells will have been isolated from the
biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
. Once a microorganism has been isolated in pure culture, it is necessary to preserve it in a viable state for further study and use in cultures called stock cultures. These cultures have to be maintained, such that there is no loss of their biological, immunological and cultural characters.
Eukaryotic cell culture
Isolation of pure cultures
For single-celled eukaryotes, such as yeast, the isolation of pure cultures uses the same techniques as for bacterial cultures. Pure cultures of multicellular organisms are often more easily isolated by simply picking out a single individual to initiate a culture. This is a useful technique for pure culture of
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
, multicellular
algae
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
, and small
metazoa
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
, for example.
Developing pure culture techniques is crucial to the observation of the specimen in question. The most common method to isolate individual cells and produce a pure culture is to prepare a streak plate. The streak plate method is a way to physically separate the microbial population, and is done by spreading the inoculate back and forth with an
inoculating loop over the solid
agar plate. Upon incubation, colonies will arise and single cells will have been isolated from the biomass. Once a microorganism has been isolated in pure culture, it is necessary to preserve it in a viable state for further study and use. Stock cultures have to be maintained, such that there is no loss of their biological, immunological and cultural characters.
See also
*
Colony-forming unit
In microbiology, colony-forming unit (CFU, cfu or Cfu) is a unit which estimates the number of microbial cells (bacteria, fungi, viruses etc.) in a sample that are viable, able to multiply via binary fission under the controlled conditions. Cou ...
*
Blood culture
A blood culture is a medical laboratory test used to detect bacteria or fungi in a person's blood. Under normal conditions, the blood does not contain microorganisms: their presence can indicate a bloodstream infection such as bacteremia or f ...
*
Microbial dark matter
Microbial dark matter comprises the vast majority of microbial organisms (usually bacteria and archaea) that microbiologists are unable to culture in the laboratory, due to lack of knowledge or ability to supply the required growth conditions. Mi ...
*
Microbial Food Cultures
*
Screening cultures
*
Sputum culture
*
Synchronous culture
*
Gellan gum
References
External links
EFFCA- European Food and Feed Cultutes Association. Information about production and uses of microbial cultures as well as legislative aspects.
{{Authority control
Microbiology terms
Cell culture