Bacterial culture media
One of the most important reasons for culturing bacteria in vitro is its utility in diagnosing
infectious diseases. Isolating a bacterium from sites in body normally known to be sterile is an
indication of its role in the disease process. Culturing bacteria is also the initial step in studying
its morphology and its identification. Bacteria have to be cultured in order to obtain antigens
from developing serological assays or vaccines. Certain genetic studies and manipulations of the
cells also need that bacteria be cultured in vitro. Culturing bacteria also provide a reliable way
estimating their numbers (viable count). Culturing on solid media is another convenient way of
separating bacteria in mixtures.
History:
Louis Pasteur used simple broths made up of urine or meat extracts. Robert Koch realized the
importance of solid media and used potato pieces to grow bacteria. It was on the suggestion of
Fannie Eilshemius, wife of Walther Hesse (who was an assistant to Robert Koch) that agar was
used to solidify culture media. Before the use of agar, attempts were made to use gelatin as
solidifying agent. Gelatin had some inherent problems; it existed as liquid at normal incubating
temperatures (35-37oC) and was digested by certain bacteria.
Composition of culture media:
Bacteria infecting humans (commensals or pathogens) are chemoorganoheterotrophs. When
culturing bacteria, it is very important to provide similar environmental and nutritional conditions
that exist in its natural habitat. Hence, an artificial culture medium must provide all the
nutritional components that a bacterium gets in its natural habitat. Most often, a culture medium
contains water, a source of carbon & energy, source of nitrogen, trace elements and some growth
factors. Besides these, the pH of the medium must be set accordingly. Some of the ingredients of
culture media include water, agar, peptone, casein hydrolysate, meat extract, yeast extract and
malt extract.
Classification:
Bacterial culture media can be classified in at least three ways; Based on consistency, based on
nutritional component and based on its functional use.
1) Classification based on consistency:
Culture media are liquid, semi-solid or solid and biphasic.
A) Liquid media: These are available for use in test-tubes, bottles or flasks. Liquid media
are sometimes referred as “broths” (e.g nutrient broth). In liquid medium, bacteria grow
uniformly producing general turbidity. Certain aerobic bacteria and those containing fimbriae
(Vibrio & Bacillus) are known to grow as a thin film called ‘surface pellicle’ on the surface of
undisturbed broth. Bacillus anthracis is known to produce stalactite growth on ghee containing
broth. Sometimes the initial turbidity may be followed by clearing due to autolysis, which is seen
© Sridhar Rao P.N (www.microrao.com)
One of the most important reasons for culturing bacteria in vitro is its utility in diagnosing
infectious diseases. Isolating a bacterium from sites in body normally known to be sterile is an
indication of its role in the disease process. Culturing bacteria is also the initial step in studying
its morphology and its identification. Bacteria have to be cultured in order to obtain antigens
from developing serological assays or vaccines. Certain genetic studies and manipulations of the
cells also need that bacteria be cultured in vitro. Culturing bacteria also provide a reliable way
estimating their numbers (viable count). Culturing on solid media is another convenient way of
separating bacteria in mixtures.
History:
Louis Pasteur used simple broths made up of urine or meat extracts. Robert Koch realized the
importance of solid media and used potato pieces to grow bacteria. It was on the suggestion of
Fannie Eilshemius, wife of Walther Hesse (who was an assistant to Robert Koch) that agar was
used to solidify culture media. Before the use of agar, attempts were made to use gelatin as
solidifying agent. Gelatin had some inherent problems; it existed as liquid at normal incubating
temperatures (35-37oC) and was digested by certain bacteria.
Composition of culture media:
Bacteria infecting humans (commensals or pathogens) are chemoorganoheterotrophs. When
culturing bacteria, it is very important to provide similar environmental and nutritional conditions
that exist in its natural habitat. Hence, an artificial culture medium must provide all the
nutritional components that a bacterium gets in its natural habitat. Most often, a culture medium
contains water, a source of carbon & energy, source of nitrogen, trace elements and some growth
factors. Besides these, the pH of the medium must be set accordingly. Some of the ingredients of
culture media include water, agar, peptone, casein hydrolysate, meat extract, yeast extract and
malt extract.
Classification:
Bacterial culture media can be classified in at least three ways; Based on consistency, based on
nutritional component and based on its functional use.
1) Classification based on consistency:
Culture media are liquid, semi-solid or solid and biphasic.
A) Liquid media: These are available for use in test-tubes, bottles or flasks. Liquid media
are sometimes referred as “broths” (e.g nutrient broth). In liquid medium, bacteria grow
uniformly producing general turbidity. Certain aerobic bacteria and those containing fimbriae
(Vibrio & Bacillus) are known to grow as a thin film called ‘surface pellicle’ on the surface of
undisturbed broth. Bacillus anthracis is known to produce stalactite growth on ghee containing
broth. Sometimes the initial turbidity may be followed by clearing due to autolysis, which is seen
© Sridhar Rao P.N (www.microrao.com)
