Culture of bacteria

Culture of bacteria

All living organism required basic nutrients for growth and development. The microorganism,like all another living organism,required nutrients for their growth and development. The nutrient preparation (food material) on or in which microorganism are grown in the laboratory is called culture medium and the growth itself is called a culture. There are different varieties of microorganism in nature. Although they have the same basic requirement, but the nutritional requirements of bacteria vary widely. Thus culture media vary in form and chemical composition. Numerous media have been developed for cultivation. Some media contains solutions of inorganic salts and may be supplemented with one or more organic compounds while other media contain complex ingredients such as extracts or digest of plant and animal tissues.

Bacteria also exhibit wide differences with respect to the physical conditions favouring their growth such as temperature, PH and gaseous environment. For the cultivation of virus and bacteriophages living host are required. Bacteria phase propagates within certain bacterial cells,where the bacterial cell serves as the medium host for the bacteriophage. Media have been classified variously using different criteria like chemical composition, physical state and utility purpose.On the basis of their composition,there are three main types of culture media;

Natural or empirical culture media

Semisynthetic culture media and

Synthetic and chemically defined culture media.

Natural or empirical culture media:

Those media whose exact chemical composition is not known is called natural or empirical culture media. These include milk urine,diluted blood vegetables, juices, meat extracts and infusions peptones.

Semisynthetic culture media:

Those media whose chemical composition is partially known are called semi-synthetic culture media.Any medium which contains agar becomes a semi-synthetic medium.Eg; Potato Dextrose agar(PDA), Czapek-Dox Agar(COA) etc. Agar is a solidifying agent.

Synthetic and chemically defined culture media:

Those media which are composed of special substances of known chemical composition are called synthetic media Eg;Czapek solution, Richard solution etc.

On the basis of physical states, media are used in three forms

Solid culture media

Semi-solid culture media

Fluid culture media

• Solid culture media:

This form of media is used mainly in Petri-dishes as plate cultures.It can also be used in bottles or tubes as stab(deep)or slope cultures. These media usually contain a concentration of 1-2% agar to yield a suitable gel. The solid culture media are used to observe the colonial appearance, shape, size of the colony and the changes in the surrounding medium which help to identify the bacteria and differentiate commensals from pathogens.Eg;Nutrient agar,blood agar,chocolate agar etc.

Study of bacteria : Microscopic characteristics

Cultural characteristics

• Semi-solid culture media:

This form of media is prepared by adding a small amount of agar 0.2-0.5%w/v to a fluid medium. Semisolid media are used mainly as the transport medium,carry-Blair transport medium etc.

• Fluid culture medium:

This form of media does not contain agar or any solidifying agent. The growth of the microorganism in these media is shown by a turbidity in the medium. Fluid culture media are used mainly as enrichment media,biochemical testing media and blood culture media. Eg;Tryptone broth, selenite f broth, Nutrient broth etc.

On the basis of utility purpose,the main types of culture media are;

• Basal media: These are simple media which support the growth of microorganisms that do not have special nutritional requirements.Basal media are also known as general purpose media. These are often used in the preparation of enriched media,to maintain the stock culture of control strains of bacteria and for subculturing pathogens from differential or selective media prior to performing biochemical and serological identification tests.
• Enriched media: These are media that are enriched with whole blood,lysed blood,serum,extra peptones,special extracts or vitamins to support the growth of pathogens that require additional nutrients or growth stimulants Eg;Blood agar,Loeffler serum medium,Tryptone soya agar etc
• Enrichment media: These are fluid media that increases the number of a pathogen by counting enrichments and or substances,that discourage the multiplication of unwanted bacteria. Eg selenite F broth is used as an enrichment medium for salmonella in faeces or urine prior to subculturing on xylose lysine deoxycholate(XLD)Agar or other entire selective media.
• Selective media: These are media which contain substances that inhibit or slow down the growth of microorganism other than the pathogens for which the media are intended. This type of media facilitates the isolation of particular species from a mixed inoculum.In recent years, antimicrobial agents have become increasingly used as the selective agent in culture media. Eg;XLD agar for salmonella and shigella,Butler medium for Campylobacter species
• Differential media: These are media to which indicators,dyes or other substances are added to differentiate microorganism. This type of media can also be known as indicator media as this media incorporate some substance that is changed visibly as a result of the metabolic activities of particular organisms. Eg TCBS agar contains the indicator bromothymol blue which differentiates sucrose fomenting from non-sucrose fermenting vibrio species. Macconkey agar contains the indicator neutral red which differentiates lactose fermenting from a non-lactose fermenting organism of an enterobacter group. Blood agar can also be described as a differential medium when it differentiates haemolytic from non-haemolytic bacteria.
• Transport media: These are mostly semi-solid media that contain ingredients to prevent the overgrowth of commensals and ensure the survival of aerobic and anaerobicthogens when specimens can not be cultured soon after collection. Their use is particularly when transporting microbiological specimens from health centres to the district microbiology laboratory. Eg Cary-Blair medium for preserving enteric pathogens,Amies transport medium for ensuring the viability of gonococci and other pathogens in the specimen collected as scabs.

Some media and their function in microbiology

1. Nutrient Media:Routine culture of bacteria
2. Blood agar:culture of streptococcus,H.influnza,N.goorrhoeae
3. SDA(Sabouraud dextrose agar:)selective media for fungi
4. Alkaline peptone water:Enrichment culture for vibrio species.
5. VRBA(violet red bile agar):Selective media for coliform
6. Robertson’s cooked meat medium:Culture of anaerobes.

SIM(Sulphide,Indole and motility test)media:to test whether the organism is the h2s producer or not,to check its ability to hydrolyze tryptophan and its motility.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.