Microbial culture media preparation is the process of mixing nutrients, agents for buffering and maintaining the osmotic balance, as well as selective inhibitors or indicators to create an agar or broth that supports the growth and the differentiation of microorganisms. Microbial culture media preparation is a routine task in the regular monitoring of spoilage and pathogenic microbes in microbiological testing.
Microbial culture media should provide optimal growth conditions for all or specific types of microorganisms. The precise composition of a medium depends on the species being cultivated and the application goal. The medium pH needs to be adjusted depending on the microorganisms. Microbial culture media is classified based on several parameters, like chemical constituents, physical nature, and function. Types of media defined by these parameters are described below.
Synthetic (Defined) medium is a media with defined chemical commonly used to culture photoautotrophs, such as cyanobacteria and photosynthetic protists. It is widely used in research, to study microorganism metabolism.
Complex media is a media containing non-defined chemical components, such as peptone, meat extract, and yeast extract, that meet the nutritional requirements of different microorganisms. It is used to culture fastidious microbes with complex nutritional requirements.
Solid medium uses 1-7% agar-agar or 10-20% gelatin to solidify the liquid broth. Solid media is used to isolate different microbes from each other, establish pure cultures, make agar slants, and make agar stabs.
Liquid medium does not contain any solidifying agents. After inoculation and incubation, cells become visible in the form of a small mass or broth blurring.
Ready-to-use medium is a solid or liquid medium supplied in plates, bottles, tubes or other containers, in ready-to-use form or ready-to-use after remelting and supplementing.
Medium prepared from commercially dehydrated formulations is a medium in dry form which requires rehydration and processing before use, resulting in either a complete medium or an incomplete medium to which supplements are added before use.
Medium prepared from individual components is a medium produced by a microbiology laboratory entirely from its individual ingredients.
On-demand medium is produced by a system that keeps highly concentrated, sterile culture medium available in a laboratory over several days. By dilution with sterile water it can produce a required amount of ready-to-use medium on demand, without the need for autoclaving.
Media preparation from dehydrated commercial formulations should be performed by following manufacturer’s instructions. The formulation of basic ingredients, like peptones, yeast extracts, agar, buffering substances, and antibiotics, is modified to achieve consistency of the medium. The required amount of dehydrated medium or individual ingredients are dissolved in distilled water by continuous stirring followed by heating (if necessary). Media containing agar should be adequately soaked with proper agitation before heating. The pH must be adjusted, and the medium is dispensed into appropriate containers for sterilization by moist heat in an autoclave. Heat-sensitive substances (e.g., proteins, enzymes) are sterilized by using membrane filters.
Culture media must be stored at specified temperatures to prevent modification of the composition, and no longer than the product shelf-life. Aseptic preparation and storage are essential to protect culture media from microbial infection. Water loss on storage can be minimized by impermeable wrapping and/or storage at 5 °C ± 3 °C. Chemical degradation, e.g. oxidation or antimicrobial loss, can be retarded by protection from light, heat, and dehydration.
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