Culture
The cultivation of cells in the laboratory. Bacteria and yeasts may be grown suspended in a liquid medium or as colonies on a solid medium; molds grow on moist surfaces; and animal and plant cells (tissue cultures) usually adhere to the glass or plastic beneath a liquid medium. Cultures must provide sources of energy and raw material for biosynthesis, as well as a suitable physical environment.
The materials supplied determine which organisms can grow out from a mixed inoculum. Some bacteria (prototrophic) can produce all their constituents from a single organic carbon source; hence they can grow on a simple medium. Other cells (auxotrophic) lack various biosynthetic pathways and hence require various amino acids, nucleic acid bases, and vitamins. Obligatory or facultative anaerobes grow in the absence of O2; many cells require elevated CO2. Cultures isolated from nature are usually mixed; pure cultures are best obtained by subculturing single colonies. Viruses are often grown in cultures of a host cell, and may be isolated as plaques in a continuous lawn of those cells. In diagnostic bacteriology, species are ordinarily identified by their ability to grow on various selective media and by the characteristic appearance of their colonies on test media. See also Bacterial growth.
Laboratory cultures are often made in small flasks, test tubes, or covered flat dishes (petri dishes). Industrial cultures for antibiotics or other microbial products are usually in fermentors of 10,000 gallons (37,850 liters) or more. The cells may be separated from the culture fluid by centrifugation or filtration.
Specific procedures are employed for isolation, cultivation, and manipulation of microorganisms, including viruses and rickettsia, and for propagation of plant and animal cells and tissues. A relatively minute number of cells, the inoculum, is introduced into a sterilized nutrient environment, the medium. The culture medium in a suitable vessel is protected by cotton plugs or loose-fitting covers with overlapping edges so as to allow diffusion of air, yet prevent access of contaminating organisms from the air or from unsterilized surfaces. The transfer, or inoculation, usually is done with the end of a flamed, then cooled, platinum wire. Sterile swabs may also be used and, in the case of liquid inoculum, sterile pipets.
The aqueous solution of nutrients may be left as a liquid medium or may be solidified by incorporation of a nutritionally inert substance, most commonly agar or silica gel. Special gas requirements may be provided in culture vessels closed to the atmosphere, as for anaerobic organisms. Inoculated vessels are held at a desired constant temperature in an incubator or water bath. Liquid culture media may be mechanically agitated during incubation. Maximal growth, which is visible as a turbidity or as masses of cells, is usually attained within a few days, although some organisms may require weeks to reach this stage. See also Chemostat; Embryonated egg culture; Tissue culture.
The materials supplied determine which organisms can grow out from a mixed inoculum. Some bacteria (prototrophic) can produce all their constituents from a single organic carbon source; hence they can grow on a simple medium. Other cells (auxotrophic) lack various biosynthetic pathways and hence require various amino acids, nucleic acid bases, and vitamins. Obligatory or facultative anaerobes grow in the absence of O2; many cells require elevated CO2. Cultures isolated from nature are usually mixed; pure cultures are best obtained by subculturing single colonies. Viruses are often grown in cultures of a host cell, and may be isolated as plaques in a continuous lawn of those cells. In diagnostic bacteriology, species are ordinarily identified by their ability to grow on various selective media and by the characteristic appearance of their colonies on test media. See also Bacterial growth.
Laboratory cultures are often made in small flasks, test tubes, or covered flat dishes (petri dishes). Industrial cultures for antibiotics or other microbial products are usually in fermentors of 10,000 gallons (37,850 liters) or more. The cells may be separated from the culture fluid by centrifugation or filtration.
Specific procedures are employed for isolation, cultivation, and manipulation of microorganisms, including viruses and rickettsia, and for propagation of plant and animal cells and tissues. A relatively minute number of cells, the inoculum, is introduced into a sterilized nutrient environment, the medium. The culture medium in a suitable vessel is protected by cotton plugs or loose-fitting covers with overlapping edges so as to allow diffusion of air, yet prevent access of contaminating organisms from the air or from unsterilized surfaces. The transfer, or inoculation, usually is done with the end of a flamed, then cooled, platinum wire. Sterile swabs may also be used and, in the case of liquid inoculum, sterile pipets.
The aqueous solution of nutrients may be left as a liquid medium or may be solidified by incorporation of a nutritionally inert substance, most commonly agar or silica gel. Special gas requirements may be provided in culture vessels closed to the atmosphere, as for anaerobic organisms. Inoculated vessels are held at a desired constant temperature in an incubator or water bath. Liquid culture media may be mechanically agitated during incubation. Maximal growth, which is visible as a turbidity or as masses of cells, is usually attained within a few days, although some organisms may require weeks to reach this stage. See also Chemostat; Embryonated egg culture; Tissue culture.
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