Growth media
The microbiologist has various choices to make if wishing to grow microorganisms in the laboratory. The biomass introduced into the growth medium is recognized as the inoculant and one of the first questions which required to be addressed is whether to inoculate onto solid or into liquid media. The medium will be chosen to reflect the origin of the inoculant that might be a mixed culture of several different species of microorganism or a pure culture of only one species. Solid media are normally held in circular sterile plastic containers with lids the solidity being provided through agar. The culture is plated onto or streaked the surface of the medium using a sterile wire or plastic loop or a sterile glass spreader respectively.
Microorganisms have an enormous metabolic diversity, so need a medium made up of components to suit. Many organisms can synthesize all their cellular components from energy sources and simple carbon , like as glucose and so will grow on a minimal synthetic or defined medium. Like a medium will contain sources of phosphate, nitrogen, calcium, sulfur, magnesium, iron, and potassium as inorganic salts and may be supplemented with trace elements such as manganese, zinc, boron, cobalt, nickel, copper, molybdenum and chromium. These components are needed in minute quantities through the organism and are used as prosthetic groups in some enzymes. A truly minimal medium will not contain any vitamins but organisms will often grow more quickly if provided with thiamine, riboflavin, nicotinic acid, calcium pantothenate, pyridoxine HCl, biotin, folic acid and vitamin B12. Again these are provided in minute quantities enough to be used during enzyme synthesis but not in sufficient amounts to act as a carbon and/or energy source.
When growing some auxotrophs or bacteria with a requirement for amino acids it may be necessary to supplement the minimal medium with some or all of the 20 possible acids. This is general in pathogens where a protected lifestyle in an animal host has led to the loss of one or all the enzymes involved in amino acid synthesis path- ways. To Identifying amino acid needs is often time-consuming and costly in that case a complex medium can be used.
Complex media are describe in the sense which absolute quantities of buffering ions are added to a solution as well as known amounts of animal, plant or yeast extracts. The general complex medium Lauria Bertoni broth contains 5 g l-1 NaCl, but carbon, energy, trace elements and other growth factors are given by 5 g l-1 yeast extract and 10 g l-1 Tryptone. The Tryptone is casein digested with pancreatic enzymes, so the exact composition in terms of molar concentrations of amino acids, short peptides, and so on is unknown and will vary among manufacturers. Similarly, yeast extract a hydrolysate of baker’s yeast is of undefined composition. Pathogenic bacteria associated with bacteremia are frequently grown on a complex medium containing overall or partially hydrolyzed blood that is not only provides the essential growth factors but can also give an indication of the presence of hemolytic organisms through clear haloes in the blood red medium around colonies.
Liquid media can be placed in a variety of containers appropriate to the oxygen needs of the organism to be grown. Anaerobes and Facultative anaerobes can be grown in bottles with gentle shaking to mix the culture through the more generally used aerophiles are generally grown in batch culture in Erlenmeyer flasks. These flasks are adapted chemistry apparatus, conical flasks among 5 ml and 5 l in volume. They are filled to 10 percent of the total volume so which the liquid medium provides sufficient surface area for oxygen transfer to the culture. The inoculation of solid and liquid media and the transfer of cultures from one container to another without the ingress of contaminating organisms have become known as aseptic method.
Media have been established over the last 100 years in both utility and composition. Differential or Selective media are of a composition which is only allows the growth of one type or group of organisms. Example for, a minimal medium containing methanol as a carbon and energy source will select for methylotrophs those organisms able to use reduced C1 compounds. The medium can be enhanced future to be diagnostic. For example solid Baird Parker medium will allow the growth of only a handful of general including Staphylococcus and Micrococcus but only Staphylococcus aureus will grow as gray-black shiny colonies with a narrow white entire margin surrounded through a zone of clearing 2–5 mm. This egg yolk colony form is used as a first indication of the presence of potential pathogens before more detailed tests are carried out.
Many microbiological media are adapted to the study of aerobes but the true anaerobes, particularly those which are damaged through exposure to oxygen requirement special culture conditions. The total exclusion of oxygen is hard, but a series of methods named after their inventor achieve this.