【正文】 Other metal inos are also needed but usually only at very low concentrations, such as Zn2+, Cu2+, Mo6+, and Co2+. These are often termed trace elements. Not all the biological functions of metal ions are known, but Fe2+, Mg2+, Zn2+, Ca2+, Mo6+, and Cu2+, are known to be cofactors for various enzymes. Most bacteria do not require Na+, but certain marine bacteria, cyanobacteria, and photosynthetic bacteria do require it. For those members of archaeobacteria known as the “ extreme halophiles”, the requirement is astonishing. They can not grow with less than 12~15% NaCl. They require this high level of NaCl for maintenance of the integrity of their cell walls and for the stability and activity of their certain enzymes. VitaminsAll microorganisms need vitamins and vitaminlike pounds. Some bacteria are capable of synthesizing vitamins from other pounds in the culture media, but others can not do so and will not grow unless the required vitamins are supplied in the media. WaterAll microorganisms require water. Nutrients must be in aqueous solution before they can enter the cells. Water is to dissolve and disperse nutrients and to provide a suitable milieu for the various metabolic reactions of a cell. Moreover, the high specific heat of water provides resistance to sudden, transient temperature changes in the environment.2 Nutritional Types of Microorganisms Although microorganisms have great diversity of nutritional requirement, they can be divided into four major groups on the basis of their utilization of energy and carbon sources. Photolithotroph or photoautotrophThey utilize light as energy source, and CO2 as major or even sole source of carbon. Photoorganotroph or photoheterotrophThey require light as energy source and organic pounds as carbon source. CH3 LightCO2 + 2 CHON [ CH2O] + 2CH3COCH3 + H2O CH3 Rhodospirillum Chemolithotroph or ChemoautotrophThey rely on inorganic chemical pounds for their energy and use CO2 as their major, or even sole source of carbon. Chemoorganotroph or ChemoheterotrophThey utilize organic pounds as their energy and carbon Source. Most bateria and all fungi belong to this group.3 Medium, Media or culture mediaum, culture media definition: an aqueous solution containing various nutrients suitable for the growth of microorganisms. Types of media Based on the chemical posiion● Synthetic mediaThe chemical position of every ingredients in the medium is clear.Medium for cultivation of ActinomycetesStarch 20g KNO3 K2HPO4 MgSO4?7H2O NaCl FeSO4?7H2O dH2O 1000ml● Natural mediaThe media containing plex natural raw materials surch as peptones, meat broth, yeast extract, plants and animal materials.Medium for cultivation of fungiPotato 200g sucrose 20g H2O 1000mlMedium for cultivation of bacteriaBeef extract peptone NaCl H2O 1000ml Based on physical status ● Broth media: liquid media● Solid media: The solidifying agent is added to the liquid media. The solidifying agent is usually agar, which at concentrations of ~% forms firm, transparent gels that are not degraded by most bacteria. Silica gel is sometimes used as an inorganic solidifying agent for cultivation of autotrophic bactoria. ● Semisolid media: Agar concentration: ~% Based on special purpose ● Selective mediaThese media provide nutrients that enhance the growth of a particular type of microorganism and do not enhance (and may even inhibit) other types of microorganisms that may be present. For instance, nitrogenfree medium (Ashby) will specifically select the growth of nitrogenfixing bacteria. ● Enriched mediaThese media contain a certain nutrient that promotes and enriches a special type of microorganism. For example, a medium in which cellulose is the only carbon source will enrich the growth of celluloseutilizing microorganism. During the process of cultivation, the noncelluloseutilizing microorganisms are eliminated gradually. This is especially useful for isolation of a bacterium when the number is very low in the sample. ● Differential mediaThe media that contain some kinds of ingredients which can differentiate various kinds of microorganisms. For example, if a mixture of bacteria is inoculated onto a bloodcontaining agar medium, some of the bacteria may hemolyze the red blood cells。 others do not. Thus one can distinquish between hemolytic and nonhemolytic bacteria on the same medium. Preparation of media The principles● The nutrients in the medium with suitable concentration and ratia must satisfy the normal growth of microorganisms.● Optimum pH for growth● In large scale protuction, the source and price of the ingredients in the medium must be considered. The procedures● Each ingredient with correct amount is dissolved in the appropriate volume of distilled H2O。● The pH of the fluid medium is determined with a pH meter and adjusted if necessary?！?If a solid medium is desired, agar is added and the medium is boiled to dissolve the agar?！?The medium is sterilized, generally by autoclaving. Some specific ingredients that are heatlabile are sterilized by filtration.4 Transport of nutrients Passive diffusionPassive diffusion is the process that solute molecules cross the membrane as a result of a difference in concentration of the molecules across the membrane. The difference in concentration ( higher outside the membrane than inside) governs the rate of inward flow of the solute molecule. With time, this concentration gradient diminishes until equilibrium is reached.Except for water and some lipidsoluble molecules, few pounds can pass through the cytoplasmic membrane by passive diffusion. In passive diffusion, no substance in the membrane interacts specifically with the solute molecule, a