The main reasons why soil is a good condition for microbial nutrition are:
1. Soil contains various nutrients needed by microorganisms.
2. A large amount of litter in the soil can provide necessary nitrogen and carbon sources for microorganisms.
3. A large number of gaps between soils provide necessary oxygen for microorganisms.
Soil is often called the natural culture medium for microorganisms.
There are many kinds of culture media, which can be divided into many kinds according to their composition, physical state and use.
(a) according to the different components.
1, natural media (composite media) This kind of media contains natural organic matter with unclear or unstable chemical composition, which is also called chemically undefined media. Beef paste peptone medium and wort medium both belong to this category. LB (luria-bertani) medium commonly used in gene cloning technology is also a natural medium, and its components are shown in Table 5.9.
Sources and main components of beef paste, peptone and yeast extract
Nutritional beef extract
A pasty substance is obtained by concentrating the tissue extract of Laiyuan lean beef.
The main components are rich in water-soluble sugars, organic nitrogen compounds, vitamins, salts and so on.
Nutritional peptone
Laiyuan is made by hydrolyzing meat, casein or gelatin with acid or protease and drying.
Powdered substance whose main ingredient is rich in organic nitrogen compounds and contains some vitamins and sugar.
Nutritional substance yeast extract
A paste obtained by concentrating a water-soluble extract from yeast cells.
The main ingredients are rich in B vitamins, as well as organic nitrogen compounds and sugars.
Commonly used natural organic nutrients include beef paste, peptone, yeast extract (Table 5. 10), bean sprout juice, corn flour, soil extract, bran, milk, serum, rice straw extract, feather extract, carrot juice, coconut juice, etc. Fecal microorganisms can use fecal water as nutrients. Natural culture medium has low cost, which is not only often used in laboratory, but also suitable for industrial large-scale microbial fermentation production.
2. Synthetic culture medium is a culture medium made of substances with completely clear chemical composition, also known as culture medium with definite chemical composition. Gao's My Media and Cha's Media belong to this type. When preparing synthetic culture medium, it has strong repeatability, but compared with natural culture medium, its cost is higher and the growth rate of microorganisms is slower, so it is generally suitable for laboratory research such as microbial nutrition demand, metabolism, classification and identification, biomass determination, strain breeding and genetic analysis.
(2) according to the physical condition.
According to the existence and content of coagulant in culture medium, culture medium can be divided into three types: solid culture medium, semi-solid culture medium and liquid culture medium.
1, solid medium (so 1id medium)
A certain amount of coagulant is added to the liquid culture medium to make it solid, which is the solid culture medium. The ideal coagulant should meet the following conditions: ① it will not be decomposed and utilized by cultured microorganisms; (2) Keep the solid state in the temperature range of microbial growth. When cultivating thermophilic bacteria, because high temperature is easy to liquefy the culture medium, coagulant is usually added to the culture medium to solve this problem. ③ The freezing point temperature of coagulant should not be too low, otherwise it is not conducive to the growth of microorganisms; ④ Coagulant has no toxic effect on cultured microorganisms; ⑤ Coagulant will not be destroyed during sterilization; ⑥ Good transparency and strong adhesion; ⑦ Convenient preparation and low price. Commonly used coagulants are agar, gelatin and silica gel. Table 5. 1 1 lists some main characteristics of agar and gelatin.
For most microorganisms, agar is the most ideal coagulant, and agar is a highly branched complex polysaccharide extracted from algae (marine agar). Gelatin is a product made from collagen, which was originally used as a coagulant. However, because its freezing point is too low, and non-specific extracellular protease produced by some bacteria and many fungi and specific collagenase produced by Clostridium can liquefy gelatin, it has been rarely used as a coagulant at present. Silica gel is a colloid formed by the condensation of inorganic sodium silicate (Na2SO3) and potassium silicate (K2SiO3) neutralized by hydrochloric acid and sulfuric acid. It contains no organic matter and is suitable for preparing culture medium for separating and cultivating autotrophic microorganisms.