Differential diagnostic environments are special mixtures of nutrients used to determine the species of microbes and to study their properties. With the growth of bacteria in differential diagnostic environments, chemical processes occur due to the presence of various enzymes in the microbial cell. Some of them are able to break down proteins , others - carbohydrates , others - to cause oxidation and reduction reactions, etc. Due to the action of enzymes in the differential diagnostic environment, appropriate changes occur.
Differential diagnostic environments can be divided into four main groups.
1. Environments containing protein and detecting the ability of microbes to break down proteins (proteolytic properties): meat-peptone gelatin "column", folded horse or bovine serum, milk, blood agar. When sowing bacteria by puncture in the meat-peptone gelatin, the "dilution of the medium" is observed by the "column" in the case of protein breakdown. When sown on a medium with serum folded, protein cleavage is determined by diluting the medium and forming cavities on its surface. Microbial cleavage of milk is detected by enlightenment or dissolution of the initially coagulated milk. The presence of hemolytic activity of the studied culture is checked by sowing it in a Petri dish on a special blood agar. As a result of the destruction of red blood cells around the colonies (for example, hemolytic streptococcus or staphylococcus ) formed zones of enlightenment.
2. Environments to detect the ability of microbes to break down carbohydrates and high-atom alcohols (Endo Medium, Levina Medium, Russell Medium, Drigalsky - Conradi Medium, Rapoport - Weintraub Medium, Shustovoy Medium). To identify these properties of microorganisms, a “variegated” series is also used, i.e., a series of test tubes containing nutrient media , including various carbohydrates, polyhydric alcohols, and an indicator. Litmus tincture or bromthymol blue is used as indicators. The decomposition of any of the carbohydrates with the formation of acid is detected by a change in the color of the indicator, the formation of gas — by filling with gas and floating a special glass float in a liquid medium. Or apply semi-liquid Hiss medium (see) with 0.5% agar with the corresponding sugars and Andrade indicator. After seeding the microbe on these media, the formation of an acid is detected by reddening of the medium, and the formation of a gas — by the appearance of its bubbles in the agar or by breaking and shifting the agar column upwards. The differential diagnostic media of the second group also include starch agar, which is used to determine the ability of microbes to break down starch, Clark's medium, etc.
3. Environments that detect the ability of microbes to discolor dyes added to the broth: methylene blue, thionin, litmus, indigo carmine , neutral red or others (Rotberger medium, Omelyansky medium). The third group also includes mediums with nitrates, which serve to determine the ability of microbes to reduce nitric acid salts (nitrates) to nitrous acid salts (nitrites) and further to ammonia or free nitrogen.
4. Environments that detect the ability of microbes to assimilate substances that are not absorbed by other microbes, for example, a medium with sodium citrate (Simons citrate agar) to distinguish E. coli, which is deprived of the ability to assimilate this medium, from other bacteria of the intestinal group or medium with sodium oleic acid to differentiate diphtheria bacillus from falsely diphtheria and diphtheroids (Agar Angering).
Differential diagnostic media also include anaerobic differentiation media, tellurite media for differentiation of diphtheria bacteria, urea media, alkaline media (Dieudonne agar) for the cultivation of Vibrio cholerae, etc. See also Microbial Identification.