Bacterial research requires meticulous work with numerous equipment and instruments. So that microorganisms in the laboratory multiplied as quickly as possible and can maintain normal life, special nutrient media are used. Their composition and biophysical conditions are suitable for the active growth of a bacterial culture.
Nutrient media. Microbiology and other applications
Colonies of bacteria in laboratory conditions are grown on Petri dishes, which are filled with jelly-like or semi-liquid contents. This is a nutrient medium, the composition and properties of which are as close as possible to natural for the qualitative growth of culture.
Such media are used in microbiological research and in medical diagnostic laboratories. The latter work most often with smears of pathogenic or conditionally pathogenic bacteria, the systematic position of which is determined directly in the institution.
Natural and synthetic environments
The basic rule for working with bacteria is the correct selection of a nutrient medium. It should be suitable for numerous criteria, including the content of micro and macro elements, enzymes, a constant value of acidity, osmotic pressure and even the percentage of oxygen in the air.
Nutrient media are classified into two large groups:
- Natural environments. Such mixtures are prepared from natural components. This can be river water, parts of plants, manure, vegetables, plant and animal tissues, yeast, etc. Such environments are characterized by a high content of natural chemicals, the diversity of which contributes to the growth of bacterial culture. Despite these obvious advantages, natural environments do not allow specialized studies with specific strains of bacteria.
- Synthetic environments. They differ in that their chemical composition is known in exact ratios of all components. Such media are prepared for a specific bacterial culture, the metabolism of which is known to the researcher in advance. Actually, for this reason it is possible to prepare a similar synthetic medium for the development of microorganisms. They are used to analyze the vital activity of bacteria. For example, you can find out what substances they release into the environment and how much. In natural environments, microorganisms will also grow, but to track any quantitative changes in the composition is impossible due to ignorance of the initial proportions of substances.
Differential diagnostic environments
In working with bacteria, not only ordinary nutrient media can be used. Microbiology is an extensive science, and therefore, when conducting research, sometimes it is necessary to make a selection of microorganisms according to some characteristic. The use of differential diagnostic media in the laboratory makes it possible to select the necessary bacterial colonies on a Petri dish according to the biochemical sign of their life.
The composition of such environments always includes the following components:
1. Nutrients for cell growth.
2. The analyzed substrate (substance).
3. An indicator that will give a characteristic color when a certain reaction occurs.
An example is the differential diagnostic nutrient medium Endo. It is used to select bacterial colonies that can break down lactose. Initially, this environment has a pinkish color. If a colony of microorganisms is not able to break down lactose, it takes the usual white color. If bacteria can cleave this substrate, they stain in a characteristic bright red color.
Elective environments
In diagnostic laboratories, smears are often carried out, which contain many different types of bacteria. Obviously, for quality work it is necessary to somehow select the colonies we need from dozens of outsiders. A breeding ground for bacteria can help here, the composition of which is ideally selected for the life of only one type of microorganism.
For example, such an elective medium is suitable only for the propagation of Escherichia coli. Then, from seeding many bacteria on a Petri dish, we will see only colonies of the same Escherichia coli and no more. Before starting work, it is necessary to know well the metabolism of the studied bacterium in order to successfully select it from a mixture of other species.
Solid, semi-liquid and liquid culture media
Bacteria can be grown not only on solid substrates. Nutrient media differ among themselves in the state of aggregation, which depends on the composition during manufacture. Initially, they all have a liquid consistency, and when gelatin or agar is added in a certain percentage, the mixture freezes.
Liquid culture media are usually found in test tubes. If it becomes necessary to grow bacteria in such conditions, add a solution with a sample of the culture and wait 2-3 days. The result may be different: a precipitate forms, a film appears, small flakes float or a cloudy solution forms.
Dense culture media are often used in microbiological studies to study the properties of bacterial colonies. Such media are always transparent or translucent so that it is possible to correctly determine the color and shape of the culture of microorganisms.
Preparation of culture media
Substrates such as meat and peptone mixtures based on broth, gelatin or agar are very simple to prepare. If you need to make a solid or semi-liquid substrate, add 2-3% or 0.2-0.3% gelatin or agar, respectively, to the liquid. They play a major role in the hardening of the mixture, but are by no means a source of nutrients. Thus, nutrient media are obtained that are suitable for the growth of a bacterial culture.