Pure cultures are the key dogma of microbiology of the 20th century. To understand the essence of this concept, it is worth remembering that bacteria are very small and morphologically difficult to distinguish. But they differ in biochemical processes, and this is precisely their main species feature. But in a normal environment, we are not dealing with one kind of bacteria, but with a whole biome - a community that affects each other, and it is impossible to single out the role of one microorganism. And here we need a pure culture or strain of a particular species.
Microbial Hunters and Agar Agar
The brilliant idea of isolating pure microbial cultures belongs to the medical microbiologist Heinrich Hermann Robert Koch (1843-1910). The one who discovered the causative agent of anthrax, cholera and tuberculosis and is deservedly considered the founder of bacteriology and epidemiology.
It was he who invented the method of pure cultures, when a diluted culture of microbes is applied to a nutrient medium based on agar-agar polysaccharide and a colony of completely identical organisms grows from one cell. It is clearly visible to the naked eye and is specific to each species.
His invention gave impetus to the development of microbiology and taxonomy of microorganisms. Indeed, it was possible to cultivate any microbe in its pure form and to study one hundred million cells as one.
Without diminishing Koch’s achievements
It is worth noting that Koch's associates and students contributed to this invention. So, the idea of using agar agar belongs to Fanny Angelina Hesse, the wife of assistant Koch - V. Hesse.
Another Koch assistant bacteriologist Julius Richard Petri (1852-1921) proposed growing bacterial colonies in flat glass cups. Today, even schoolchildren know about Petri dishes.
Dogma of Microbiology
Pure (ashenic) culture - a set (population or strain) of microorganisms that have identical morphological and biochemical properties and are descendants of one cell.
The selection of pure culture involves the implementation of three stages:
- Obtaining and accumulation of microorganism culture.
- Isolation of pure culture.
- Identification and verification of the purity of the culture.
Pure crop isolation methods
In microbiology, the following methods are used to obtain an axenic culture of organisms:
- Mechanical methods (inoculation on Petri dishes using a spatula or loop, inoculation by diluting the material in agar — plate distributions, separation method based on the mobility of microorganisms).
- Biological - a method in which laboratory animals sensitive to a pathogen are infected. So pure cultures of bacteria are isolated from the body of mice (for example, pneumococci and bacilli tularemia).
- Methods that are based on the selective resistance of microorganisms to certain factors. When heated, for example, all spore-forming bacteria die, and non-spore-forming bacteria remain in a pure culture. When exposed to acids, bacteria sensitive to them die, and acid-resistant (for example, tuberculosis bacilli) survive. Exposure to antibiotics leaves a pure culture of microorganisms insensitive to it on the medium. Creating an oxygen and oxygen-free environment will separate aerobes from anaerobes.
What is it for?
Pure crops are applied:
- In scientific taxonomy in the classification (determination of the phylogenetic place in the system) of microorganisms.
- In the study of heredity and variability of organisms.
- With infectious diagnostics and the identification of pathogens.
- By isolating a pure culture of bacteria that spoil food.
- By production of vitamins, enzymes, antibiotics, serums and vaccines.
- In the food industry (production of bread, wine, kvass and beer (acetic bacteria and unicellular yeast fungi), lactic acid products (lactobacilli and lactic acid bacteria)).
- In biotechnology and in the study of viruses.
In nature, everything is completely wrong
In the 90s of the last century, regarding pure cultures, everything suddenly changed. It turned out that when the microorganisms of two pure strains are combined in one test tube, they behave completely differently than they do individually. Biochemical processes of their life affect (suppress or stimulate) each other. This is exactly what happens in natural biomes.
The conclusion is simple: the properties of a pure culture in a laboratory cannot be extrapolated to natural biomes.
Genomic revolution
Another blow was dealt by genomic identification of microorganisms. Initially, for the genomic analysis of microorganisms by molecular geneticists, a portion of ribosomal RNA common for all bacteria was selected. In accordance with the differences in the nucleotide sequence in this nucleic acid, all bacteria were distributed according to the phylogenetic relationship.
It was then that it turned out that the cultural strains and those bacteria that we studied make up about 5% of all bacteria that inhabit our planet. And, unlike cultural strains, we do not know anything about their properties and characteristics of biochemistry.
Having found the corresponding sequence in the genome of a natural strain, we can only place it on a phylogenetic tree and assume that in nature it has the same properties as the nearest related strain of the clean line.
So what's next?
So far, in the future, sequencing of the bacterial genome in one single cell. Today it is expensive and very difficult. And therefore, clean lines remain the “gold reserve” of microbiology.
Although the difficulties remain. For example, the bacteria of “black smokers” located at the bottom of the ocean have recently been investigated. The microorganism was described and sequenced by its genome without isolation of a pure culture.
A similar situation exists with bacteria living in the depths of gold mines. It turned out that this is a clean line of microorganisms - the descendants of one bacterium.
However, these organisms do not grow on nutrient media, and so far no one has managed to grow a colony of the pure strain.
Biotechnology News
In the development of this branch of applied knowledge, humanity faces many questions. And not only biological, but also ethical. How much can a person change the world around him and not harm him? The question remains open.
But today, biotechnology is being introduced into our lives. So, bacterial strains that are able to feed on plastic and decompose it have already been bred. While they do it slowly. But scientists are working on their genome. No one is surprised that all human insulin is “made” by us for genetically modified E. coli bacteria.
And artificial biosynthesis already today supplies us with biogas and biofuels in the form of high molecular weight carbohydrates of natural origin (vital products of bacteria, simple mushrooms that process the biomass of our waste into fuel, energy, chemicals).
Arable land and fresh water are today the most important component of limited natural resources. New biotechnology (bioremediation) offers the possibility of using microorganisms to restore their potential capabilities and remove polluting factors.
And that’s all - the future that has already come.