The smallest organisms on our planet are prokaryotic bacterial cells. Perhaps their size is one of the reasons why they are capable of a huge increase in numbers. It is estimated that normally the reproduction of bacteria occurs every 20-30 minutes. Naturally, the number of forming cells is simply unbelievable! For a day in terms of mass, the offspring from one individual will be about 5 tons.
However, due to various limiting factors, such an increase does not occur. What is the reason for such a rapid multiplication of these organisms? Let's try to figure this out.
Methods of reproduction of bacteria
There are several of them. The main factor that determines one or another variant of the reproduction process is external conditions. How do bacteria multiply under favorable conditions? Two ways:
- vegetative (asexual);
- by budding.
These are faster, simpler, and evolutionarily ancient methods. However, with such methods there is no exchange of genetic information, therefore, daughter individuals are an exact maternal copy.
But how do bacteria multiply if the parameters of the external sphere are unfavorable? Then a special process begins, having the same basis as the sexual reproduction of multicellular organisms — the exchange of genetic information due to the recombination of sections of DNA molecules of neighboring nucleoids in the cells.
Consider each of these options in more detail and find out the features of the artificial cultivation of populations of microorganisms.
Asexual reproduction and its features
Each bacterial cell performs all vital processes:
- breathing
- eats;
- moves around;
- absorbs and spends energy;
- growing;
- developing.
The result of all life is the process of asexual reproduction, as a result of which the mother cell gives rise to new individuals and gradually dies.
Process description
How bacteria grow vegetatively can be briefly described in a few paragraphs.
- The bacterium reaches a certain size and supply of nutrients. As a result, she is ready to start the process.
- Then the cell begins to elongate longitudinally, that is, to lengthen.
- In this case, a transverse septum (constriction) is formed in the middle, which begins to converge toward the center of the cell, squeezing it.
- Inside this, the process of replication of the DNA molecule occurs, that is, the division of the nucleoid.
- Genetic material at both ends of the molecule is attached to the walls of the bacterial cell.
- The main and replicated DNA diverge at different poles.
- The constriction closes and the cell is divided in half. Thus, two daughter individuals are formed.
- Missing structures (flagella, mucus capsules, etc.) are completed separately for each representative.
- The cell nucleoid is the first to divide, and cytokinesis occurs after it.
Obviously, the growth and reproduction of bacteria of this type is a very simple and quick process that does not require preparation and any specific features.
However, in some species (for example, in a hay rod-shaped bacterium), the process does not end there. Two daughter cells form a bridge between each other, through which DNA freely passes from one to the other. Meeting each other, nucleic acid molecules exchange sites, which leads to mutations. After that, the DNA returns to its cell, only already changed, with new sections built into the chain. The bridge collapses, each individual begins an independent existence.
Types of Cell Division
There are several other features in how bacteria reproduce asexually. When the process is completed, three scenarios are possible.
- Cytokinesis and nucleoid division occurs earlier than constrictions have time to form, and the cells diverge in two structures. Therefore, in these cases, the formation of diplococci, streptococci, staphylococci, rod-shaped multicellular formations and other combined forms occurs.
- Normal separation. In this case, timely and harmonious vegetative division of both the cells themselves and the genetic material and cytoplasm occurs. As a result, two normal daughter cells are obtained from one mother.
- The ongoing division of genetic material is far ahead of the separation of the cells themselves. As a result, multinucleoid forms are formed.
After the formation of any of the above colonies, cells in many of them also tend to separate from each other and independent existence.
Methods for disconnecting cells from each other
Features of the reproduction of bacteria in a vegetative way provide for different options for the separation of cells. There are three of them.
- Cells break the cytoplasmic bridge between them and are repelled from each other (breaking apart). Example: anthrax sticks.
- Accurate separation by sliding one individual over the surface of another. Example: Escherichia.
- Section. A method in which one cell describes a V shape around another. Example: Diphtheria bacteria.
On this, the features of asexual (vegetative) reproduction of bacterial individuals end.
Sexual reproduction: features
How is the reproduction of bacteria sexually, was established only in 1946. Prior to this, only the option already considered by us was known. It was believed that the answer to the question of how real bacteria multiply is unequivocal: only asexually by dividing cells in two.
However, experiments on Escherichia coli strains showed that its cells are capable of conjugation. This is the name of the special process of exchange of genetic material. And this is a direct sign of sexual reproduction.
As such, there are no female and male cells among bacteria. However, there is always one that contains the initial DNA (male) and one that will receive it (female). The whole process is as follows.
- Two cells come together and establish contact.
- The contact point is called drank, and it is a hollow fibrous tube. It is produced by the "male" cell.
- Then, through the formed channel, the transfer of father's DNA to the mother cell begins to occur.
- Here, the genetic material is recombined, that is, exchanged sites. Each molecule completes its own missing chain.
- The process is very slow, so during its passage the "male" cell is able to replicate its DNA several times.
- As a result, new individuals are formed that have recombinant genetic material with signs of both male and female cells. And also some initial paternal cells remain.
Bacterial colony growth
In order to study how the growth and reproduction of bacteria occurs, special nutrient media suitable for each species are prepared. The strains are seeded on them and, subject to certain sterile conditions, in order to avoid competition when other microorganisms are shared, the changes occurring and the growth of entire colonies are observed.
If this process is not limited by limiting factors , then population growth proceeds in a logarithmic progression. The death of cells in a natural way - only in arithmetic.
Spore formation
Hearing that microbes are able to form special structures - spores, many think that this is another option for how bacteria multiply. However, it is not.
A spore is only a temporary state of rest of a cell in which it is able to survive adverse environmental conditions. Sometimes even decades. This is in no way connected with the processes of reproduction of offspring.