an extensive group of unicellular microorganisms characterized by the absence of a cell nucleus surrounded by a membrane. At the same time, the genetic material of the bacterium (deoxyribonucleic acid, or DNA) occupies a very definite place in the cell - a zone called the nucleoid. Organisms with such a cell structure are called prokaryotes ("prenuclear"), in contrast to all others - eukaryotes ("truly nuclear"), whose DNA is located in the nucleus surrounded by a membrane.
Bacteria, formerly considered microscopic plants, are now separated into an independent kingdom of Monera - one of five in the current classification system, along with plants, animals, fungi and protists.
Fossil evidence.
Bacteria are probably the oldest known group of organisms. Layered stone structures - stromatolites - dated in some cases to the beginning of the Archeozoic (Archean), i.e. emerged 3.5 billion years ago - the result of the vital activity of bacteria, usually photosynthesizing, the so-called. blue-green algae. Such structures (bacterial films saturated with carbonates) are still formed, mainly off the coast of Australia, the Bahamas, in the Gulf of California and the Persian Gulf, but they are relatively rare and do not reach large sizes, because they feed on herbivorous organisms, for example, gastropods. Nowadays, stromatolites grow mainly where these animals are absent due to high salinity of water or for other reasons, however, before the appearance of herbivorous forms in the course of evolution, they could reach enormous sizes, making up an essential element of oceanic shallow water, comparable to modern coral reefs. In some ancient rocks, tiny charred spheres have been found that are also believed to be the remains of bacteria. The first nuclear, i.e. eukaryotic, cells evolved from bacteria about 1.4 billion years ago.
Ecology.
There are a lot of bacteria in the soil, at the bottom of lakes and oceans - wherever they accumulate organic matter... They live in cold weather, when the thermometer is slightly above zero, and in hot acidic springs with temperatures above 90 ° C. Some bacteria tolerate very high salinity; in particular, they are the only organisms found in the Dead Sea. In the atmosphere, they are present in water droplets, and their abundance there usually correlates with the dustiness of the air. So, in cities, rainwater contains much more bacteria than in rural areas. There are few of them in the cold air of the highlands and polar regions; nevertheless, they are found even in the lower layer of the stratosphere at an altitude of 8 km.
The digestive tract of animals is densely populated with bacteria (usually harmless). Experiments have shown that they are not necessary for the vital activity of most species, although they can synthesize some vitamins. However, in ruminants (cows, antelopes, sheep) and many termites, they are involved in the digestion of plant foods. In addition, the immune system of an animal raised under sterile conditions does not develop normally due to the lack of stimulation by bacteria. The normal bacterial "flora" of the intestine is also important for the suppression of harmful microorganisms that enter it.
STRUCTURE AND LIFE OF BACTERIA
Bacteria are much smaller than the cells of multicellular plants and animals. Their thickness is usually 0.5-2.0 microns, and their length is 1.0-8.0 microns. Some forms can hardly be discerned by the resolution of standard light microscopes (about 0.3 μm), but species with a length of more than 10 μm and a width that also go beyond the indicated range are known, and a number of very thin bacteria can exceed 50 μm in length. On the surface corresponding to the point set with a pencil, a quarter of a million average-sized representatives of this kingdom will fit.
Structure.
According to the morphological features, the following groups of bacteria are distinguished: cocci (more or less spherical), bacilli (rods or cylinders with rounded ends), spirillae (rigid spirals) and spirochetes (thin and flexible hair-like forms). Some authors tend to combine the last two groups into one - spirilla.
Prokaryotes differ from eukaryotes mainly in the absence of a formed nucleus and in the typical case of only one chromosome - a very long circular DNA molecule attached at one point to the cell membrane. Prokaryotes also lack membrane-surrounded intracellular organelles called mitochondria and chloroplasts. In eukaryotes, mitochondria generate energy during respiration, and photosynthesis occurs in chloroplasts. In prokaryotes, the entire cell (and, first of all, the cell membrane) takes on the function of mitochondria, and in photosynthetic forms, the chloroplast at the same time. Like eukaryotes, inside the bacterium there are small nucleoprotein structures - ribosomes, which are necessary for protein synthesis, but they are not associated with any membranes. With very few exceptions, bacteria are unable to synthesize sterols - important components of eukaryotic cell membranes.
Outside the cell membrane, most bacteria are clad with a cell wall somewhat resembling a cellulose wall plant cells, but consisting of other polymers (they include not only carbohydrates, but also amino acids and substances specific to bacteria). This membrane prevents the bacterial cell from bursting when water enters it through osmosis. There is often a protective mucous capsule on top of the cell wall. Many bacteria are equipped with flagella, with which they actively swim. Bacterial flagella are simpler and somewhat differently than similar structures of eukaryotes.
Sensory functions and behavior.
Many bacteria have chemical receptors that register changes in the acidity of the environment and the concentration of various substances, such as sugars, amino acids, oxygen and carbon dioxide. Each substance has its own type of such "taste" receptors, and the loss of any of them as a result of mutation leads to partial "taste blindness". Many motile bacteria also respond to temperature fluctuations, and photosynthetic species respond to changes in light. Some bacteria perceive the direction of the magnetic field lines, including the Earth's magnetic field, with the help of magnetite particles (magnetic iron ore - Fe 3 O 4) present in their cells. In water, bacteria use this ability to swim along the lines of force in search of a favorable environment.
Conditioned reflexes are unknown in bacteria, but they have a certain kind of primitive memory. While swimming, they compare the perceived intensity of the stimulus with its previous value, i.e. determine whether it has become more or less, and, based on this, keep the direction of movement or change it.
The direction in science called microbiology deals with the study of the structure, reproduction, classification and systematics of microorganisms. There are a lot of bacteria on Earth. Microbiologists have identified up to ten thousand species of prokaryotes. They are completely different, differ in properties, nutritional characteristics, structure. Some are used in industry, others are necessary to maintain the respiration of plants, while others live inside a person, ensuring the correct functioning of systems and organs. There are microorganisms that can withstand very strong radiation, and there are those that die at low temperatures.
Most bacteria, their structure and reproduction are studied by microbiologists, refer to unicellular prokaryotes. Only a few species have two or more cells. In shape, microorganisms can be:
- round;
- in the form of sticks;
- twisted;
- star-shaped;
- cubic.
The main components of prokaryotes:
- does not differ from the membrane in eukaryotic cells.
- Mesosome. With its help, hereditary material is attached.
- A nucleotide is an incompletely formed nucleus of prokaryotes, which contains chromosomes.
- Ribosomes are non-membrane organelles that occupy up to 40% of the cell.
Instead of a nucleus, bacteria cells have a nucleotide. This is a molecule that is responsible for the transfer of genetic information, and also includes plasmids. Single-celled microorganisms do not include such membrane organelles as the Golgi apparatus, mitochondria.
Bacterial metabolism
In the cells of bacteria, like other organisms, the synthesis of proteins, lipids and carbons must be carried out. The metabolism of unicellular organisms, as well as their structure, differs from the processes occurring in eukaryotic cells. Autotrophs and heterotrophs are distinguished. The first of them are capable of synthesizing substances necessary for normal life from inorganic compounds. Heterotrophs only transform organic matter.
- Fermentation is a reaction resulting in the formation of special molecules. Their significance is that they transfer phosphoric acid residues to ADP.
- Breathing is the process by which ATP is synthesized. If eukaryotic cells use oxygen for breathing, then prokaryotic cells can breathe due to mineral or organic compounds.
- Photosynthesis of microorganisms can be carried out with or without oxygen. Instead of oxygen, some bacteria use bacteriochlorophyll for photosynthesis, which is due to their structure.
Bacteria, which can carry out photosynthesis only in an oxygen-free way, lack a photosystem. More recently, scientists have identified a group of microorganisms that receive the energy necessary for normal life from reactions that use hydrogen.
How microorganisms transmit hereditary material
There are three main ways bacteria pass on hereditary material. They depend on the structure of prokaryotes.
- Transformation. This is when bacterial donors transmit their hereditary information directly to recipients.
- Transduction is the process of transferring hereditary material from a donor bacterium to a recipient bacterium through phages.
- Conjugation. This is when genetic information is transferred from one bacterium to another through direct contact.
Most of the microorganisms from among, but by budding or division. There is a difference in how gram-positive and gram-negative microorganisms multiply, which is due to the structural features. For the simplest organisms, sexual reproduction is also characteristic, but only in the most primitive form. Their cells don't even fuse. The exchange of hereditary material occurs in the process of genetic recombination.
As a result of the fusion of two donor cells, one cell is formed, which contains the genetic information of both of them. As a result of the displacement of genes, a qualitatively new unicellular organism is formed, which is necessary for the normal course of their evolution.
Genetic apparatus
Genes are responsible for the taxonomy of microorganisms, their specificity as a species, and functions. In prokaryotic cells, genes are located on the chromosome, the only closed DNA molecule. Due to the specific structure of bacteria, the processes of translation and transcription are coupled in their cells. The mRNA, which has just been synthesized, immediately binds to the ribosomes, since the region where the chromosome is located is not surrounded by a membrane.
Plasmids are also carriers of genetic information. It is a closed-loop DNA containing genes that manifest only under specific conditions.
In bacteria, genes are transferred horizontally. When the conjugation process takes place, the transfer of genetic information occurs directly from the donor to the recipient. In the process of horizontal transfer, the formation of new genes does not occur, this is characteristic of mutation, but the formation of gene combinations takes place.
Bacteria differ from other organisms by the features of their structure, reproduction, and vital activity. It depends on this which species and series they will belong to. Knowledge from microbiology is also necessary in order to be able to identify a disease, as there are a number of bacteria that cause their development. The study of unicellular microorganisms is necessary, since their importance in nature and human life cannot be overestimated. There is a very large number of different microorganisms, but they all have their own differences and characteristic features.
Reproduction of bacteria by division is the most common method of increasing the size of the microbial population. After division, bacteria grow to their original size, which requires certain substances (growth factors).
The methods of reproduction of bacteria are different, but for most of their species a form of asexual reproduction by the method of division is inherent. Bacteria rarely reproduce by budding. Sexual reproduction of bacteria is present in a primitive form.
Figure: 1. The photo shows a bacterial cell in the stage of division.
The genetic apparatus of bacteria
The genetic apparatus of bacteria is represented by a single DNA - the chromosome. DNA is closed in a ring. The chromosome is localized in a nucleotide that does not have a membrane. There are plasmids in the bacterial cell.
Nucleoid
The nucleoid is analogous to the nucleus. It is located in the center of the cell. It contains DNA - the carrier of hereditary information in a folded form. The unwound DNA reaches a length of 1 mm. The nuclear substance of a bacterial cell does not have a membrane, a nucleolus and a set of chromosomes; it does not divide by mitosis. The nucleotide is doubled before division. During division, the number of nucleotides increases to 4.
Figure: 2. The photo shows a bacterial cell on a cut. A nucleotide is visible in the central part.
Plasmids
Plasmids are autonomous molecules coiled into a ring of double-stranded DNA. Their mass is much less than the mass of a nucleotide. Despite the fact that hereditary information is encoded in the DNA of plasmids, they are not vital and necessary for the bacterial cell.
Figure: 3. The photo shows a bacterial plasmid.
Division stages
After reaching a certain size inherent in an adult cell, division mechanisms are triggered.
DNA replication
DNA replication precedes cell division. Mesosomes (folds of the cytoplasmic membrane) hold DNA until the process of division (replication) is complete.
DNA replication is carried out using enzymes by DNA polymerases. During replication, hydrogen bonds in 2-helical DNA are broken, as a result of which two daughter single-stranded ones are formed from one DNA. Subsequently, when the daughter DNAs took their place in the separated daughter cells, they are restored.
As soon as DNA replication is complete, as a result of synthesis, a constriction appears, dividing the cell in half. First, the nucleotide undergoes division, then the cytoplasm. Cell wall synthesis completes division.
Figure: 4. Scheme of division of a bacterial cell.
DNA exchange
In the hay bacillus, the DNA replication process ends with the exchange of 2 DNA sections.
After cell division, a bridge is formed, along which the DNA of one cell passes into another. Then both DNAs are intertwined. Some pieces of both DNA stick together. In places of adhesion, DNA fragments are exchanged. One of the DNA is bridged back into the first cell.
Figure: 5. Variant of DNA exchange in hay bacillus.
Types of bacterial cell division
If cell division is ahead of the separation process, then multicellular rods and cocci are formed.
With synchronous cell division, two full-fledged daughter cells are formed.
If a nucleotide divides faster than the cell itself, then multi-nucleotide bacteria are formed.
Methods for separating bacteria
Division by breaking
Division by breaking is typical of anthrax bacilli. As a result of this division, the cells break at the junction points, breaking the cytoplasmic bridges. Then they repel each other, forming chains.
Sliding separation
In sliding division, after division, the cell becomes detached and, as it were, slides over the surface of another cell. This method of separation is typical for some forms of Escherichia.
Split split
During split division, one of the divided cells with its free end describes an arc of a circle, the center of which is the point of its contact with another cell, forming a Roman five or cuneiform (corynebacterium diphtheria, listeria).
Figure: 6. In the photo, bacteria are rod-shaped, forming chains (anthrax rods).
Figure: 7. In the photo there is a sliding way of separating Escherichia coli.
Figure: 8. Split method for separating corynebacteria.
View of bacterial clumps after division
Clusters of dividing cells have a varied shape, which depends on the direction of the division plane.
Globular bacteria arranged one at a time, two by two (diplococci), in packages, in chains, or as bunches of grapes. Rod-shaped bacteria - in chains.
Spiral bacteria - chaotic.
Figure: 9. In the photo there are micrococci. They are round, smooth, white, yellow and red in color. In nature, micrococci are ubiquitous. They live in different cavities of the human body.
Figure: 10. In the photo, the bacteria are diplococci - Streptococcus pneumoniae.
Figure: 11. In the photo, the bacteria sarcina. Coccoid bacteria are combined into bags.
Figure: 12. In the photo there are streptococci bacteria (from the Greek “strepto” - a chain). Arranged in chains. They are the causative agents of a number of diseases.
Figure: 13. In the photo the bacteria are "golden" staphylococci. They are arranged like "bunches of grapes". The clusters are golden in color. They are the causative agents of a number of diseases.
Figure: 14. In the photo, the twisted bacteria Leptospira - the causative agents of many diseases.
Figure: 15. The photo shows rod-shaped bacteria of the genus Vibrio.
Bacteria division rate
The rate of division of bacteria is extremely high. On average, one bacterial cell divides every 20 minutes. Within only one day, one cell forms 72 generations of offspring. Mycobacterium tuberculosis divides slowly. The whole process of fission takes them about 14 hours.
Figure: 16. The photo shows the process of cell division of streptococcus.
Sexual reproduction of bacteria
In 1946, scientists discovered sexual reproduction in a primitive form. In this case, gametes (male and female germ cells) are not formed, however, some cells exchange genetic material ( genetic recombination).
Gene transfer occurs as a result conjugation - unidirectional transfer of a part of genetic information in the form plasmids upon contact of bacterial cells.
Plasmids are small DNA molecules. They are not associated with the genome of chromosomes and are capable of doubling autonomously. The plasmids contain genes that increase the resistance of bacterial cells to adverse environmental conditions. Bacteria often pass these genes to each other. The transfer of genetic information to bacteria of a different species is also noted.
In the absence of a true sexual process, it is conjugation that plays a huge role in the exchange of useful signs. This transfers the ability of bacteria to exhibit drug resistance. For humanity, the transmission of antibiotic resistance between disease-causing populations is especially dangerous.
Figure: 17. The photo shows the moment of conjugation of two Escherichia coli.
Phases of development of the bacterial population
When sowing on a nutrient medium, the development of the bacterial population goes through several phases.
Initial phase
The initial phase is the period from the moment of sowing to their growth. On average, the initial phase lasts 1 to 2 hours.
Reproduction delay phase
This is the phase of intense bacterial growth. Its duration is about 2 hours. It depends on the age of the culture, the period of adaptation, the quality of the nutrient medium, etc.
Logarithmic phase
This phase is marked by a peak in the rate of reproduction and increase in the bacterial population. Its duration is 5 - 6 hours.
Negative acceleration phase
In this phase, a decline in the rate of reproduction is noted, the number of dividing bacteria decreases and the number of dead bacteria increases. The reason for the negative acceleration is the depletion of the nutrient medium. Its duration is about 2 hours.
Stationary maximum phase
In the stationary phase, an equal number of dead and newly formed individuals is noted. Its duration is about 2 hours.
Doom acceleration phase
During this phase, the number of dead cells progressively increases. Its duration is about 3 hours.
Logarithmic death phase
During this phase, bacterial cells die off at a constant rate. Its duration is about 5 hours.
The phase of decreasing the rate of withering away
In this phase, the remaining living bacterial cells enter a dormant state.
Figure: 18. The figure shows the growth curve of the bacterial population.
Figure: 19. The photo shows a blue-green colony of Pseudomonas aeruginosa, a yellow micrococcal colony, a blood-red colony of Bacterium prodigiosum, and a black Bacteroides niger colony.
Figure: 20. The photo shows a colony of bacteria. Each colony is the offspring of a single cell. There are millions of cells in a colony. the colony grows in 1 - 3 days.
Division of magnetically sensitive bacteria
In the 1970s, sea bacteria were discovered that had a sense of magnetism. Magnetism allows these amazing creatures to navigate along the lines of the Earth's magnetic field and find sulfur, oxygen and other substances it needs so much. Their "compass" is represented by magnetosomes, which are composed of a magnet. As they divide, magnetically sensitive bacteria divide their compass. In this case, the constriction during division becomes clearly insufficient, therefore the bacterial cell bends and makes a sharp fracture.
Figure: 21. The photo shows the moment of division of the magnetically sensitive bacteria.
Bacterial growth
At the beginning of the division of a bacterial cell, two DNA molecules diverge to different ends of the cell. Then the cell is divided into two equal parts, which are separated from each other and increase to the original size. The division rate of many bacteria averages 20 to 30 minutes. Within only one day, one cell forms 72 generations of offspring.
The mass of cells in the process of growth and development quickly absorbs nutrients from the environment. This is facilitated by favorable environmental factors - temperature regime, sufficient amount of nutrients, the required pH of the environment. Aerobic cells require oxygen. For anaerobes, it is dangerous. However, the unlimited multiplication of bacteria in nature does not occur. Sunshine, dry air, lack of food, heat environment and other factors have a detrimental effect on the bacterial cell.
Figure: 22. The photo shows the moment of cell division.
Growth factors
For the growth of bacteria, certain substances (growth factors) are needed, some of which are synthesized by the cell itself, and some come from the environment. The need for growth factors is different for all bacteria.
The need for growth factors is a constant feature, which makes it possible to use it for the identification of bacteria, preparation of culture media and use in biotechnology.
Bacterial growth factors (bacterial vitamins) are chemical elements, most of which are water-soluble B vitamins. This group also includes hemin, choline, purine and pyrimidine bases and other amino acids. In the absence of growth factors, bacteriostasis occurs.
Bacteria use growth factors in minimal amounts and unchanged. A number of chemicals in this group are part of cellular enzymes.
Figure: 23. The photo shows the moment of division of the rod-shaped bacteria.
The most important bacterial growth factors
- Vitamin B1 (thiamine)... Takes part in carbohydrate metabolism.
- Vitamin B2 "(riboflavin)... Takes part in redox reactions.
- Pantothenic acid is an integral part of coenzyme A.
- Vitamin B6 (pyridoxine)... Takes part in the exchange of amino acids.
- Vitamins B12 (cobalamins are substances containing cobalt). They take an active part in the synthesis of nucleotides.
- Folic acid... Some of its derivatives are part of enzymes that catalyze the synthesis of purine and pyrimidine bases, as well as some amino acids.
- Biotin... Participates in nitrogen metabolism, and also catalyzes the synthesis of unsaturated fatty acids.
- Vitamin PP (a nicotinic acid). Participates in redox reactions, the formation of enzymes and the metabolism of lipids and carbohydrates.
- Vitamin H (para-aminobenzoic acid). It is a growth factor for many bacteria, including those inhabiting the human intestine. Folic acid is synthesized from para-aminobenzoic acid.
- Gemin... It is an integral part of some enzymes that take part in oxidation reactions.
- Choline... Takes part in the reactions of lipid synthesis of the cell wall. It is a supplier of the methyl group in the synthesis of amino acids.
- Purine and pyrimidine bases (adenine, guanine, xanthine, hypoxanthine, cytosine, thymine and uracil). Substances are needed mainly as components of nucleic acids.
- Amino acids... These substances are the constituents of cell proteins.
The need for growth factors of certain bacteria
Auxotrophs to ensure life, they need the supply of chemicals from the outside. For example, Clostridia are unable to synthesize lecithin and tyrosine. Staphylococci need lecithin and arginine. Streptococci need the intake of fatty acids - components of phospholipids. Corinebacteria and Shigella need niacin. Staphylococcus aureus, pneumococci and brucella need vitamin B1. Streptococci and tetanus bacilli - in pantothenic acid.
Prototrophs independently synthesize the necessary substances.
Figure: 24. Different environmental conditions affect the growth of bacterial colonies in different ways. Left - stable growth in the form of a slowly expanding circle. On the right - fast growth in the form of "shoots".
Studying the needs of bacteria for growth factors allows scientists to obtain a large microbial mass, which is so necessary for the manufacture of antimicrobial drugs, serums and vaccines.
Read more about bacteria in the articles:
Reproduction of bacteria is a mechanism for increasing the number of microbial populations. Division of bacteria is the main way of reproduction. After division, the bacteria should reach the size of adults. Bacteria grow by rapidly absorbing nutrients from their environment. For growth, certain substances (growth factors) are required, some of which are synthesized by the bacterial cell itself, and some come from the environment.
By studying the growth and reproduction of bacteria, scientists are constantly discovering the beneficial properties of microorganisms, the use of which in everyday life and in production is limited only by their properties.