Microorganisms (microbes) are considered to be unicellular organisms, the size of which does not exceed 0.1 mm. Representatives of this large group may have different cellular organization, morphological features and metabolic capabilities, that is, the main feature that unites them is size. The term "microorganism" itself is not endowed with a taxonomic sense. Microbes belong to a wide variety of taxonomic units, and other representatives of these units can be multicellular and reach large sizes.
General approaches to the classification of microorganisms
As a result of the gradual accumulation of factual material about microbes, it became necessary to introduce rules for their description and systematization.
The classification of microorganisms is characterized by the presence of the following taxa: domain, phylum, class, order, family, genus, species. In microbiology, scientists use the binomial system of object characteristics, that is, the nomenclature includes the names of the genus and species.
For most microorganismsan extremely primitive and universal structure is characteristic, therefore, their division into taxa cannot be carried out only according to morphological features. Functional features, molecular biological data, patterns of biochemical processes, etc. are used as criteria.
Identification features
To identify an unknown microorganism, research is being carried out to study the following properties:
- Cell cytology (primarily belonging to pro- or eukaryotic organisms).
- Morphology of cells and colonies (under specific conditions).
- Cultural characteristics (growth characteristics on different media).
- The complex of physiological properties on which the classification of microorganisms by type of respiration (aerobic, anaerobic) is based
- Biochemical signs (presence or absence of certain metabolic pathways).
- A set of molecular biological properties, including taking into account the nucleotide sequence, the possibility of hybridization of nucleic acids with the material of type strains.
- Chemotaxonomic indicators, which take into account the chemical composition of various compounds and structures.
- Serological characteristics (reactions "antigen - antibody"; especially for pathogenic microorganisms).
- Presence and nature of sensitivity to specific phages.
Systematics and classification of microorganisms related to prokaryotes is carried out using the "Burgey's Guide to the Systematics of Bacteria". The identification is carried out usingBurgey's determinant.
Different ways of classifying microbes
To determine the taxonomic affiliation of an organism, several methods of classifying microorganisms are used.
In the formal numeric classification, all features are considered equally significant. That is, the presence or absence of a particular feature is taken into account.
Morphophysiological classification involves the study of the totality of morphological properties and features of the course of metabolic processes. In this case, it is endowed with meaning and significance of a particular property of an object. The placement of a microorganism in one or another taxonomic group and the assignment of a name depend primarily on the type of cellular organization, cell and colony morphology, and growth patterns.
Accounting for functional characteristics provides for the possibility of using various nutrients by microorganisms. Also important is the dependence on certain physical and chemical factors of the environment, and in particular the ways of obtaining energy. There are microbes that require chemotaxonomic studies to identify them. Pathogenic microorganisms require serodiagnosis. The qualifier is used to interpret the results of the above tests.
In molecular genetic classification, the structure of the molecules of the most important biopolymers is analyzed.
Procedure for identifying microorganisms
Today, the identification of a particular microscopic organism begins withisolation of its pure culture and analysis of the nucleotide sequence of 16S rRNA. Thus, the place of the microbe on the phylogenetic tree is determined, and the subsequent specification by genus and species is carried out using traditional microbiological methods. A coincidence value of 90% allows you to determine the genus, and 97% - species.
Even clearer differentiation of microorganisms by genus and species is possible using polyphyletic (polyphase) taxonomy, when the determination of nucleotide sequences is combined with the use of information at various levels, up to ecological. That is, a search for groups of similar strains is carried out first, followed by determining the phylogenetic positions of these groups, fixing the differences between the groups and their nearest neighbors, and collecting data to differentiate the groups.
Main groups of eukaryotic microorganisms: algae
This domain includes three groups where there are microscopic organisms. We are talking about algae, protozoa and fungi.
Algae are unicellular, colonial or multicellular phototrophs that carry out oxygenic photosynthesis. The development of a molecular genetic classification of microorganisms belonging to this group has not yet been completed. Therefore, at the moment, in practice, the classification of algae is used based on the composition of pigments and reserve substances, the structure of the cell wall, the presence of mobility and the method of reproduction.
Typical representatives of this group areunicellular organisms belonging to dinoflagellates, diatoms, euglenoids and green algae. All algae are characterized by the formation of chlorophyll and various forms of carotenoids, but the ability to synthesize other forms of chlorophylls and phycobilins in the representatives of the group manifests itself in different ways.
The combination of these or those pigments determines the staining of cells in different colors. They can be green, brown, red, golden. Cell pigmentation is a species characteristic.
Diatoms are unicellular planktonic forms, in which the cell wall looks like a silicon bivalve shell. Some representatives are capable of movement by the type of sliding. Reproduction is both asexual and sexual.
The habitats of unicellular euglena algae are freshwater reservoirs. They move with the help of flagella. There is no cell wall. Able to grow in darkness due to the process of oxidation of organic substances.
Dinoflagellates have a special structure of the cell wall, it consists of cellulose. These planktonic unicellular algae have two lateral flagella.
For microscopic representatives of green algae, habitats are fresh and marine water bodies, soil and the surface of various terrestrial objects. There are non-motile species, and some are capable of locomotion using flagella. Just like dinoflagellates, green microalgae have a cellulose cell wall. The storage of starch in cells is characteristic. Reproduction is both asexual and sexualway.
Eukaryotic Organisms: Protozoa
The basic principles of classification of microorganisms belonging to the protozoa are based on morphological characteristics that vary greatly among representatives of this group.
The ubiquitous distribution, the maintenance of a saprotrophic or parasitic lifestyle largely determines their diversity. Food free-living protozoa are bacteria, algae, yeast, other protozoa and even small arthropods, as well as the dead remains of plants, animals and microorganisms. Most representatives do not have a cell wall.
They can lead a stationary life or move around with the help of various devices: flagella, cilia and prolegs. There are several more groups within the taxonomic group of protozoa.
Representatives of the simplest
Amoebae feed by endocytosis, move with the help of pseudopods, the essence of reproduction lies in the primitive division of the cell in two. Most amoebas are free-living aquatic forms, but there are some that cause disease in humans and animals.
Infusoria cells have two different nuclei, asexual reproduction consists in transverse division. There are representatives for which sexual reproduction is characteristic. A coordinated system of cilia takes part in the movement. Endocytosis is carried out by capturing food in a special oral cavity, and the remains are excreted throughhole at the back end. In nature, ciliates live in water bodies polluted with organic substances, as well as in the rumen of ruminants.
Flagellates are characterized by the presence of flagella. The absorption of dissolved nutrients is carried out by the entire surface of the CPM. Division occurs only in the longitudinal direction. Among flagellates, there are both free-living and symbiotic species. The main symbionts of humans and animals are trypanosomes (cause sleeping sickness), leishmania (cause difficult-to-heal ulcers), giardia (lead to intestinal disorders).
Sporozoans have the most complex life cycle of all protozoans. The most famous representative of sporozoans is the malarial plasmodium.
Eukaryotic microorganisms: fungi
Classification of microorganisms according to the type of nutrition refers representatives of this group to heterotrophs. Most are characterized by the formation of mycelium. Respiration is usually aerobic. But there are also facultative anaerobes that can switch to alcoholic fermentation. Reproduction methods are vegetative, asexual and sexual. It is this feature that serves as a criterion for further classification of fungi.
If we talk about the significance of representatives of this group, then the most interesting here is the combined non-taxonomic group of yeasts. This includes mushrooms that do not have a mycelial growth stage. There are many facultative anaerobes among yeasts. However, there are also pathogenic species.
Main groups of prokaryotic microorganisms:archaea
Morphology and classification of prokaryotic microorganisms combines them into two domains: bacteria and archaea, whose representatives have many significant differences. Archaea do not have peptidoglycan (murein) cell walls typical of bacteria. They are characterized by the presence of another heteropolysaccharide - pseudomurein, which does not contain N-acetylmuramic acid.
Archaea are divided into three phyla.
Features of the structure of bacteria
The principles of classification of microorganisms that unite microbes in this domain are based on the structural features of the cell membrane, and in particular the content of peptidoglycan in it. There are currently 23 phyla in the domain.
Bacteria are an important link in the cycle of substances in nature. The essence of their significance in this global process is the decomposition of plant and animal remains, the purification of water bodies polluted with organic matter, and the modification of inorganic compounds. Without them, the existence of life on Earth would be impossible. These microorganisms live everywhere, their habitat can be soil, water, air, the human body, animals and plants.
According to the shape of cells, the presence of devices for movement, the articulation of cells among themselves, this domain is carried out within the subsequent classification of microorganisms. Microbiology considers the following types of bacteria based on the shape of the cells: round, rod-shaped, filamentous, convoluted, spiral. According to the type of movement, bacteria can be immobile, flagellated, or move due to excretion.mucus. Based on the way cells articulate with each other, bacteria can be isolated, linked in the form of pairs, granules, and branching forms are also found.
Pathogenic microorganisms: classification
Pathogenic microorganisms are many among rod-shaped bacteria (causative agents of diphtheria, tuberculosis, typhoid fever, anthrax); protozoa (malarial plasmodium, toxoplasma, leishmania, giardia, trichomonas, some pathogenic amoeba), actinomycetes, mycobacteria (causative agents of tuberculosis, leprosy), molds and yeast-like fungi (causative agents of mycoses, candidiasis). Mushrooms can cause all kinds of skin lesions, for example, different types of lichen (with the exception of herpes zoster, in which the virus is involved). Some yeasts, being permanent inhabitants of the skin, do not have a detrimental effect under normal immune system conditions. However, if the activity of the immune system is reduced, then they cause the appearance of seborrheic dermatitis.
Pathogenicity groups
The epidemiological danger of microorganisms is the criterion for grouping all pathogenic microbes into four groups corresponding to four risk categories. Thus, the groups of pathogenicity of microorganisms, the classification of which is given below, are of the greatest interest to microbiologists, since they directly affect the life and he alth of the population.
The safest, 4th pathogenicity group, includes microbes that do not pose a threat to the he alth of an individual (or the risk of this threat is negligiblesmall). That is, the risk of infection is very small.
3rd group are characterized by a moderate risk of infection for an individual, a low risk for society as a whole. Such pathogens could theoretically cause disease, and even if they do, there are proven effective treatments, as well as a set of preventive measures that can prevent the spread of infection.
The 2nd pathogenicity group includes microorganisms that pose a high risk for the individual, but low for society as a whole. In this case, the pathogen can cause severe disease in humans, but it does not spread from one infected person to another. Effective methods of treatment and prevention are available.
1st pathogenicity group is characterized by a high risk for both the individual and society as a whole. A pathogen that causes severe disease in a human or animal can be easily transmitted in a variety of ways. Effective treatments and preventive measures are generally not available.
Pathogenic microorganisms, the classification of which determines their belonging to one or another pathogenicity group, cause great damage to public he alth only if they belong to the 1st or 2nd group.
