Microorganisms (microbes) are considered unicellular organisms whose size does not exceed 0.1 mm. Representatives of this large group may have different cellular organization, morphological characters and metabolic capabilities, that is, the main feature that unites them is size. The term "microorganism" itself is not endowed with a taxonomic meaning. Microbes belong to the most diverse 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 a binomial system for characterizing an object, that is, the nomenclature includes the names of the genus and species.
Most microorganisms are characterized by an extremely primitive and universal structure, therefore, their division into taxa cannot be carried out only by morphological characters. Functional features, molecular biological data, biochemical flow patterns, etc. are used as criteria.
Identification Features
To identify an unknown microorganism, studies are conducted to study the following properties:
- Cell cytology (primarily belonging to pro- or eukaryotic organisms).
- Morphology of cells and colonies (in specific conditions).
- Cultural characteristics (growth characteristics on different media).
- The complex of physiological properties on which the classification of microorganisms according to the type of respiration (aerobic, anaerobic) is based
- Biochemical signs (the presence or absence of certain metabolic pathways).
- A set of molecular biological properties, including taking into account the sequence of nucleotides, the possibility of hybridization of nucleic acids with the material of typical strains.
- Chemotaxonomic indicators, implying consideration of the chemical composition of various compounds and structures.
- Serological characteristics (antigen-antibody reactions; especially for pathogenic microorganisms).
- The presence and nature of sensitivity to specific phages.
The systematics and classification of microorganisms related to prokaryotes is carried out using the “Bergey Guide to the Systematics of Bacteria”. And identification is carried out using the identifier Bergee.
Different ways to classify germs
To determine the taxonomic affiliation of an organism, apply several methods for the classification of microorganisms.
With a formal numerical classification, all attributes are considered equally significant. That is, the presence or absence of one or another sign is taken into account.
Morphophysiological classification involves the study of the totality of morphological properties and characteristics of the metabolic processes. In this case, the meaning is given to the significance of a property in an object. The placement of a microorganism in a particular taxonomic group and the assignment of a name depend primarily on the type of cell organization, morphology of cells and colonies, as well as the nature of growth.
Consideration of functional characteristics provides the possibility of using microorganisms of various nutrients. Also important is the dependence on certain physico-chemical environmental factors, and in particular, ways of generating energy. There are microbes for the identification of which chemotaxonomic studies are necessary. Pathogens need serodiagnosis. A determinant is used to interpret the results of the above tests.
Molecular genetic classification analyzes the structure of the molecules of the most important biopolymers.
The identification of microorganisms
Nowadays, the identification of a specific microscopic organism begins with the isolation of its pure culture and analysis of the 16S rRNA nucleotide sequence. Thus, the location of the microbe on the phylogenetic tree is carried out, and the subsequent concretization by genus and species is carried out using traditional microbiological methods. A coincidence value of 90% allows you to determine the gender, and 97% - the species.
An even clearer differentiation of microorganisms by genus and species is possible when using polyphyletic (polyphase) taxonomy, when the determination of nucleotide sequences is combined with the use of information of various levels, up to environmental. That is, a preliminary search for groups of similar strains is carried out, followed by determination of the phylogenetic positions of these groups, fixing the differences between the groups and their closest neighbors, and collecting data to differentiate the groups.
The main groups of eukaryotic microorganisms: algae
This domain includes three groups where there are microscopic organisms. We are talking about algae, protozoa and mushrooms.
Algae are unicellular, colonial or multicellular phototrophs that carry out oxygenic photosynthesis. The development of the molecular genetic classification of microorganisms belonging to this group is not yet completed. Therefore, at the moment, in practice, classification of algae is applied 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 are unicellular organisms belonging to dinoflagellate, diatoms, euglena and green algae. The formation of chlorophyll and various forms of carotenoids is characteristic of all algae, but the ability to synthesize other forms of chlorophyll and phycobilins in the representatives of the group manifests itself in different ways.
The combination of certain pigments determines the staining of cells in various colors. They can be green, brown, red, golden. Cell pigmentation is a species characteristic.
Diatoms are unicellular planktonic forms in which the cell wall has the appearance of a silicon bivalve carapace. Some representatives are capable of moving along the type of sliding. Reproduction is both asexual and sexual.
The habitats of unicellular euglenae algae are freshwater bodies of water. Move with the help of flagella. There is no cell wall. Able to grow in the dark 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, the habitats are fresh and marine bodies of water, soil and the surface of various terrestrial objects. There are still species, and some are capable of movement using flagella. Like dinoflagellates, green microalgae have a cellulose cell wall. The storing of starch in cells is characteristic. Reproduction is carried out both asexually and sexually.
Eukaryotic organisms: protozoa
The basic principles for the classification of microorganisms belonging to the simplest are based on morphological characteristics, which vary greatly among representatives of this group.
The ubiquity, the conduct 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 dead remains of plants, animals and microorganisms. Most representatives have no cell wall.
They can lead a motionless lifestyle or move around with the help of various devices: flagella, cilia and pseudopods. Within the taxonomic group of protozoa there are several more groups.
Representatives of protozoa
Amoebas carry out nutrition by endocytosis, move with the help of pseudopods, the essence of reproduction consists in primitive cell division in two. Most amoebas are free-living water forms, but there are also those that cause diseases in humans and animals.
There are two different nuclei in the cells of the ciliates; asexual reproduction consists in transverse fission. There are representatives who are characterized by sexual reproduction. A coordinated cilia system takes part in the movement. Endocytosis is carried out by the capture of food by a special oral cavity, and the remains are removed through an opening at the posterior end. In nature, ciliates live in bodies of water contaminated with organic substances, as well as rumen of ruminants.
Flagella are characterized by the presence of flagella. The absorption of dissolved nutrients is carried out by the entire surface of the CPM. The 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 healing ulcers), Giardia (lead to intestinal disorders).
Among the simplest sporozoans, the most difficult life cycle. The most famous representative of sporozoans is the malarial plasmodium.
Eukaryotic microorganisms: fungi
Classification of microorganisms by type of nutrition classifies representatives of this group as heterotrophs. For most, the formation of mycelium is characteristic. Breathing is usually aerobic. But there are optional anaerobes, which can switch to alcoholic fermentation. Methods of reproduction - vegetative, asexual and sexual. It is this trait that serves as a criterion for further classification of fungi.
If we talk about the significance of the representatives of this group, then the most interesting is the combined non-taxonomic group of yeast. It includes fungi that lack the mycelial growth stage. Among yeast, many facultative anaerobes. However, pathogenic species are also found.
The main groups of microorganisms-prokaryotes: archaea
The 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, in which there is no N-acetylmuramic acid.
Archeans 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 structural features of the cell membrane, and in particular, the content of peptidoglycan in it. Currently there are 23 phylums 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 residues, the purification of organically polluted water bodies, and the modification of inorganic compounds. Without them, the existence of life on Earth would have become impossible. These microorganisms live everywhere, their habitat may be soil, water, air, the human body, animals and plants.
According to the shape of the cells, the presence of devices for movement, the articulation of cells between themselves of this domain, a subsequent classification of microorganisms is carried out inside. Microbiology considers the following types of bacteria based on the shape of the cells: round, rod-shaped, filamentous, crimped, spiral-shaped. By the type of movement, bacteria can be motionless, flagellate or move due to mucus secretion. Based on the method of combining cells with each other, bacteria can be isolated, coupled in the form of pairs, granules, branching forms also occur.
Pathogens: classification
There are many pathogenic microorganisms among rod-shaped bacteria (pathogens of diphtheria, tuberculosis, typhoid fever, anthrax); protozoa (plasmodium malaria, toxoplasma, leishmania, giardia, trichomonas, some pathogenic amoeba), actinomycetes, mycobacteria (pathogens of tuberculosis, leprosy), mold and yeast-like fungi (pathogens of mycoses, candidiasis). Mushrooms can cause all kinds of skin lesions, for example, various types of lichen (with the exception of herpes zoster, in the appearance of which the virus is involved). Some yeasts, being permanent inhabitants of the skin, do not adversely affect the normal functioning of the immune system. However, if the activity of immunity is reduced, then they cause the appearance of seborrheic dermatitis.
Pathogenicity groups
The epidemiological danger of microorganisms is a criterion for combining all pathogenic microbes into four groups corresponding to four risk categories. Thus, the pathogenicity groups of microorganisms, the classification of which is given below, are of the greatest interest to microbiologists, since they directly affect the life and health of the population.
The safest, 4th group of pathogenicity includes microbes that do not pose a threat to the health of an individual (or the risk of this threat is negligible). That is, the risk of infection is very small.
3rd group is characterized by a moderate risk of infection for an individual, low risk for society as a whole. Such pathogens theoretically can cause the disease, and even if this happens, there are proven effective methods of treatment, as well as a set of preventive measures that can prevent the spread of infection.
The 2nd group of pathogenicity includes microorganisms that represent high risk indicators for an individual, but low for society as a whole. In this case, the pathogen can cause a serious illness in a person, however, it does not spread from one infected to another. Effective methods of treatment and prevention are available.
The 1st group of pathogenicity is characterized by a high risk for both the individual and society as a whole. A pathogen that causes a serious illness in a person or animal can be easily transmitted in various ways. Effective treatments and preventive measures are usually not available.
Pathogenic microorganisms, the classification of which determines their belonging to a particular group of pathogenicity, bring great damage to the health of society, only if they belong to the 1st or 2nd group.