FUNDAMENTALS OF MICROBIOLOGY - V. M. Samygin - 2015

CHAPTER 8. GENETIC FOUNDATIONS OF MICROBIOLOGY

8.1. GENETIC APPARATUS OF Prokaryotes

Information regarding the traits intrinsic to an Organism is concentrated in its genetic apparatus, which ensures their preservation and reproduction during The process of organismal multiplication. The genetic apparatus possesses high stability and precision in the mechanisms governing its functioning. However, this stability is not absolute, as that would preclude any possibility of change and evolutionary transformation. Consequently, the genetic apparatus must ensure stability while, on the other hand, remaining sufficiently plastic, i.e., possessing the capacity for Variability.

In prokaryotes, all genetic material is localized within a single chromosome, although under certain conditions bacterial Cells may contain multiple copies of the chromosome. Chromosomes are Nucleoproteins (Nucleic Acids and Proteins) that serve as carriers of the genes determining the hereditary Properties of the organism. Genes are arranged linearly within the chromosome. The Primary Structure that stores information and transmits it by inheritance is DNA. A Gene is a fragment of a DNA molecule that controls the synthesis of a single protein or peptide. Genetic information concerning all traits intrinsic to The Cell is encoded within the genes. A bacterial chromosome contains up to 4,000 individual genes.

The Introduction/4.html">Prokaryotic Cell chromosome is essentially a macromolecule of DNA organized into a nucleoid. The total sum of genes possessed by a cell is termed its genotype. The genotype maintains relative constancy under any conditions, which makes it possible to distinguish different species of microorganisms from one another.

Along with the chromosome, genetic material in Bacteria may be contained in non-chromosomal genetic elements—Plasmids—located either in the Cell Cytoplasm or in a state integrated with the chromosome. In their free state, plasmids are circular DNA molecules that are stably transmitted to the progeny of bacterial cells independently of the chromosomal DNA.

8.2. Heredity and Variability

The storage and transmission of information are associated with the genetic apparatus of the microbial cell, which enables The transfer of species-specific traits from parent to offspring. The preservation of specific organismal properties over a succession of generations is termed heredity. Microorganism variability is commonly understood as the capacity of cells to alter their species-specific traits. Heredity and variability are two inextricably linked processes that form the foundation of all living things. A trait (or property) is referred to in genetics as a phen, and their combination in a specific environment is termed a phenotype. Consequently, variability is the capacity to alter the phenotype. Under the exact same hereditary makeup, a microorganism may exhibit multiple phenotypes depending on The Nature of Nutrition, aeration, Temperature, and other cultivation conditions.

Shifts in phenotype under changing environmental conditions may occur without the involvement of the genotype or as a consequence of its damage. Variability in which the phenotypic shift of microbes occurs without altering the genotype is termed non-hereditary, phenotypic variability, or modification. Modifications arise as adaptive responses of bacterial cells to environmental changes, allowing them to adapt rapidly and maintain population numbers at a viable level. Once the corresponding stimulus that caused their formation is removed, the bacteria revert to their original phenotype.

A standard manifestation of modification is the splitting of a homogeneous population into several types. This phenomenon is known as microbial dissociation. Typically, dissociation arises under conditions unfavorable to the initial population. This form of variability is generally transient, and the microorganism subsequently reacquires its original phenotype.

Variability in which the phenotypic shift is associated with a preceding structural alteration in the genotype is termed hereditary or genotypic. This form of variability is inherited and relatively rarely reverts to the original phenotype. The most prominent forms of variability are presented in Fig. 8.

Modifications are not accompanied by Changes in the genome and are rapidly lost. Modifications depend on environmental conditions and are subdivided into:

- morphological, in which cell shape, dimensions, and The ability to form capsules and spores are altered.

- cultural — associated with the ability to produce pigment (Serratia marcescens), and non-heritable alterations in colony type on solid nutrient media.

- PHYSIOLOGICAL AND BIOCHEMICAL, in which the corresponding properties of microorganisms are altered. A characteristic example is the production by bacteria of adaptive Enzymes only in the presence of a specific substance (substrate). Another example is the decrease in virulence properties of bacteria upon serial passage on artificial nutrient media. Passing such a culture through the organism of a susceptible animal is sufficient to fully restore its virulence properties.

During genotypic variability, alterations of genes in the chromosome invariably occur and are passed on by inheritance. Genotypic variability can arise As a result of Mutations and genetic recombination.

In a broad sense, a mutation can be defined as a suddenly arising, heritable change in the genetic apparatus of the cell. From a genetic perspective, mutations are alterations in the DNA nucleotide sequence manifested as a hereditarily fixed loss or modification of a trait or

group of traits. They are rooted in errors of hereditary information copying. Phenotypic manifestations of mutations may include changes in bacterial cell Morphology, temperature sensitivity, requirements for growth factors (Amino Acids, Vitamins), decreased virulence, and The Emergence of Antibiotic Resistance. Mutations in bacteria can cause both the loss of function and the acquisition of new traits.

Class="center">Fig. 8. Forms of microorganism variability

A distinction is made between cytoplasmic mutations, affecting non-chromosomal genetic elements (plasmids), and nuclear, or chromosomal mutations, associated with alterations in the nucleoid. The most important chromosomal rearrangements include:

- loss of chromosome segments (deletions),

- doubling (duplication) or multiplication (Amplification) of the number of individual genes or groups of genes,

- insertions of a chromosome segment into new locations (Transpositions),

- exchange of chromosomal segments (translocations),

- alteration in the arrangement order of genes on a chromosome (inversions).

A bacterium with DNA different from that of the original cell is called a mutant. If the resulting mutation confers an advantage in the Struggle for Existence, mutants survive and produce numerous offspring. Conversely, if the mutation confers no advantage, the mutant perishes. Accordingly, based on their phenotypic consequences, mutations are classified into neutral, conditionally lethal, and lethal. Neutral mutations do not manifest phenotypically as any alterations in traits. Mutations that lead to an alteration rather than a loss of physiological activity are called conditionally lethal. Depending on environmental conditions, cells may retain their viability or, conversely, lose it. Lethal mutations are characterized by the complete loss of the ability to synthesize metabolites vital for the cell (most commonly enzymes). Based on their phenotypic manifestation, mutations can be subdivided into morphological and biochemical. The former include mutations accompanied by the loss of morphological elements of the bacterial cell: flagella, capsule, spores, Cell wall. The majority of mutations that manifest as the loss of the ability to synthesize enzymes necessary for the Formation of primary or secondary metabolites are considered biochemical.

According to the extent of damage, mutations can be point mutations, when the damage is restricted to a single nucleotide pair, and extended mutations. Chromosomal mutations are those that cause the appearance of a new trait through the alteration of two or more chromosomal regions, while Gene Mutations are caused by the appearance of a new trait due to the alteration of a single gene.

An important characteristic of mutants is their ability to revert, i.e., to undergo reverse mutation to the initial phenotype. Mutants that arise as a result of reversion are called revertants.

Mutations always arise within a population of individuals, often without any visible impacts on the population. Such mutations, whose causes are unknown, are called spontaneous or uncontrolled. They can lead to both favorable and unfavorable genetic changes. Spontaneous mutations occur at a low frequency (10^-8).

The proportion of mutants in a microbial population can be sharply increased by subjecting them to mutagenesis, i.e., exposure to factors of a physical, chemical, or biological nature capable of inducing mutations. Factors that cause mutations are called mutagens, and mutations caused by mutagens are termed induced.

Physical mutagens include ultraviolet and ionizing radiation, magnetic fields, and temperature. Among chemical substances, the strongest mutagens are organic peroxides, acridine Dyes, ethyleneimines, alkylating agents, nitric acid, nitrogenous base analogues, etc. Nitrosoguanidine and nitrosomethylurea exhibit the highest efficacy. Biological mutagens include phages, Antibiotics, and phytoncides. Under METABOLISM/18.html">The Influence of mutagens, the mutation frequency increases up to 100,000-fold.

The second type of hereditary variability includes changes that occur in prokaryotes as a result of the recombination of genetic material, involving an interaction between two genomes that leads to The formation of DNA recombinants and the generation of a progeny genome combining The genes of both parents. In other words, the process of genetic recombination involves the pooling of genes from two different cells. There are Three Main Mechanisms leading to the recombination of prokaryotic genetic material: conjugation, transformation, and Transduction. In this process, a portion of the genetic material is transferred from the donor cell to the recipient cell.

During conjugation, the transfer of genetic material occurs via direct contact between two cells. In this process, a bridge forms between the two bacteria, and genetic material (usually plasmid DNA) passes from one cell to the other.

Transformation is the transfer of genetic information via DNA isolated from a donor cell. Bacterial Transformation can occur when DNA is transferred from one cell to another during the cultivation of bacterial cultures on media containing dead cells, filtrates, or extracts of other cultures (or pure DNA preparations). In such cases, bacteria acquire the traits of the microorganisms with which they were cultured and subsequently pass these properties on to their offspring. However, only closely related microorganisms are capable of integrating DNA into the cell's genetic apparatus.

During transduction, gene transfer from one cell to another is mediated by a bacteriophage (bacterial virus). In this process, a small segment of the bacterial chromosome is incorporated into the phage particle. When this particle infects a new bacterium, it injects the genetic material of the previous host (the donor cell) into it. Thus, the recipient cell becomes a carrier of the genetic information of the donor cell.

A distinction is made between generalized (nonspecific), specialized, and abortive transduction. Generalized transduction characterizes the bacteriophage-mediated transfer of a fragment from any part of the bacterial chromosome. Specialized transduction is observed when the phage DNA integrates into the bacterium to form a prophage. In abortive transduction, the introduced donor DNA fragment does not integrate into the recipient chromosome, but remains in the cytoplasm and Functions autonomously there. Subsequently, it is passed to one of the daughter cells and is eventually lost in the progeny.

During genetic recombination, the DNA molecule is not destroyed but merely modified; therefore, unlike mutations, the rearrangement of genetic structures does not cause lethal effects, and the phenotypic expression of the new trait is detected immediately after the integration of exogenous DNA. The frequency of recombination varies (10^-1–10^-9) and depends on the efficiency of DNA transfer.