GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

7. PROKARYOTE SYSTEMATICS

7.3. THE SPECIES CONCEPT IN BACTERIOLOGY

Giving a precise definition of a bacterial species is quite challenging due to their biological characteristics. While PLANT AND ANIMAL species are defined by well-defined morphological and physiological differences—specifically the inability of individuals from two populations to interbreed—these criteria are absent in Bacteria. Bacteria show little morphological diversity, reproduce asexually, feature plasmid-mediated heredity, and possess The ability to exchange genetic material even among distantly related groups. To approach the species concept in bacteriology, we must first review the historical development of this concept in biology as a whole.

Spontaneous ideas about the species of living organisms emerged in ancient times. With The Development of biology as a science in the pre-Darwinian period, two main concepts arose: nominalism and typology. Nominalists believed that species do not actually exist in reality. The typological concept can be described as morphological, since morphological similarity serves as the primary criterion for a species.

In the 1940s, through the work of Ernst Mayr, the biological species concept was formed. It is based on the premise that species consist of populations, are real, and possess internal genetic integration because all individuals (members) of a species share a common genetic program developed historically through evolution. According to the biological concept, a species is a group of interbreeding organisms that form natural populations and are reproductively isolated from other groups (i.e., they cannot interbreed with members of other groups).

The Main criteria for a species in biology were established as follows:

physiological (genetic criterion) — free interbreeding within the species and the inability to interbreed (reproductive isolation) with other species;

geographical, which implies that each species has a specific geographic range;

ecological — within its range, each species occupies a specific niche and is characterized by a particular type of interaction with individuals of other species and within its own species.

Among microbiologists, the view that bacterial species do not truly exist is widespread. This stems from a lack of information about the actual existence of bacteria in nature, as well as the considerable physiological diversity of natural isolates. Furthermore, it became evident that bacteria can exchange Genetic information via Plasmids, phages, and transformation. It turned out that certain conjugative plasmids can be transferred and maintained across different taxonomic groups of bacteria (for example, plasmids of the P-1 incompatibility group). This led many microbiologists to conclude that bacterial genomes are neither as protected nor as stable as eukaryotic ones. In reality, as geneticists studied bacterial Gene transfer mechanisms in greater detail, it became clear that such Conclusions were inaccurate. Factors limiting the penetration of foreign genes into Prokaryotic Cells include: restriction-modification systems; the activity level of Enzymes responsible for recombination mechanisms; the outer membrane system, through which foreign DNA can enter only via specific mechanisms encoded by conjugative plasmids; and a specific gene sequence within the Chromosomes.

Based on this, we can conclude that the criteria defining eukaryotic species are entirely applicable to bacteria. In other words, a bacterial species is a group of organisms that share a similar set and order of genes on their chromosome and are capable (at least potentially) of exchanging chromosomal genes and undergoing Homologous Recombination with high frequency.

However, this interpretation of the bacterial species concept remained unofficial for a long time. For instance, R.Y. Stanier and N.R. Krieg, in the Introduction to the ninth edition of Bergey's Manual of Systematic Bacteriology, define a bacterial species as a collection of strains that share many properties in common and differ significantly from other strains. They do acknowledge, however, that DNA Homology levels—an indicator of genetic relatedness—can be used for a more precise species definition. Practical guidelines on this matter were set forth in the decisions of the Committee on Ad Hoc Taxonomy of Bacteria, published as early as late 1987. Specifically, these decisions state that a species is the only taxonomic unit that can be defined phylogenetically. A species should include strains with a DNA-DNA relatedness level of 70% or higher, and a ΔT_m of 5 ºC or lower. Phenotypic characteristics must be consistent with this definition and may diverge from the phylogenetic species concept only as an exception.

The Application of Molecular biology Methods in microbiology has refined researchers' views on The Nature of bacterial species. In 2002, a meeting of the International Committee on Systematics of Prokaryotes took place. It recommended determining The nucleotide sequences of genes encoding 16S rRNA and Proteins, alongside DNA-DNA Hybridization, as the primary molecular criteria for species delineation. The Committee cited the definition of a species formulated by R. Rosselló-Móra and R. Amann (2001): "A species is a category that circumscribes a genomically coherent group of strains sharing a high degree of similarity in independent features under defined standard conditions."