Current knowledge of the taxonomy and classification of representatives from the order Legionellales (Legionellaceae, Coxiellaceae) Phylum Pseudomonadota

Abstract

Since the development of microbiology as a science, the identification, systematics and classification of microorganisms have been carried out based on the study of phenotypic characteristics. In the second half of the 20th century, after the discovery of the structure of DNA and polymerase chain reaction, the era of molecular biological research began in microbiology. Since 2002, the use of a complex of molecular biological and phylogenetic methods has been recommended as a guiding approach in the classification and taxonomy of prokaryotes, which has led to reclassification in some taxa. The representation of the taxonomy of Legionella pneumophila (order Legionellales, class Gammaproteobacteria) was formulated after the outbreak of legionnaires’ disease (legionellosis) in the USA in 1976. The description of the causative agent of Q-fever – Coxiella burnetii occurred 40 years earlier, but its classification and taxonomic status in the order Rickettsiales of the class Alphaproteobacteria were presented incorrectly based on the study of available phenotypic characteristics in the 1930s and 1950s. This determined the «fate» of C. burnetii and Q-fever for half a century, included in the list of rickettsiae and rickettsioses traditionally studied by rickettsiologists. As a result of the application of molecular biological methods in the 1990s, in the order Legionellales was replenished by several representatives of the order Rickettsiales, three representatives of which (C. burnetii, Rickettsiella grilli and Wolbachia persica) were moved to the class Gammaproteobacteria. Analysis of the 16S rRNA gene sequence led to the reclassification of Coxiella into a separate genus of gamma-proteobacteria of the phylum Proteobacteria (now Pseudomonadota) along with the genera Legionella, Francisella and Rickettsiella with representatives of which have the greatest phylogenetic relationship. Sequencing of the 16S rRNA gene of C. burnetii isolates from different geographical regions revealed only three nucleotide substitutions, indicating that the nucleotide sequences of these strains are closely related (the degree of homology is 99%), which confirms the phylogenetic uniformity of the genus Coxiella within a single species. A comparative analysis of the proteome of representatives of the order Legionellales and a phylodendrogramm constructed during the study of the main proteins of representatives of the genera Legionella, Coxiella, Acquicella and Rickettsiella (Diplorickettsia) allowed the formation of various clades confirming their monophyla. The C. burnetii genome encodes 24 similar components out of 27 involved in the pathogenesis of L. pneumophila by type IVB secretory system (type IVB secretory systems). Verification of the taxonomic position of C. burnetii is an example of the use of molecular biological methods to solve problems of taxonomy and classification of prokaryotes. It is advisable to use an integrated approach based on the application of «classical» methods of bacteriology and new molecular biological and phylogenetic methods, which will allow to obtain an objective understanding of the microorganisms associated with the human population.