T. pyogenes belongs to Gram-positive, facultative anaerobic, pleomorphic, non-spore forming microorganisms, currently classified as a representative of the genus Trueperella. Species from this genus have been separated from Arcanobacterium to constitute a new taxonomic unit. The primary basis for the reclassification of these microorganisms was provided by the results of investigations concerning the presence and types of isoprenoid quinones in the bacterial cell wall. Biochemical studies were subsequently extended to include the analyses of the 16S rRNA gene sequence, the peptidoglycan structure, the G+C content in the DNA strand, the profile of lipid acids, the cell wall sugar composition, the type of muramyl acyl groups, the polar lipid profile, and the menaquinone composition. Owing to a variety of virulence factors, T. pyogenes is able to colonise many tissues, causing clinical symptoms in various organs and systems. The main virulence factor in T. pyogenes is pyolysin (PLO), an intracellular toxin with cytolytic activity against various types of cells, particularly neutrophils and macrophages. The microorganism also exhibits the ability to adhere to host cells, which ensures its persistence and survival on the mucous membrane surface. The main role in the adhesion process of T. pyogenes is played by the neuraminidases NanH and NanP. Additionally, with their ability to attach to the components of the extracellular matrix (ECM) of the host cell, surface proteins also serve as an adhesion factor. Fimbriae are an important virulence factor of T. pyogenes. The function of the hitherto recognized fimbrial proteins FimB, FimA, and StrA consists in facilitating bacterial penetration through the cell membrane. During infections caused by T. pyogenes a characteristic bacterial growth is observed, known as biofilm formation, which results in resistance to antibacterial agents and mechanisms of anti-infective host immune response, including phagocytosis. Virulence factors of T. pyogenes also include specific calcium-dependent proteins exhibiting proteolytic activity, which play a significant role in the infectious process and tissue damage, microbial escape from the host’s immune defence, and the modulation of the immune system during infectious and inflammatory processes. The identification of the types and mechanisms of action of individual virulence factors of T. pyogenes at the molecular level will provide a basis for the development of a rational veterinary medical strategy for the treatment of animals infected with these microorganisms.
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