Genetic diversity and spatial segregation of Francisella tularensis subsp. holarctica in Germany

Francisella tularensis is an intracellular coccobacillus and the causative agent of tularemia, a zoonotic disease with a wide host range. Among F. tularensis subspecies, especially F. tularensis subsp. holarctica is of clinical relevance for European countries. The study presented herein, focuses namely on genetic diversity and spatial segregation of F. tularensis subsp. holarctica in Germany, as still limited information are available. The investigation is based on the analysis of 34 F. tularensis subsp. holarctica isolates plus one draft genome sequence from an F. tularensis subsp. holarctica outbreak was included. The isolates were cultured from sample material being that of human patients (n = 25) and wild animals (n = 9). For 6 out of 34 isolates, epidemiological links between disease onset in humans and tick-bits could be established showing again the importance of arthropod linked transmission of tularemia in Germany. The genomes from strains investigated herein were assigned to three of four major F. tularensis subsp. holarctica clades: B.4, B.6 and B.12. Thereby, B.6 and B.12 clade members were predominantly found. Also, it turned out that eight isolates which caused pneumonia in patients clustered into B.6 clade, postulated to be more virulent for hares if compared to B.12 clade members. Altogether, 19 different subclades were assigned to clade B.6 (Biovar I, Erythromycin sensitive) and B.12 (Biovar II, Erythromycin resistant). Additional 17 isolates without a yet final subclade defintion. The results achieved herein, clearly point out that the given genetic diversity in Germany seems to be comparably high to those found in other European countries including Scandinavian regions. A spatial segregation of B.6 and B.12 strains was found to be likely, B.12 clade members were predominantly found in eastern parts and B.6 members more in western to southern parts of Germany.