Comparative genomics reveals pathogen-and symbiont specific genes in Francisella and Coxiella

Ticks are vectors of human and animal pathogens. Additionally, ticks harbor endosymbionts and other commensal bacteria. Coxiella and Francisella endosymbionts are tick-associated bacteria closely related to human pathogens Coxiella burnetii, the agent of Q fever, and Francisella tularensis, which causes tularemia. Prior studies showed genomes of endosymbionts encode genes for the synthesis of B vitamins and cofactors but have lost virulence-related genes. The aim of this study was to compare the genes present in pathogens and endosymbionts to identify metabolic pathways that have been differentially retained between the two lineages. The genome portal BioCyc was used to analyze the genomes of Francisella endosymbionts in Ornithodoros moubata and Argas arbereus ticks, pathogens F. tularensis subsp. tularensis and F. tularensis subsp. novicida, Coxiella endosymbiont in Rhipicephalus microplus, human pathogen C. burnetii RSA 493 Nine Mile phase I and laboratory strain C. burnetii RSA 439 Nine Mile Phase II. Analysis showed that endosymbionts possess pathways for the biosynthesis of amino acids, transporters of inorganic ions, LoIA-lipoprotein complex and Lipopolysaccharide (LPS). Some genes for the transport of LPS a major structural unit of gram-negative bacteria were notably absent in the genomes of pathogens. Metabolic processes absent from tick symbionts but present in pathogens include pathways for the metabolism of 2"-deoxycytidine, thymidine, inorganic nutrients, pyruvate and glucose oxidation. Additionally, virulence genes for outer membrane and periplasmic proteins were absent in symbiont genomes. This knowledge from the biochemical processes in endosymbionts could be useful in designing new approaches towards the control of ticks and tick-borne diseases.