Duron et al.
Tick-bacteria mutualism depends on B vitamin synthesis pathways.
Curr. Biol. 2018; 28, 1-7
High throughput 16S rDNA sequencing of the African tick Ornithodorus moubata revealed that a strain of Francisella (strain F-OM) could be detected in all examined larvae (n=100), most nymphs (19/20), and all adult ticks (n=10). The highest symbiotic density of strain F-OM was found in the Malpighian tubules, and in female ovaries the density of Francisella F-OM was 144 x higher than in male testes. A controlled transovarial transmission of the bacterial strain through the egg cytoplasm was suggested by the authors. Francisella F-OM may provide vitamin B to the ticks, which they need because blood as food source is rich in proteins and lipids but scare in B vitamins. Antibiotic treatment of nymphs drastically reduced the number of Francisella F-OM and hampered the emergence of adult females and males. The antibiotic effect on the ticks was not observed when vitamin B was added to the blood meal. Nymphs treated with antibiotics exhibited physical abnormalities compared to nymphs treated with antibiotics but supplemented with vitamin B. In the latter group, the mortality rate was lower (less than 5.1%) compared to 15.8% in the other group. The F-OM genome has conserved genes involved in the production of seven B vitamins, while compared to other Francisella species, the strain F-OM exhibits a substantial level of genome reduction and genes associated with virulence are missing or have been pseudogenized. The observed pattern of Ornithodorus moubata unveils an essential role to supply the ticks, which are highly adapted to an obligate blood feeding cycle, with B vitamins.