A Francisella tularensis Chitinase Contributes to Bacterial Persistence and Replication in Two Major U.S. Tick Vectors

Abstract

Nearly 100 years after the first report of tick-borne tularemia, questions remain about the tick vector(s) that pose the greatest risk for transmitting Francisella tularensis (Ft), the causative agent of tularemia. Additionally, few studies have identified genes/proteins required for Ft to infect, persist, and replicate in ticks. To answer questions about vector competence and Ft transmission by ticks, we infected Dermacentor variabilis (Dv),Amblyomma americanum (Aa), and Haemaphysalis longicornis (Hl; invasive species from Asia) ticks with Ft, finding that although Aa ticks initially become infected with 1 order of magnitude higher Ft, Ft replicated more robustly in Dv ticks, and did not persist in Hl ticks. In transmission studies, both Dv and Aa ticks efficiently transmitted Ft to naïve mice, causing disease in 57% and 46% of mice, respectively. Of four putative Ft chitinases, FTL1793 is the most conserved among Francisella sp. We generated a ΔFTL1793 mutant and found that ΔFTL1793 was deficient for infection, persistence, and replication in ticks. Recombinant FTL1793 exhibited chitinase activity in vitro, suggesting that FTL1793 may provide an alternative energy source for Ft in ticks. Taken together, Dv ticks appear to pose a greater risk for harboring and transmitting tularemia and FTL1793 plays a major role in promoting tick infections by Ft.

Tick extracellular vesicles enable arthropod feeding and promote distinct outcomes of bacterial infection

“…We observed high bacterial burdens in the blood (6 × 10 5 CFU/mL), lungs, livers, and spleens (3 × 10 5 – 1.9 × 10 6 CFU/mg tissue). Importantly, our results corroborated with a reproducible and quantitative model of F. tularensis in D. variabilis 57 , 58 and revealed that the presence of F. tularensis inside ticks was comparable to what has been described on Martha’s Vineyard 59 . Martha’s Vineyard is an island off the coast of Massachusetts where epizootic transmission of tularemia by ticks has occurred since 2001 60 .…”

Seasonal Dynamics of Tick Species in the Ecotone of Parks and Recreational Areas in Middlesex County (New Jersey, USA)

“…The public health impact of H. longicornis in the USA is still unclear. Vector competence experiments have shown that H. longicornis is not a competent vector of Borrelia burgdorferi —the agent of Lyme disease [ 47 ]; Anaplasma phagocytophilum —the agent of human granulocytic anaplasmosis [ 48 ]; or Francisella tularensis —the agent of tularemia [ 49 ]. In contrast, H. longicornis is a competent vector of Rickettsia rickettsii —the causative agent of Rocky Mountain spotted fever [ 50 ] and Heartland virus [ 51 ].…”

Analysis of Microorganism Diversity in Haemaphysalis longicornis From Shaanxi, China, Based on Metagenomic Sequencing

“…The different abundances may be related to the carrying capacity of H. longicornis . Previous studies have found that F. tularensis did not persist in H. longicornis after artificial infection ( Tully and Huntley, 2020 ). However, Coxiella has a symbiotic relationship with H. longicornis .…”