Submitted to: Infection and Immunity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 2, 2011
Publication Date: September 19, 2011
Citation: Reif, K., Palmer, G.H., Ueti, M.W., Scoles, G.A., Noh, S.M. 2011. Dermacentor andersoni transmission of Francisella tularensis subsp. novicida reflects bacterial colonization, dissemination and replication coordinated with tick feeding.. Infection and Immunity. 79(12):4941-6. Interpretive Summary: Pathogens transmitted by ticks cause significant morbidity and mortality in animals and humans. Bacterial pathogens in diverse taxonomic classes share the common feature of invasion and replication in and transmission from a tick vector. Despite the importance of tick transmission in maintaining and spreading these pathogens, little is known about the molecular underpinnings of bacterial colonization in the tick and subsequent transmission. The tools required to identify these bacterial mechanisms are largely unavailable. In contrast to this limitation for tick-borne bacterial pathogens, genome-wide libraries have allowed for relatively rapid identification of genes required for specific functions in a broad array of bacterial pathogens. Recently, a mutant library of Francisella tularensis subsp. novicida has been developed and used to in vivo negatively select and thus identify genes required for pulmonary and systemic infection in mice. We asked whether F. novicida could serve as a model bacterium to identify pathogen determinants of tick colonization using the transposon mutant library. The data reported in this paper establish that F. novicida can colonize and be transmitted from Dermacentor andersoni ticks, and that D. andersoni can serve as a valid tool to negatively select F. novicida mutants which have an impaired growth phenotype in the tick. This model will allow for the identification of genes and thus gene products required for tick colonization.
Technical Abstract: Ticks serve as biological vectors for a wide variety of bacterial pathogens which must be able to efficiently colonize specific tick tissues prior to transmission. The bacterial determinants of tick colonization are largely unknown, a knowledge gap attributed in large part to the paucity of tools to genetically manipulate these pathogens. In this study, we demonstrated that Francisella tularensis subsp. novicida, for which a complete two-allele transposon mutant library has been constructed, initially infects the midgut of 100% of acquisition fed Dermacentor andersoni nymphs with stable colonization and replication during a subsequent molt.. Increased dissemination to and florid replication within the salivary gland was closely linked to a second (transmission) feed and culminated in bacterial secretion into the saliva and successful transmission. Testing multiple mutants resulted in total bacterial levels similar to that observed for single mutants. However, there was evidence of a bottleneck during colonization resulting in a founder effect in which the most successful mutant varied when comparing individual ticks. Thus it is essential to assess mutant success at the level of the tick population rather than in individual ticks. The ability of F. novicida to recapitulate the key physiological events by which bacteria colonize and are transmitted by ixodid ticks provides a new genome-wide approach to identify required pathogen molecules and pathways. The identification of specific genes and, more importantly, conserved pathways that function at the tick-pathogen interface will accelerate the development of new methods to block transmission.