Abstract
ABSTRACTThe Gram-negative obligate intracellular bacterium Rickettsia parkeri is an emerging tick-borne human pathogen. Recently, R. parkeri Sca2 and RickA have been implicated in adherence and actin-based motility in vertebrate host cell infection models; however, the rickettsia-derived factors essential to tick infection are unknown. Using R. parkeri mutants lacking functional Sca2 or RickA to compare actin polymerization, replication, and cell-to-cell spread in vitro, similar phenotypes in tick and mammalian cells were observed. Specifically, actin polymerization in cultured tick cells is controlled by the two separate proteins in a time-dependent manner. To assess the role of Sca2 and RickA in dissemination in the tick host, Rickettsia-free Amblyomma maculatum, the natural vector of R. parkeri, was exposed to wild-type, R. parkeri rickA::tn, or R. parkeri sca2::tn bacteria, and individual tick tissues, including salivary glands, midguts, ovaries, and hemolymph, were analyzed at 12 h and after continued bloodmeal acquisition for 3 or 7 days postexposure. Initially, ticks exposed to wild-type R. parkeri had the highest rickettsial load across all organs; however, rickettsial loads decreased and wild-type rickettsiae were cleared from the ovaries at 7 days postexposure. In contrast, ticks exposed to R. parkeri rickA::tn or R. parkeri sca2::tn had comparatively lower rickettsial loads, but bacteria persisted in all organs for 7 days. These data suggest that while RickA and Sca2 function in actin polymerization in tick cells, the absence of these proteins did not change dissemination patterns within the tick vector.
Highlights
The Gram-negative obligate intracellular bacterium Rickettsia parkeri is an emerging tick-borne human pathogen
Similar phenotypes were observed by comparing vertebrate and tick host cell backgrounds, and while all strains were able to disseminate in the tick after acquisition, the wild-type strain resulted in a greater bacterial load with a diminished ability to persist in tick reproductive tissue
High-magnification images of actin-based motility (ABM) in ISE6 cells were visualized at 48 hpi, demonstrating a similarity to that previously shown in mammalian cells [7,8,9, 12, 13]
Summary
The Gram-negative obligate intracellular bacterium Rickettsia parkeri is an emerging tick-borne human pathogen. Ticks exposed to R. parkeri rickA::tn or R. parkeri sca2::tn had comparatively lower rickettsial loads, but bacteria persisted in all organs for 7 days These data suggest that while RickA and Sca function in actin polymerization in tick cells, the absence of these proteins did not change dissemination patterns within the tick vector. It has recently been demonstrated that disruption of two key proteins leading to R. parkeri actin-based motility (ABM) negatively impacts intracellular bacterial movement and dissemination from cell to cell in in vitro models of mammalian infection [7] One of these proteins, RickA, is a nucleation promoting factor that functions by activating the host cell Arp2/3 complex to mediate actin branching and ABM [7, 8]. Similar phenotypes were observed by comparing vertebrate and tick host cell backgrounds, and while all strains were able to disseminate in the tick after acquisition, the wild-type strain resulted in a greater bacterial load with a diminished ability to persist in tick reproductive tissue
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