Miller, J. W. | bioRxiv Science

bioRxiv2018

A complex symbiosis involving within species variation in the response of Dictyostelium amoebae to Burkholderia bacteria

Recent symbioses, particularly facultative ones, are well suited for unravelling the evolutionary give and take between partners. Here we look at variation in wild-collected samples of the social amoeba Dictyostelium discoideum and their relationships with bacterial symbionts, Burkholderia hayleyella and Burkholderia agricolaris. Only about a third of field-collected amoebae carry a symbiont. We cured and cross-infected D. discoideum hosts with different symbiont association histories and then compared the responses of the amoebae to each symbiont type. Before curing, field-collected clones did not vary significantly in overall fitness, but infected hosts produced morphologically different multicellular structures. After curing and re-infecting, host fitness declined overall. However, natural B. hayleyella hosts suffered fewer fitness costs when re-infected with B. hayleyella, indicating that they have evolved mechanisms to tolerate their naturally acquired symbiont. Exploring relationships between endosymbionts and hosts that vary within species may also reveal much about disease dynamics.

microbiology↗

bioRxiv2018

In Vivo Measurements of the Frequency-Dependent Impedance of the Spinal Cord

Improved knowledge of the electrode-tissue impedance will be useful in optimizing the clinical protocols and resulting efficacy of the existing and emerging approaches to spinal cord stimulation. Toward that end, the complex impedance (amplitude and phase) of in vivo ovine spinal cord tissue was measured at the electrode-pial subdural surface interface from 5 Hz to 1 MHz, and with the bi-polar electrodes oriented both parallel and perpendicular to the rostral-caudal axis of the spinal cord. At stimulation frequencies above 10 kHz, most of the impedance then becomes resistive in nature and the phase diference between the stimulation signal and the resulting current drops to {approx} 10{ring}, thus maximizing power transfer to the tissues. Also, at these higher frequencies, the current pulse maintains significantly greater fidelity to the shape of the stimulation signal applied across the electrodes. Lastly, there were lower impedances associated with parallel as opposed to perpendicular orientation of the electrodes, thus reflecting the effects of fiber orientation within the spinal cord. Impedance differences of this kind have not been reported with epidural stimulation because of the electrical shunting effects of the intervening layer of relatively high conductivity cerebrospinal fluid. These observations provide a quantitative basis for improved models of spinal cord stimulation and suggest certain advantages for direct intradural stimulation relative to the standard epidural approaches. (Some figures in this article are in colour only in the electronic version)

biophysics↗