Effects of nystatin on discrete unit 150-3000 Hz extracellular spikes in biofilm forming Hericium erinaceus liquid cultures recorded using custommicroelectrode arrays

Extracellular electrical potentials have been observed in a number of filamentous mycelial species with incommensurable and non-stereotypic features. In Basidiomycetes, detecting these signals reliably is dependent on the properties of the cell wall and plasma membrane and requires implementation of microelectrode array hardware, filtering and spike sorting methods. In this paper, we present recording methods for detection of discrete unit extracellular spikes in biofilm forming liquid cultures of Hericium erinaceus. We utilised custom designed microelectrode arrays (MEAs) with passive planar hard gold microelectrodes and individual radius of 100 {micro}m in recordings at a sample rate of 30 kHz. Triplicate recordings of mycelial samples in a double shielded electromagnetic and RF shield box were conducted for wild-type, ionophore assays and fungicidal assays. The recordings were analysed offline using the Kilosort4 sorting algorithm resulting in detection of discrete unit spikes with milliseconds durations. The clustered spike waveforms for the wild-type triplicates were estimated to have a mean trough-to-peak-time of 2.68 {+/-} 0.087 ms and width at half maximum of 0.8 {+/-} 0.031 ms across a combined total of 418 spiking units. Ionophore assays using nystatin solution (10,000 units/ml) exhibited significant statistical differences including a reduction of total units to 97. A decrease in the trough-to-peak time of the mean waveform (1.97 {+/-} 0.32 ms) and an increase in the width at half maximum (2.7 {+/-} 2.45 ms) were also observed. Nystatin was found reduce the mean extracellular spike amplitude from 173.06 {micro}V in the wild-type to 25.76 {micro}V in the assay. Physiological disruption of the cell wall and plasma membrane was confirmed by environmental scanning microscopy comparison of triplicates at 90 % humidity. In comparison, a fungicidal assay utilising 12% w/v H2O2 solution resulted in zero spiking units and absence of discrete unit activity across all channels in triplicate recordings.