University of Ljubljana

About: University of Ljubljana is a education organization based out in Ljubljana, Slovenia. It is known for research contribution in the topics: Population & Liquid crystal. The organization has 17210 authors who have published 47013 publications receiving 1082684 citations. The organization is also known as: Univerza v Ljubljani.

Extremophilic fungi in arctic ice: a relationship between adaptation to low temperature and water activity

Abstract: Little is known about fungal diversity in extremely cold regions. Low temperatures induce the formation of ice crystals and therefore also the creation of low water activity (aw). These are the dominant factors in external chemistry that influence microbial biota in cold regions. Therefore, we have used selective low water activity media plus low incubation temperatures for the isolation of fungi from an Arctic environment. In comparison with the highest values of colony forming units (CFU) obtained on mesophilic media, considerably higher fungal CFU per litre of water were detected on low aw media, ranging from 1000 to 3000 l � 1 in seawater, 6000 to 7000 l � 1 in melted sea ice and up to 13,000 l � 1 in melted glacier ice. The dominant taxa were ascomycetous and basidiomycetous yeasts, melanized fungi, mainly represented by the genera Cladosporium and Aureobasidium plus different species of the genus Penicillium. Preliminary taxonomic analyses revealed several new species and varieties. Further characterisations are needed to determine whether this diversity is due to geographic isolation, ecological conditions or independent evolutionary origin. 2003 Elsevier Ltd. All rights reserved.

Theoretical evaluation of the distributed power dissipation in biological cells exposed to electric fields.

Abstract: The paper deals with the power dissipation caused by exposure of biological cells to electric fields of various frequencies. With DC and sub-MHz AC frequencies, power dissipation in the cell membrane is of the same order of magnitude as in the external medium. At MHz and GHz frequencies, dielectric relaxation leads to dielectric power dissipation gradually increasing with frequency, and total power dissipation within the membrane rises significantly. Since such local increase can lead to considerable biochemical and biophysical changes within the membrane, especially at higher frequencies, the bulk treatment does not provide a complete picture of effects of an exposure. In this paper, we theoretically analyze the distribution of power dissipation as a function of field frequency. We first discuss conductive power dissipation generated by DC exposures. Then, we focus on AC fields; starting with the established first-order model, which includes only conductive power dissipation and is valid at sub-MHz frequencies, we enhance it in two steps. We first introduce the capacitive properties of the cytoplasm and the external medium to obtain a second-order model, which still includes only conductive power dissipation. Then we enhance this model further by accounting for dielectric relaxation effects, thereby introducing dielectric power dissipation. The calculations show that due to the latter component, in the MHz range the power dissipation within the membrane significantly exceeds the value in the external medium, while in the lower GHz range this effect is even more pronounced. This implies that even in exposures that do not cause a significant temperature rise at the macroscopic, whole-system level, the locally increased power dissipation in cell membranes could lead to various effects at the microscopic, single-cell level.

Equivalent Pulse Parameters for Electroporation

Abstract: Electroporation-based applications require the use of specific pulse parameters for a successful outcome. When recommended values of pulse parameters cannot be set, similar outcomes can be obtained by using equivalent pulse parameters. We determined the relations between the amplitude and duration/number of pulses resulting in the same fraction of electroporated cells. Pulse duration was varied from 150 ns to 100 ms, and the number of pulses from 1 to 128. Fura 2-AM was used to determine electroporation of cells to Ca2+. With longer pulses or higher number of pulses, lower amplitudes are needed for the same fraction of electroporated cells. The expression derived from the model of electroporation could describe the measured data on the whole interval of pulse durations. In a narrower range (0.1-100 ms), less complex, logarithmic or power functions could be used instead. The relation between amplitude and number of pulses could best be described with a power function or an exponential function. We show that relatively simple two-parameter power or logarithmic functions are useful when equivalent pulse parameters for electroporation are sought. Such mathematical relations between pulse parameters can be important in planning of electroporation-based treatments, such as electrochemotherapy and nonthermal irreversible electroporation.