Antimicrobial potential of cold-adapted bacteria and fungi from Polar Regions

The Earth's biosphere is predominantly cold in time (about 85 % of the year) being exposed to temperatures below 5 °C. Polar Regions constitute about 14 % of cold habitats on Earth. Such low-temperature environments often undergo a combination of environmental stresses including desiccation, nutrients limitation, high salinity, adverse solar radiation and low biochemical activity (Pearce 2012 ). Even though such harsh conditions preclude life in most of its forms, cold habitats have been successfully colonized by numerous organisms, especially by microorganisms that predominate over other organisms in terms of both biodiversity and biomass (Feller and Gerday 2003 ; Margesin 2007 ; Pearce 2012 ). On the basis of their cardinal temperatures, cold-adapted microorganisms are frequently distinguished in psychrophilic and psychrotolerant (or cold-loving and cold-tolerant, respectively) (Morita 1975 ). By defi nition, the optimal growth temperature of psychrophiles is < 15 °C, whereas they are not able to grow above 20 °C. On the other side, psychrotolerants can grow over a wide temperature range with the fastest growth rates being above 20 °C. Accordingly, the heat-sensitive true psychrophilic microorganisms inhabit permanently cold habitats, whereas psychrotolerant are overrepresented inenvironments undergoing seasonal or diurnal thermal fl uctuations (Margesin 2007 ). In global terms, psychrotolerants exhibit a much wider distribution than psychrophiles (Pearce 2012 ). Since cold-adapted microorganisms have been subjected to a number of environmental stresses on a long timescale, they have evolved a variety of structural and physiological modifi cations to ensure survival in restrictive environmental conditions (Pearce 2012 ). These include the production of cold-active enzymes with a more fl exible 3D structure at low temperature, cold-acclimation (CAPs) and coldshock proteins (CSPs), the incorporation of high amounts of unsaturated fatty acids and carotenoids in cell membranes to maintain optimum fl uidity and permeability, and the synthesis of cryoprotective substances (Margesin et al. 2007; Russell 2008 ). In addition to cellular modifi cations, the recently assessed cold-active antagonistic properties of cold-adapted microorganisms may reduce the presence of competitive microorganisms thus contributing to the microbial adaptation to permanently low temperatures (Lo Giudice et al. 2007a ; Mangano et al. 2009 ; Prasad et al. 2011 ; Bell et al. 2013 ). On the other side, such capability has highlighted the possibility to use cold-adapted microorganisms as a novel source of industrial exploitable antimicrobial compounds. Thus, many different, complex and sophisticated survival strategies, which are quite relevant for the ecology of cold-adapted microorganisms, might render them valuable resources also for biotechnological purposes (Cavicchioli et al. 2002 ).