Is there a common water-activity limit for the three domains of life?

Archaea and Bacteria constitute a majority of life systems on Earth but have long been considered inferior to Eukarya in terms of solute tolerance. Whereas the most halophilic prokaryotes are known for an ability to multiply at saturated NaCl (water activity (a w) 0.755) some xerophilic fungi can germinate, usually at high-sugar concentrations, at values as low as 0.650-0.605 a w. Here, we present evidence that halophilic prokayotes can grow down to water activities of <0.755 for Halanaerobium lacusrosei (0.748), Halobacterium strain 004.1 (0.728), Halobacterium sp. NRC-1 and Halococcus morrhuae (0.717), Haloquadratum walsbyi (0.709), Halococcus salifodinae (0.693), Halobacterium noricense (0.687), Natrinema pallidum (0.681) and haloarchaeal strains GN-2 and GN-5 (0.635 a w). Furthermore, extrapolation of growth curves (prone to giving conservative estimates) indicated theoretical minima down to 0.611 a w for extreme, obligately halophilic Archaea and Bacteria. These were compared with minima for the most solute-tolerant Bacteria in high-sugar (or other non-saline) media (Mycobacterium spp., Tetragenococcus halophilus, Saccharibacter floricola, Staphylococcus aureus and so on) and eukaryotic microbes in saline (Wallemia spp., Basipetospora halophila, Dunaliella spp. and so on) and high-sugar substrates (for example, Xeromyces bisporus, Zygosaccharomyces rouxii, Aspergillus and Eurotium spp.). We also manipulated the balance of chaotropic and kosmotropic stressors for the extreme, xerophilic fungi Aspergillus penicilloides and X. bisporus and, via this approach, their established water-activity limits for mycelial growth (∼0.65) were reduced to 0.640. Furthermore, extrapolations indicated theoretical limits of 0.632 and 0.636 a w for A. penicilloides and X. bisporus, respectively. Collectively, these findings suggest that there is a common water-activity limit that is determined by physicochemical constraints for the three domains of life

Antibacterial properties of wild edible and non-edible mushrooms found in Zimbabwe

Mushrooms have been used extensively in traditional medicine as antimicrobial, antiviral and antitumor agents. Infectious diseases remain a major threat to human health, due to global antimicrobial resistance. This has led to an increase in the search for new and potent antimicrobial substances. The aim of the present study was to investigate the antimicrobial activity of the aqueous (cold and hot) and organic solvents (methanol, ethanol and acetone) extracts of ten mushroom species collected from the woodlands in Zimbabwe against common local bacterial isolates Escherichia coli, Salmonella typhi, Staphylococcus aureus and Streptococcus pneumoniae using agar disc diffusion method. The crude extracts of the mushrooms exhibited antibacterial properties to all the bacteria tested. Extracts obtained from ethanol were the most effective tested against bacteria (36.5%), followed by methanol (30.8%) and acetone (30.8%). Aqueous extracts exhibited the lowest effect on bacterial growth inhibition (1.9%), despite including the extract with the highest inhibitory activity (14 mm). The acetone extract of Cantharellus symoensii had the second highest inhibitory value of 11.5 mm followed by the methanol extract from Cantharellus miomboensis and the ethanol extracts of Ganoderma lucidum and C. symoensii with values 11.0, 10.67 and 10.0 mm, respectively. Cantharellus heinemannianus and C. symoensii had the highest effect on inhibition of bacteria as indicated by the different extracts showing high inhibitory properties ranging from 8-14 mm [15.4% (8) each] followed by G. lucidum [13.5% (7)], while Boletus edulis, Coprinus sp. and Trametes strumosa had the least [5.8% (3) each]. The positive results of screening local mushrooms for antibacterial activity forms the basis for further phytochemical studies and development of antimicrobial agents against common human bacterial and fungal infections