Abstract
The cold stress biology of psychrotrophic Pseudomonas strains isolated from the rhizosphere of Himalayan plants have been explored to evaluate their cryotolerance characteristcs. Pseudomonas strains were examined for stress metabolites, viz., exopolysaccharide (EPS) production, intracellular sugar, polyols and amino acid content, ice nucleation activity, and their freezing survival at -10 and -40°C, respectively. High freezing survival was observed for the Pseudomonas strains that were grown at 4°C prior to their freezing at -10 or -40°C. Increased EPS production was noticed when Pseudomonas strains were grown at lower temperatures, i.e., 4 and 15°C, in comparison with their optimal growth temperature of 28°C. All Pseudomonas strains showed low level of type-III class ice nucleation activity at -10°C after 96 h. Considerable differences were noticed in accumulated contents of various intracellular sugars, polyols, amino acids for all Pseudomonas strains when they grown at two different temperatures, i.e., 4 and 28°C, respectively. The unusual complement of stress protectants especially, raffinose, cysteine and aspartic acid that accumulated in the bacterial cells at low temperature was novel and intriguing finding of this study. The finding that raffinose is a key metabolite accumulated at low temperature is an exciting discovery, and to the best of our information this is first report ever signifying its role in bacterial cold tolerance.Electronic supplementary materialThe online version of this article (doi:10.1186/2193-1801-2-667) contains supplementary material, which is available to authorized users.
Highlights
Microorganisms have a range of evolutionary adaptations and physiological acclimation mechanisms that allow them to survive and remain active in the conditions of environmental stress
Bacterial growth and freeze survival Freezing survival studies of Pseudomonas strains revealed that strains which were grown at 4°C prior to freezing separately at −10 and −40°C demonstrated significantly higher freezing survival rather than cultures which were grown at 28°C prior to freezing (Figure 1)
It was observed that Pseudomonas strains grown at low temperature (e.g., 4°C) have a survival advantage upon freezing tolerance compared to their optimal growth temperature (28°C)
Summary
Microorganisms have a range of evolutionary adaptations and physiological acclimation mechanisms that allow them to survive and remain active in the conditions of environmental stress. It has been suggested that trehalose, glycerol and sorbitol are the major cryoprotectants for prokaryotic cells to response the freezing damage, thereby. In this regard, regulatory proteins and key metabolic enzymes require adjustments to cope with the temperature shifts in order to maintain a balanced microbial growth at the new environmental temperature. Regulatory proteins and key metabolic enzymes require adjustments to cope with the temperature shifts in order to maintain a balanced microbial growth at the new environmental temperature Under such conditions, the synthesis of specific cryoprotectant molecules might be enhanced that act as chemical chaperons and protect the cellular proteins from freezing temperature. Very little is known about the possibility of other molecules responsible for the survival of bacteria subjected to freezing challenge by an adaptation of the microbial cells to low temperatures in psychrotolerant/psychrophilic bacteria (Margesin and Schinner 1999; Mishra et al 2010)
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