Effects of Osmotic Stress and Temperature on Microbial Growth

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Effect of Osmotic Stress and Temperature on Microbial Growth BIO 3400-002L – Microbiology Lab

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Effect of Osmotic Stress and Temperature on Microbial Growth Luiz Felipe Isidoro

ABSTRACT

Evolution allowed primitive forms of life to develop proteins and enzymes that made it possible for life to evolve under environments with hostile conditions, such as high salt and heat. More specifically, some bacteria selected genes that code for peptides with stronger intermolecular forces, coping with extreme heat, or for compatible solutes, which accumulate to compensate osmotic stress. The present study utilizes multiple bacterial strains to assess their ability to overcome unfavorable conditions and promote growth. To achieve this goal, three species of bacteria were incubated under different salt concentrations, and six were used in two separate procedures where incubation took place at various temperatures. These organisms were classified based on the observations made after the assays conducted.

INTRODUCTION

For the most primordial forms of life, osmotic stress and variations in temperature, when not fatal, were harmful for their growth. Nowadays, however, microorganisms can be found almost anywhere we can think of, independently of the conditions inherent to each of these places. It took billions of years of evolution to result in bacteria and archaea adapting to harsh conditions, such as extreme heat or

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hypertonicity, for example. During this long period of time, novel proteins were positively selected according to each environment, whose nature would keep their function at their optimum.

In a high salt environment, there is a gradient of water flowing from the interior of a microbial cell to the medium, in order to establish isotonicity; conversely, in low salt enviroments, the water gradient is from the medium into the cell. Although both lead to alterations in the shape of the cell (shrinking and bursting, respectively), which ultimately can result in cell death, some microbes developed traits to stop this water gradient: halophiles, for example, can synthesize proteins in enough quantities to increase the intracellular tonicity, causing a tonic equilibrium with the outside environment. Evolution also leaded to microorganisms developing stronger cell walls, increasing the resistance to a massive inflow of water in low salt environments.

Temperature is also another factor that plays a role in microbial life and growth: in cold temperatures, biochemical activity is reduced, and therefore active sites of enzymes cannot catalyze essential reactions efficiently due to the lack of necessary molecular vibrations. On the other hand, high temperatures can be harmful because some intermolecular forces that keep proteins at their proper tertiary and quaternary structures can be disrupted. Microorganisms evolved to cope with extreme temperatures in a variety of ways.

Thermophiles produce enzymes that catalyze the formation of more branched, saturated phospholipids, avoiding that their cell membranes melt; they also selected proteins with stronger intermolecular forces and with more incorporation of cysteine residues, increasing the amount of disulfide bonds.

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Psychrophiles, on the other hand, produce a higher amount of unsaturated, unbranched membrane phospholipids. These characteristics represent less

calorimetric energy from such phospholipids, resulting in a lower melting point, thus allowing the membrane to be fluid under cold environments.

The present experiment was designed to observe these temperature effects upon culturing of Escherichia coli, Micrococcus luteus, Bacillus stearothermophilus, Bacillus subtilis, Staphylococcus aureus, and Pseudomonas aeruginosa, as well as the osmotic stress effects on Chromobacter salexigens, E. coli, and S. aureus....
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