Wann ist es Mikroorganismen zu salzig oder zu süß? / VAAM - Vereinigung für Allgemeine und Angewandte Mikrobiologie e.V.

When is it too salty or too sweet for microorganisms?

Each type of bacteria can handle salt and sugar differently. However, bacteria cannot tolerate an infinite amount of salt or sugar in their immediate environment, as this removes water from the cells.

This property allows food to be preserved with salt or sugar. Salt is used to preserve meat or fish, for example, while sugar is used as a preservative in jam.

Substances such as sugar and salt have a strong ability to bind water. This can be visualised if we imagine a cell that ends up in the sea, i.e. in salt water. The salt concentration in the surroundings of the cell is now higher than in the cell. Water therefore flows out of the cell in order to equalise the imbalance of salt concentration inside and outside the cell. As a result, a cell can "die of thirst" if too much water is removed from it. The reverse is true in a freshwater lake, where the concentration of substances in the cell is higher than in the area surrounding the cell. Water then flows into the cell from outside to equalise the concentration of substances: The cell threatens to burst. This equalisation of different substance concentrations is called osmosis.

But some microorganisms have developed strategies to cope with high osmotic pressure. Bacteria and fungi that are adapted to environments with a high salt content are known as halophilic (Greek halos, salt and philia, love). Some halophilic microorganisms store salt in the cell (mainly in the form of potassium chloride, as this salt dries out the inside of the cell much less than the normal table salt sodium chloride) and thus reduce the osmotic pressure. However, all metabolic processes within these microorganisms must be permanently adapted to higher salt concentrations.

Another possibility is the accumulation of soluble particles (compatible solutes, e.g. sugars or amino acids), which can reduce the osmotic pressure but do not harm the processes in the cell. This more flexible strategy is utilised by microorganisms that have to cope with frequently fluctuating salt concentrations.

One interesting example is the 2017 Microbe of the Year: Halobacterium salinarum lives in waters with a salt content of over 20 per cent and absorbs potassium chloride into the cell interior in order to survive there. This enables it to survive for hundreds of years even on salt crystals.

Karl Urban: Überleben im Salz - Ein Mikroorganismus entrinnt der Trockenheit, Deutschlandfunk 6/2012

Christine Ziegler: Angepasste Individualisten: Bakterielle Transporter im Osmostress, BIOspektrum 4/2010