Does Salt Water Kill Bacteria?





Salinity is an abiotic factor along with light, temperature, and nutrients. This can affect the survival of unicellular organisms like bacteria.

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All water have a certain degree of salinity. Whereas freshwater has a salinity of about 0.5 parts per thousand, estuaries, where freshwater mixes with ocean salty water, have a salinity of between 0.5 and 30 parts per thousand (ppt). The ocean, on the other hand, has an average salinity of 35 parts per thousand.

Like many other organisms, different types of bacteria have adapted to living in different types of habitats. Bacteria species capable of surviving and growing under high salt concentrations are known as halophiles. However, there are many other species that are incapable of surviving under such conditions.

 





How Different Types of Bacteria respond to Saltwater



Adaptations of Halophilic Bacteria


Generally, there are two main strategies that halophilic bacteria use to survive in salty water.


These include:

 

  • Salting-in (salt-in) - As the name suggests, this strategy relies on taking salt into the cell (potassium chloride). Bacteria take potassium chloride into the cell while removing sodium ions. This allows the organisms to maintain the same concentration of salt inside as outside the cell thus preventing the organism from losing water and dying. For bacteria like Salinibacter ruber and Halanaerobiales, this is largely dependent on the sodium and chloride pumps. While the sodium pump pushes out sodium, potassium is brought into the cell. The chloride pump is used to accumulate inorganic salt into the cell for the same purpose.

 

In these bacteria, structural proteins are modified for this function. They have been shown to have different types of amino acids and fewer alpha-helices.

 

  • Salting-out (salt-out) - Essentially, the salt-in strategy relies on taking in inorganic salt into the cell in order to balance the concentration outside the cell. Salting-out largely relies on accumulating organic molecules into the cell so as to create hypertonic conditions within the cell. These may include sugars or amino acids among other molecules produced by the cell or obtained from the surroundings. Balancing the concentration of salt outside the cell prevents the cell from being deprived of water through osmosis.

 




Halophilic bacteria are divided into three main categories that include:

 

  •      Slight halophiles - Tolerate 0.34 to 0.85 M salt
  •      Moderate halophiles - Tolerate 0.94 to 3.4 M salt
  •      Extreme halophiles - Tolerate 3.4 to 5.1 M salt



For all halophilic bacteria, all adaptive strategies serve to prevent water from leaving the cell and shrinking (resulting in the death of the cell).





Bacterial Death in Salty Water



 

In isotonic conditions, the osmotic pressure inside the cell is equal to that outside the cell. For this reason, the cell membrane in bacteria remains attached to the cell wall and there is no net movement of water particles. This is due to the fact there is no greater osmotic pressure on either side to influence the movement of water. 

In a hypotonic solution, water moves/flows into the cell because solute concentration inside the cell is higher than outside the cell. Though the cell swells as more water enters the cell, the cell wall counters the osmotic pressure which prevents the cell from bursting.

When some bacteria (non-halophilic bacteria) are placed in a saline hypertonic solution, they lose water due to higher saline concentration outside the cell than inside the cell. As water is lost, the cell shrinks as the membrane detaches from the cell wall.

 

Plasmolysis can occur in both Gram-negative and Gram-positive cells.


As a result of water efflux across the membrane (in hypertonic solutions), turgor pressure is lowered.

 


Unlike other molecules like oxygen and carbon dioxide, water is a polar molecule. For this reason, despite being small in size, water molecules cannot easily diffuse through the membrane. In this case, it's worth noting that some water molecules will slowly diffuse through the membrane. For the most part, water is transported in and out of the cell through channels known as Aquaporins (AQP).

Aquaporins can be found in animals and plants as well as bacteria. This type of transport is known as facilitated diffusion. As compared to simple diffusion, it involves the use of specific transport channels. However, like simple diffusion, substances are transported down the concentration gradient. For this reason, the movement of molecules like water is still largely dependent on differences in concentration inside and out of the cell.



Given that the cytoplasm is between 60 and 86% water, the loss of water under hypertonic (high saline concentration outside the cell) conditions results in the death of many cells.

While water plays an important role in supporting the cellular structure (maintains shape and structure by filling the intracellular environment), it's also involved in a variety of chemical reactions.

Water is used in photosynthesis among some photosynthetic bacteria (E.g. Cyanobacteria) as well as influencing how amino acids are folded. Water in these cells has also been shown to act as a buffer thus protecting the organisms from the effects of substances like acids and bases.

Water is an essential element of bacterial cells and necessary for survival. Therefore, by losing water under highly saline conditions, a variety of cellular processes, as well as the general structure of the cell, are affected which can, in turn, result in the death of the organism.


* Given that salty water can kill some bacterial cells, gargling salty water is sometimes used as a means of killing some mouth bacteria.  It's worth noting that this is not an effective strategy for killing harmful mouth bacteria that cause tooth decay and gum disease as salty water does not kill all bacteria.


* Though gargling salt water is not an effective strategy of killing harmful mouth bacteria, rinsing the mouth with a warm solution of salty water is recommended by the American Dental Association as a means of keeping the site of dental procedures clean and minimizing chances of an infection. 

 






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References



 Nagina Parmar and Ajay Singh. (2014). Geomicrobiology and Biogeochemistry.

 

Nina Gunde-Cimerman, Ana Plemenitaš, and Aharon Oren. (2018). Strategies of adaptation of microorganisms of the three domains of life to high salt concentrations.

 

S. M. Abdulkarim, A. B. Fatimah and J. G. Anderson. (2009).  Effect of salt concentrations on the growth of heat-stressed and unstressed

Escherichia coli.

 

 

Links

https://www.medicalnewstoday.com/articles/325238

https://www.sciencedirect.com/topics/immunology-and-microbiology/halophiles

 

 

 





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