Salt and Microorganisms
Name: Yona Kim
How and why exactly does sodium chloride affects microorganisms?
or bacteria? In another word, how does preservative(salt) works?
All organisms with a semipermeable membrane are subject to osmotic pressure,
or the effect of water moving in and out of the cell. Bacteria have a cell
membrane and a cell wall. Bacteria must live in an aqueous (watery)
environment. Most often this is a hypotonic environment, in other words, the
concentration of water outside the cell is greater than the concentration of
water inside the cell. This causes the net movement of more water into the
cell than outside. If the bacterium did not have a cell wall, this could
cause the cell to burst. (In fact, many antibiotics work by causing an
ineffective cell wall to be made, which allows the bacterial cell to burst
under water pressure). So why does salt work as a preservative? Because
when the outside environment around a cell is salty, then the concentration
of water in the solution is less than inside the cell and water tends to
leave the cell. This causes the cell to dehydrate, which eventually kills
the cell. By subjecting bacteria to a salty environment, it keeps them from
growing. Some bacteria however, have adapted to living in salty
environments, such as Staph. bacteria a common skin inhabitant. Your skin
tends to be salty-this is one way your body protects you against bacteria on
your skin. But even Staph can't live in highly salty surroundings, such as
salted foods like ham, etc.
The major effect of salt as a preservative is that it withdraws water from
microorganisms if the external salt concentration is high enough. The
microbes would shrivel and die, spores would not be killed but would not
be able to germinate. High concentrations of sugar have the same effect.
The physical term for this is hypertonic tension. Some bacteria have
learned to cope with high salt concentrations and can live in saline
waters. Fortunately they are not pathogenic (do not cause disease) so we
need not worry about them.
Curator of the Virtual Museum of Bacteria
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Update: June 2012