For millennia, humans have used salt for seasoning and preserving food, a practice that hints at its ability to combat microscopic life. Sodium chloride, or common table salt, can kill many types of bacteria. Under the right conditions, salt creates an environment where many microbes cannot survive by affecting the movement of water across their cell membranes.
The Science of Salt and Bacteria
The antimicrobial action of sodium chloride results from a physical process called osmosis. Bacterial cells contain water and are enclosed by a semi-permeable membrane that allows water to pass through but blocks salt ions. When a bacterium is in a high-salt environment, the concentration of salt outside the cell is much greater than inside.
This imbalance triggers osmosis, causing water to move out of the bacterium to the saltier environment. This rapid outflow of water causes dehydration in a process called plasmolysis, where the cell shrinks as its membrane pulls away from the cell wall.
This severe dehydration prevents proteins and enzymes from functioning, which halts metabolic processes. The disruption of these cellular functions ultimately leads to the death of the bacterial cell.
Factors Influencing Effectiveness
The ability of salt to kill bacteria depends on its concentration. For osmosis to have a lethal effect, the salt concentration must be significantly higher outside the bacterial cell, creating a hypertonic solution. Lower concentrations might only inhibit microbial growth, while a concentration of around 20% is sufficient to kill most common spoilage bacteria.
Salt is not equally effective against all bacteria, as some have adapted to high-salinity environments. Halotolerant bacteria can withstand high salt concentrations even if they do not require it for growth. For example, some strains of Staphylococcus aureus can survive in salt concentrations up to 20%. Other organisms, known as halophiles, are “salt-loving” and require high salt concentrations to thrive.
Common Uses of Salt as an Antimicrobial
This principle has been applied for centuries in food preservation. Curing meats like bacon and ham involves rubbing the surface with salt, which draws moisture out of the meat and any bacteria present. This creates a dry environment where spoilage microbes cannot flourish. A similar principle applies to pickling, where vegetables are submerged in a high-salt brine.
Beyond the kitchen, salt is used in home remedies like gargling with salt water to soothe a sore throat. The saline solution draws water out of inflamed tissues, which can reduce swelling. It also creates a temporary hypertonic environment in the mouth and throat that reduces the number of local bacteria, providing some relief.