Salt Tolerance Test- Principle, Procedure, Results, Uses


A salt tolerance test is a microbiological test that determines the ability of bacteria to grow in a high salt concentration environment. It is based on the principle that salt affects the osmotic pressure and membrane permeability of bacterial cells, and only some bacteria have developed mechanisms to adapt and survive in such conditions. The test is useful for differentiating some bacterial groups, such as enterococci, aerococci, and staphylococci, from other similar-looking bacteria that are not salt tolerant .

The test is performed by inoculating a broth medium that contains 6.5% sodium chloride (NaCl) and a pH indicator, such as bromocresol purple, with the bacterial culture. The broth also contains a fermentable carbohydrate, such as glucose or dextrose, that serves as a substrate for bacterial metabolism. After incubation, the growth and color of the broth are observed. A positive result is indicated by turbidity (cloudiness) and/or a color change from purple to yellow, which means that the bacteria can grow and produce acid in the high salt medium. A negative result is indicated by no growth and no color change, which means that the bacteria are inhibited or killed by the high salt concentration .

Some advantages of the salt tolerance test are:

  • It is simple, inexpensive, and easy to perform.
  • It can provide a rapid presumptive identification of some bacterial groups.
  • It can help in the diagnosis of some infections caused by salt-tolerant pathogens.

Some limitations of the salt tolerance test are:

  • It is not a confirmatory test and requires other biochemical or molecular tests for complete identification of bacteria.
  • It may give false positive results with some bacteria that can grow in high salt but do not belong to the target groups, such as some beta-hemolytic streptococci or pediococci.
  • It may give false negative results with some bacteria that are slow growing or have low inoculum in the high salt medium.
  • It may be difficult to interpret the results if there is no color change but slight turbidity in the broth .

The salt tolerance test is an important tool for microbiologists and clinicians to differentiate and identify some bacteria that have clinical significance. For example, enterococci are common causes of urinary tract infections, endocarditis, wound infections, and bacteremia, and they are often resistant to many antibiotics. Aerococci are emerging pathogens that can cause urinary tract infections, endocarditis, septicemia, and meningitis. Staphylococci are well-known causes of skin and soft tissue infections, abscesses, pneumonia, osteomyelitis, septic arthritis, endocarditis, toxic shock syndrome, and food poisoning .

In this article, we will discuss the objectives, principle, requirements, procedure, results, quality control, precautions, applications, and limitations of the salt tolerance test in detail. We will also provide some examples of common salt-tolerant pathogenic cocci bacteria that can be identified by this test.