The effect to which salt concentration causes changes in bacterial growth depends on the osmotic balance required for such growth. Some bacteria require an astonishingly high level of salt to begin growth, whereas other bacteria would be immediately killed in high levels of salt.
Some marine bacteria, such as members from the Vibrio genera, require a certain amount of salt to grow. Other bacteria, such as E. coli, can grow either in the presence or absence of salt.
Bacteria That Require Salt
The bacterial species that require salt, like Vibrio, have an internal osmolyte concentration that requires salt on the outside of the cell to maintain osmotic balance over the cell membrane. Without salt, these organisms would "burst" as water rushed into the cell to try and achieve this balance.
Some bacteria, such as E. coli, can grow without salt present but can also tolerate small amounts of salt in their growth medium. E. coli may have optimal growth in the absence of salt, but in the presence of salt it will grow, just at an attenuated rate.
Highly Salt-Tolerant Bacteria
According to the article "Bacterial Food Poisoning" by food technologist Al Wagner Jr. at Texas A&M University, Listeria species are able to grow over an extremely large range of salt concentration, up to 30.5 percent. This level of tolerance can affect the food industry's attempts to use salt as a natural way to control the presence of microorganisms.
Direct Correlation Between Salt and Growth
For bacteria with salt tolerance, growth and salt concentration have a direct correlation. As the amount of salt in the growth medium increases, bacterial growth decreases. For bacteria that require salt, a bell curve of growth is observed.