In general baking terms, salt and live yeast should never meet directly because only bad things can happen. The salt will instantly and irrevocably kill the yeast, rendering it completely useless for gas generation. This is why bakers will keep at least one layer of ingredients between the live activate yeast and any added salt. When salt is deliberately added to live yeast, however, it becomes an entirely new food ingredient known as autolyzed yeast.
The interaction between salt and live yeast creates a chemical process called autolysis. Autolysis is essentially the self-destruction or self-digestion of an organism by its own enzymes. Salt does not "kill" yeast as much as it causes the live yeast's digestive enzymes to eat themselves. The result is an inactive yeast with a different concentration of proteins. This new food product can be further processed to create a yeast extract.
Many people may not recognize autolyzed yeast as an ingredient in many of their favorite processed foods, but they might recognize an ingredient derived from it: MSG, or monosodium glutamate. Both are used as flavor enhancers in processed foods. Autolyzed yeast extract is also used by itself in processed food spreads such as Vegemite. Because of its high protein content, this yeast generally has a hearty flavor similar to beef, which is why it is often used to give processed foods a meaty or savory undertone.
Both MSG and autolyzed yeast enhance the flavor of food by chemically altering the consumer's taste buds. Even though they are both often used for the same purpose, they are not exactly the same. Autolyzed yeast has a substantial amount of glutamate, but it comes nowhere close to the amount found in MSG. MSG is very concentrated and almost entirely composed of glutamates.
There is some controversy over the use of autolyzed yeast and other flavor enhancers, however. A number of people are extremely sensitive to MSG, which can trigger a condition known as a "Chinese restaurant headache." Specifically, the sodium and the glutamates can trigger an allergic reaction in certain people.
US Food and Drug Administration (FDA) requires manufacturers to list added MSG as an ingredient on the packaging of products. MSG is also found naturally in some foods and when a food product contains naturally found MSG, the FDA does not require that it be listed on the packaging. However, manufacturers cannot claim that their products are free of MSG, even if the MSG is found naturally in the ingredients.