how is microbial fermentation used to preserve foods?

Answer 1

Preservation by Fermentation
Food fermentation is usually carried out with the use of micro-organisms, producing alcoholic, acetic or lactic end products as a result of their action on simple sugars present.
These reactions take place in the absence of oxygen, that is anaerobically, and are responsible for the production of wine, vinegar, yoghurt and pickled vegetables among others.
Having pure cultures of the desirable microorganism in the food products being fermented is critical to obtaining “clean”, characteristic flavours of end products.

Answer 2

Microbes like bacteria and yeasts ferment the sugars in the foods, producing an acidic environment that inhibits or retards the growth of other bacteria that would make the food inedible.

There's a very interesting article about fermented foods and their value in developing countries at:

http://www.fao.org/docrep/x0560e/x0560e06.htm