Principle of preservation by pickling is based on

The principle of preservation by pickling is based on Lactic acid fermentation.
Pickling is the process of preserving edible products in an acid solution, usually vinegar, or in salt solution (brine). In the latter case, the acid that does the preservative action (lactic acid mainly) is produced by fermentation.
Pickles are produced through the fermentation of fruits and vegetables in brine or in their juices that contain certain salt concentrations with Lactic Acid Bacteria. The process of pickling is also known as bringing and the resulting foods as pickles.