See YIB on page b030 for a method for making your own yeast extract. Commercial preparation of yeast extract begins with a tanker load of waste yeast from a brewery. This process of self-lysis occurs in all cells at death. The wonderfully flavored yeast extract is then used for icecream syrup, candy, breakfast foods, gravy to add flavor and vitamins, sugars, and nucleic acids and other nutrients. A tiny part of the production is used in bacterial medium to replace some or all of the meat extract.
Yeast extract is high in B-complex vitamins.
It is easy to make. If neccesary you can grow your own yeast, but you can usually buy compressed yeast at any bakery. I usually comes in one pound foil wrapped bricks. Yeast Autolysate is often added to conventional media to supply growth factors for fastidious microbes. Incubate at 50C for 24 hours with occasion or mechanical stirring to autolyse the cells.