Bacillus subtilis, also known as hay bacillus or grass bacillus is a well-known and very common bacterium found in soil, water, air and decomposing plant matter. Bacillus subtilis is rod-shaped and has the ability to form a tough, protective endospore, allowing the organism to tolerate extreme environmental conditions. It is highly responsive to genetic mutation, and therefore has numerous experimental uses in a laboratory setting, but more for genetic research as opposed to health research.

Of interest to orchid growers is the Bacillus subtilis strain QST 713 (marketed as QST 713 or Serenade).  

This strain has a natural fungicidal activity and is used as a biological control agent. It is not a mutation but a natural occurring bacterium that is been used to control plant diseases and fungal pathogens, such us: blight, gray mould, mildew, early leaf spot, bacterial spot, botrytis.  Strain QST 713 controls the growth of certain harmful bacteria and fungi in various ways, probably either by competing for nutrients or directly colonizing and attaching to fungal pathogens.

I use it to control Botrytis and Anthracnoce.