Abstract: A method for rendering bacteria dormant is provided which includes the steps of growing bacteria under aerobic conditions in an aqueous media in such a manner that the so-produced bacteria include at least one species of Pseudemonas bacteria which is capable of enzymatically reducing nitrate to nitrogen and one species of Rhodopseudomonas bacteria which is capable of photosynthetically forming red pigment; dissolving an effective amount of at least one compound selected from the group consisting of the sulfides of sodium and potassium in the bacteria containing media; and exposing the sulfide containing media under anaerobic conditions to light for a period of time sufficient to cause red photosynthetic pigment to develop in the Rhodopseudomonas bacteria. The product produced is a suspension of dormant bacteria.