The mechanism of aggregate formation by Selenomonas ruminantium

Summary The mechanism of granule formation by Selenomonas ruminantium was investigated. A basic protein has been isolated from the lysate of S. ruminantium which triggers cluster formation (small aggregates of 20–100 cells) of suspended cells. Evidence is presented that these basic proteins were of ribosomal origin. It is suggested that ribosomes are released into the culture broth by lysis and that the associated basic proteins are subsequently dissociated by high monovalent cation concentrations. It was found that these positively charged basic proteins interact with the negatively charged lipopolysaccharide of the organism to form the clusters. Adding lysate to suspended cells, followed by lowering of the pH from 5.8 to 4.5 also induced clustering. At dilution rates exceeding the maximum growth rate clusters were retained in anaerobic gas-lift reactors and grew into granules (1–3 mm). It is postulated that granules evolve from clusters. Within the clusters, lysis and a low pH are induced due to diffusion limitations. As a consequence dividing cells are entrapped within the clusters, resulting in growth.