AGGREGATION IN AZOSPIRILLUM-BRASILENSE - EFFECTS OF CHEMICAL AND PHYSICAL FACTORS AND INVOLVEMENT OF EXTRACELLULAR COMPONENTS

A medium for consistent induction of aggregation of Azospirillum brasi lense cells was developed and used to study the effects of chemical an d physical factors as well as extracellular components involved in thi s phenomenon. Growth of A. brasilense strain Cd in a high C:N medium u sing fructose and ammonium chloride as C and N sources, respectively, resulted in flocculation visible to the naked eye after 24 h. No cell aggregates were formed after 72 h growth in low C:N medium. Aggregatin g cells, but not cells grown under low C:N, accumulated high amounts o f poly-beta-hydroxybutyrate and the cell envelope contained a well-def ined electron-dense layer outside the outer membrane. Suspending the a ggregates in 0.2 or 0.5 M urea was the only treatment effective for di srupting aggregates. The concentration of exopolysaccharide produced b y four different strains of A. brasilense, differing in their capacity to aggregate, strongly correlated with the extent of aggregation. Ele ctrophoretic protein profiles from different fractions of aggregating and non-aggregating cells were compared. Differences were observed in the pattern of low-molecular-mass proteins and in the polar flagellin that has previously been proposed to be involved in adhesion processes . However, a mutant lacking both lateral and polar flagella showed the strongest aggregation. The involvement of polysaccharides and/or prot eins in aggregation of A. brasilense is discussed.