Evidence that fibrillar anchoring is essential for Azospirillum brasilense Cd attachment to sand

Inoculation and incubation of wild type Azospirillum brasilense Cd (agg⁺) in pure quartz sand resulted in cell attachment to sand particles by a network made up of various sizes and shapes of fibrillar material. Inoculation of sand with an aggregate-deficient mutant of strain Cd (agg^?) resulted in no detectable fibrillar formation. Initial attachment ratio between agg⁺ cells and agg^? cells was 4:1. However, similar bacterial populations developed in the sand. Rinsing the sand, colonized by either strains, had a greater effect on agg^?; decreasing adsorption from 8.1 to 1.4%. Prolonged rinsing entirely desorbed its cells from the sand. Long bacterial incubation in sand decreased the attachment ratio between agg⁺ and agg^? from 3.4:1 to 2.9:1 and decreased desorption (by rinsing) from 10:1 to 6:1. Agitation increased bacterial population size (from 2 × 10⁷ to 4 × 10⁸ cfu g^?1) and decreased the proportion of attachment of agg⁺ cells (from 29.2 to 9.8%). A decrease in attachment was being of higher magnitude in the non-aggregating mutant (from 5 to 0% adsorption). Protease treatment of sand colonized with either bacteria decreased attachement of agg⁺ (from 27.4 to 7.1%) and released proteinaceous compound(s) into the sand only in the agg⁺ strain. Addition of NaEDTA to sand before inoculation, decreased attachment of agg⁺ (from 24.2 to 14%) but had no effect on agg^?. Addition of low amcunt of clay (montmorillonite) to sand significantly increased adsorption of agg^? to the sand particles (from 8.8 to 98.3%). Survival period of agg⁺ cells in sand was slightly longer than that of agg^? cells. It is proposed that bacterial fibrils are essential for anchoring of A. brasilense to sand.