Aggregate formation in a freshwater bacterial strain induced by growth state and conspecific chemical cues

We investigated the induction of aggregate formation in the freshwater bacterium Sphingobium sp. strain Z007 by growth state and protistan grazing. Dialysis bag batch culture experiments were conducted in which these bacteria were grown spatially separated from bacteria or from co-cultures of bacteria and predators. In pure cultures of Sphingobium sp. strain Z007, the concentrations of single cells and aggregates inside and outside the dialysis membranes developed in a similar manner over 3 days of incubation, and the proportions of aggregates were highest during the exponential growth phase. Cell production of Sphingobium sp. strain Z007 was enhanced in the presence of another isolate, Limnohabitans planktonicus, from an abundant freshwater lineage (R-BT065) outside the bags, and even more so if that strain was additionally grazed upon by the bacterivorous flagellate Poterioochromonas sp. However, the ratios of single cells to aggregates of Sphingobium sp. strain Z007 were not affected in either case. By contrast, the feeding of flagellates on Sphingobium sp. strain Z007 outside the dialysis bags led to significantly higher proportions of aggregates inside the bags. This was not paralleled by an increase in growth rates, and all cultures were in a comparable growth state at the end of the experiment. We conclude that two mechanisms, growth state and the possible release of infochemicals by the predator, may induce aggregate formation of Sphingobium sp. strain Z007. Moreover, these infochemicals only appeared to be generated by predation on cells from the same species.