CELL DENSITY-REGULATED RECOVERY OF STARVED BIOFILM POPULATIONS OF AMMONIA-OXIDIZING BACTERIA

The speed of recovery of cell suspensions and biofilm populations of t he ammonia oxidizer Nitrosomonas europaea, following starvation was de termined. Stationary-phase cells, washed and resuspended in ammonium-f ree inorganic medium, were starved for periods of up to 42 days, after which the medium was supplemented with ammonium and subsequent growth was monitored by measuring nitrite concentration changes. Cultures ex hibited a lag phase prior to exponential nitrite production, which inc reased from 8.72 h (no starvation) to 153 h after starvation for 42 da ys. Biofilm populations of N. europaea colonizing sand or soil particl es in continuous-flow, fixed column reactors were starved by continuou s supply of ammonium-free medium. Following resupply of ammonium, star ved biofilms exhibited no lag phase prior to nitrite production, even after starvation for 43.2 days, although there was evidence of cell lo ss during starvation. Biofilm formation will therefore provide a signi ficant ecological advantage for ammonia oxidizers in natural environme nts in which the substrate supply is intermittent, Cell density-depend ent phenomena in a number of gram-negative bacteria are mediated by N- acyl homoserine lactones (AHL), including N- (3-oxohexanoyl)-L-homoser ine lactone (OHHL). Addition of both ammonium and OHHL to cell suspens ions starved for 28 days decreased the lag phase in a concentration-de pendent manner from 53.4 h to a minimum of 10.8 h, AHL production by N . europaea was detected by using a luxR-luxAB AHL reporter system. The results suggest that rapid recovery of high-density biofilm populatio ns may be due to production and accumulation of OHHL to levels not pos sible in relatively low-density cell suspensions.