Stability of bacterial coaggregates in extreme environments.

The stability of coaggregates formed by the interaction of Acinetobacter johnsonii S35 and Oligotropha carboxidovorans S23, both of which are nonflocculating sludge bacteria, was evaluated by a spectrophotometric assay of aggregation index and phase contrast microscopy. Exposure of coaggregates to conditions of various pH (3-11) or temperature (10-5 degrees C) did not cause significant deflocculation and high aggregation index (85-93%) was still maintained as compared to 93.3% under normal conditions. Further, heat pretreatment of A. johnsonii S35 and O. carboxidovorans S23 cells at 60 or 80 degrees C did not prevent their subsequent coaggregation indicating that the interacting cell surface structures are heat-resistant. However, dual exposure of coaggregates to low temperature (10 degrees C) and extreme pH caused disruption of coaggregates coupled with the reduction in aggregation index below 28% (pH 3) and 49% (pH 11). Use of sterile filtrate of settled sewage (SFSS) in place of electrolyte solution as coaggregating medium produced usual large coaggregates (above 100 microm) indicating reproducibility of this coaggregation in sewage treatment plants. SFSS coaggregates incurred lesser deflocculation (50 and 85%, respectively) at extreme pH (3 and 11, respectively) and low temperature. This study demonstrated that although bacterial coaggregates are strong and stable against individual pH or temperature stress, a dual stress might deteriorate the performance and quality of treated waters.