Aerobic granules with inhibitory strains and role of extracellular polymeric substances

Microorganisms compete with other species by secreting antimicrobial compounds. The compact structure of aerobic granules was generally assumed to provide spatial isolation, resulting in the co-occurrence of diverse strains that have similar or dissimilar functions. No studies have investigated whether stable, mature aerobic granules can be formed with two mutually inhibitory strains. The strain Acinetobacter sp. I8 competes with Bacillus sphaericus I5 in a well-mixed environment, but can form stable and mature granules at 400 mg L(-1) phenol by repeatedly replenishing fresh medium in a sequencing batch reactor. The supernatants collected from the I8 medium in its exponential-growth phase or from the I5+I8 medium cultivated for 12 or 24h significantly inhibited I5 growth. Addition of tightly bound extracellular polymeric substances (TBEPS) or loosely bound extracellular polymeric substances (LBEPS) extracted from I5+I8 granules effectively suppressed the inhibitory effects of I8 on I5. The TBEPS or LBEPS physically separate strain I5 from I8 in the granule, and effectively adsorb the inhibitory substance(s) in the suspension.