Flocculation is an aggregation phenomenon of microbial cells in which they form flocs or flakes. In this study, it was found that addition of glycerol to a complex glucose medium promoted spontaneous floc formation by an Escherichia coli degP-deficient mutant strain (Delta degP) in a dose-dependent manner. In the presence of 10% (v/v) glycerol, the amount of floc formation (quantified as floc protein) reached its maximum value (230 mg/L), five times that in its absence. 10% (v/v) glycerol was the limit concentration that does not inhibit cell growth of Delta degP strain. Glycerol was not consumed by Delta degP cells during floc formation. To provide media having nearly the same viscosity as that containing 10% (v/v) glycerol, carboxymethyl cellulose (CMC) or polyvinylpyrrolidone (PVP) were added to medium as viscosifying agents. Floc formation was not promoted by increasing the medium viscosity with CMC or PVP. However, addition of ethylene glycol also significantly promoted floc formation in the same manner as glycerol. Addition of short-chain polyols decreased the number of viable Delta degP cells in the floc structure and enhanced outer membrane vesicle (OMV) production by Delta degP cells; polyols-induced damage on the outer membrane of Delta degP cells may contribute to the promoted floc formation. (C) 2020, The Society for Biotechnology, Japan. All rights reserved.