Enhanced floc formation by degP-deficient Escherichia coli cells in the presence of glycerol

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 (ΔdegP) 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 ΔdegP strain. Glycerol was not consumed by ΔdegP 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 ΔdegP cells in the floc structure and enhanced outer membrane vesicle (OMV) production by ΔdegP cells; polyols-induced damage on the outer membrane of ΔdegP cells may contribute to the promoted floc formation.

Knockout of pgdS and ggt gene changes poly-γ-glutamic acid production in Bacillus licheniformis RK14-46

Poly-gamma-glutamic acid (γ-PGA) is a water-soluble, nontoxic biocompatible polymer, which is extensively used in medicines, foodstuffs, cosmetics, and in water treatment. We previously isolated a novel γ-PGA producing strain Bacillus licheniformis RK14 from soil and developed a hyper-producing mutant strain RK14-46 by an ethyl methanesulfonate (EMS) treatment. In this study, endo-type (pgdS) and exo-type γ-PGA hydrolases (ggt) were disrupted by integrating plasmids into the genomic DNA of B. licheniformis RK14-46 strain. Unexpectedly, we observed strong inhibition of γ-PGA production following deletion of the pgdS gene, suggesting that pgdS is essential for γ-PGA biosynthesis in strain RK14-46, and in its parent strain RK14. In contrast, γ-PGA production increased by the deletion of the ggt gene and reached 39 g/L in the presence of 90 g/L glucose and elevated oxygen supply. Furthermore, γ-PGA from the ggt-disrupted mutant (Δggt) maintained a larger molecular mass throughout the culture period, whereas that from the original RK14-46 strain had degraded after glucose consumption. γ-PGA-containing culture supernatants from Δggt strain showed greater flocculation efficiency in sewage sludge than supernatants from the RK14-46 strain, reflecting greater production of γ-PGA with larger molecular mass by the Δggt strain. This is the first report concerning the deletion of pgdS and ggt genes in B. licheniformis strain and the properties of γ-PGA obtained from the mutant strain.

Inducing flocculation of non-floc-forming Escherichia coli cells

The present article reviews several approaches for inducing flocculation of Escherichia coli cells. The common industrially used bacterium E. coli does not naturally have floc-forming ability. However, there are several approaches to induce flocculation of E. coli cells. One is induction by flocculants-polyvalent inorganic salts, synthetic polymeric flocculants, or bio-based polymeric materials, including polysaccharide derivatives. Another method is the induction of spontaneous flocculation by changing the phenotypes of E. coli cells; several studies have shown that physical treatment or gene modification can endow E. coli cells with floc-forming ability. Coculturing E. coli with other microbes is another approach to induce E. coli flocculation. These approaches have particular advantages and disadvantages, and remain open to clarification of the flocculation mechanisms and improvement of the induction processes. In this review, several approaches to the induction of E. coli flocculation are summarized and discussed. This review will be a useful guide for the future development of methods for the flocculation of non-floc-forming microorganisms.