The production of bacterial cellulose in Gluconacetobacter xylinus regulated by luxR overexpression of quorum sensing system.

Quorum sensing is a mechanism that facilitates cell-to-cell communication. Through signal molecular density for signal recognition, which leads to the regulation of some physiological and biochemical functions. Gluconacetobacter xylinus CGMCC 2955, which produces bacterial cellulose (BC), synthesizes the LuxR protein belonging to the LuxI/LuxR type QS system. Here, a luxR overexpression vector was transformed into G. xylinus CGMCC 2955. The overexpression of luxR increased the yield of BC by 15.6% after 16 days static culture and reduced the cell density by 15.5% after 120-h-agitated culture. The glucose was used up by G. xylinus-pMV24-luxR at 72-h-agitated fermentation, which 12 h earlier than the wild-type (WT). The total N-acylhomoserine lactones (AHL) content of the luxR-overexpressing strain and the WT strain attained 1367.9 ± 57.86 mg/L and 842.9 ± 54.22 mg/L, respectively. The C12-HSL and C14-HSL contents of G. xylinus-pMV24-luxR were 202 ± 21.66 mg/L and 409.6 ± 0.91 mg/L, which were significantly lower than that of WT. In contrast, C6-HSL showed opposite results. The difference of AHL content proved that overexpression of luxR improved the binding of AHL and showed preference for some specific AHL. The metabolic results demonstrated that upon glucose exhaustion, the consumption of gluconic acid was promoted by luxR overexpression, and the content of D- ( +)-trehalose, an antiretrograde metabolite, increased significantly. KEY POINTS: • The overexpression of luxR increased the yield of bacterial cellulose • The content of signal molecules was significantly different • Differential metabolites were involved in multiple metabolic pathways.

Applications of cellulose and chitin/chitosan derivatives and composites as antibacterial materials: current state and perspectives.

The bacterial infections have always a serious problem to public health. Scientists are developing new antibacterial materials to overcome this problem. Polysaccharides are promising biopolymers due to their diverse biological functions, low toxicity, and high biodegradability. Chitin and chitosan have antibacterial properties due to their cationic nature, while cellulose/bacterial cellulose does not possess any antibacterial activity. Moreover, the insolubility of chitin in common solvents, the poor solubility of chitosan in water, and the low mechanical properties of chitosan have restricted their biomedical applications. In order to solve these problems, chemical modifications such as quaternization, carboxymethylation, cationization, or surface modification of these polymers with different antimicrobial agents, including metal and metal oxide nanoparticles, are carried out to obtain new materials with improved physiochemical and biological properties. This mini review describes the recent progress in such derivatives and composites with potential antibacterial applications.

Identification of Quorum-Sensing Molecules of N-Acyl-Homoserine Lactone in Gluconacetobacter Strains by Liquid Chromatography-Tandem Mass Spectrometry.

Many Gram-negative bacteria can regulate gene expression in a cell density-dependent manner via quorum-sensing systems using N-acyl-homoserine lactones (AHLs), which are typical quorum-sensing signaling molecules, and thus modulate physiological characteristics. N-acyl-homoserine lactones are small chemical molecules produced at low concentrations by bacteria and are, therefore, difficult to detect. Here, a biosensor system method and liquid chromatography-tandem mass spectrometry were combined to detect and assay AHL production. As demonstrated by liquid chromatography-tandem mass spectrometry, Gluconacetobacter xylinus CGMCC No. 2955, a Gram-negative acetic acid-producing bacterium and a typical bacterial cellulose (BC) biosynthesis strain, produces six different AHLs, including N-acetyl-homoserine lactone, N-butanoyl-homoserine lactone, N-hexanoyl-homoserine lactone, N-3-oxo-decanoyl-homoserine lactone, N-dodecanoyl-homoserine lactone, and N-tetradecanoyl-homoserine lactone. Gluconacetobacter sp. strain SX-1, another Gram-negative acetic acid-producing bacterium, which can synthesize BC, produces seven different AHLs including N-acetyl-homoserine lactone, N-butanoyl-homoserine lactone, N-hexanoyl-homoserine lactone, N-3-oxo-octanoyl-homoserine lactone, N-decanoyl-homoserine lactone, N-dodecanoyl-homoserine lactone, and N-tetradecanoyl-homoserine lactone. These results lay the foundation for investigating the relationship between BC biosynthesis and quorum-sensing systems.

Structure-Dependent Antibacterial Activity of Amino Acid-Based Supramolecular Hydrogels.

Bacterial infections are currently a major concern to human health. Amino acid-based supramolecular polymer hydrogels, which boast intrinsic antibacterial activity, are an important solution due to their good biocompatibility, cost effectiveness, and tunable structural properties. Herein, we reported three types of transparent supramolecular hydrogel with intrinsic antibacterial activity from self-assembly of commercially available Fmoc-tryptophan (Fmoc-W), Fmoc-methionine (Fmoc-M), and Fmoc-tyrosine (Fmoc-Y). The resulting hydrogels selectively inhibited the growth of Gram-positive bacteria. Moreover, the order of antibacterial activity was Fmoc-W hydrogel > Fmoc-M hydrogel > Fmoc-Y hydrogel. The critical aggregation concentration (CAC) values were found at concentrations of approximately 0.0293, 0.1172, and 0.4688 mM for Fmoc-W, Fmoc-M, and Fmoc-Y, respectively. Transmission electron microscope (TEM) images revealed rigid and aligned nanofibers for Fmoc-W hydrogel, while flexible nanofibers for Fmoc-M hydrogel and Fmoc-Y hydrogel. The results indicated that stronger aggregation capability of the gelator and the synergistic nanostructural morphology with more rigid and aligned nanofibers can lead to higher antibacterial activity of its corresponding hydrogel. In addition, the molecular arrangements of Fmoc-amino acids in the hydrogels may also contribute to their antibacterial activity. These results can guide the rational design, fabrication, and future application of other self-assembled amino acid-based hydrogels with excellent antibacterial activity.

A Lambda Red and FLP/FRT-Mediated Site-Specific Recombination System in Komagataeibacter xylinus and Its Application to Enhance the Productivity of Bacterial Cellulose.

Komagataeibacter xylinus has received increasing attention as an important microorganism for the conversion of several carbon sources to bacterial cellulose (BC). However, BC productivity has been impeded by the lack of efficient genetic engineering techniques. In this study, a lambda Red and FLP/FRT-mediated site-specific recombination system was successfully established in Komagataeibacter xylinus. Using this system, the membrane bound gene gcd, a gene that encodes glucose dehydrogenase, was knocked out to reduce the modification of glucose to gluconic acid. The engineered strain could not produce any gluconic acid and presented a decreased bacterial cellulose (BC) production due to its restricted glucose utilization. To address this problem, the gene of glucose facilitator protein (glf; ZMO0366) was introduced into the knockout strain coupled with the overexpression of the endogenous glucokinase gene (glk). The BC yield of the resultant strain increased by 63.63-173.68%, thus reducing the production cost.

Enhancing Medium-Chain Fatty Acid Ethyl Ester Production During Beer Fermentation Through EEB1 and ETR1 Overexpression in Saccharomyces pastorianus.

Esters are important flavor compounds in alcoholic beverages. Although they are present at trace levels, esters play a key role in the formation of flavors, especially fruity flavors, in beverages. Low ester contents result in bland beer and unpleasant flavor. In this study, three recombinant strains, ethanol O-acyltransferase encoding EEB1 overexpression strain (31194:: EEB1), 2-enoyl thioester reductase encoding ETR1 overexpression strain (31194:: ETR1), and EEB1- ETR1 co-overexpression strain (31194:: EEB1:: ETR1), were constructed. Ethyl hexanoate production by 31194:: EEB1 and 31194:: EEB1:: ETR1 was 106% higher than that by the parental strain. Further, ethyl octanoate production by 31194:: EEB1 and 31194:: EEB1:: ETR1 was enhanced by 47 and 41%, respectively, compared with that of parental strain 31194. However, no difference was observed between 31194:: ETR1 and the parental strain in terms of ethyl hexanoate and ethyl octanoate production. This indicates that although EEB1 overexpression in Saccharomyces pastorianus enhanced ethyl hexanoate and ethyl octanoate production, ETR1 expression levels did not affect the extracellular concentrations of these esters.

Preparation and characterization of a photocatalytic antibacterial material: Graphene oxide/TiO2/bacterial cellulose nanocomposite.

In this study, bacterial cellulose (BC) was used as a matrix to synthesize graphene oxide/Titanium dioxide (GOTiO2)-based hybrid materials. It was indicated by X-ray diffraction and selected area electron diffraction that the crystal structure of GOTiO2 was a mixed phase containing anatase and rutile. TiO2 nanoparticles were of 10-30nm diameters and densely anchored on graphene oxide sheets. Superior photocatalytic performance of the GOTiO2 was achieved under near UV excitation. The photocatalytic efficiency was optimized through controlling an appropriate calcined temperature. The obtained GOTiO2 nanoparticles were filled into porous BC matrix (GOTiO2/BC), and the photocatalytic properties of GOTiO2 nanoparticles were well maintained. Consistent with photocatalytic performance of TiO2, GOTiO2/BC generated reactive oxygen species after near ultraviolet irradiation. No dark cytotoxicity was observed at the long incubation time. In parallel, following exposure of Staphylococcus aureus cells to GOTiO2 and irradiation, a significant decrease in cell viability, as well as an increased production of reactive oxygen species was observed, which induced cellular death. The results indicated that GOTiO2/BC possess an excellent photodynamic antibacterial activity.