Influence of exopolysaccharides on the electrophoretic properties of the model cyanobacterium Synechocystis

Quorum sensing regulation in Microcystis aeruginosa: Insights into AHL-mediated physiological processes and MC-LR production

Quorum sensing (QS) is a widespread regulatory mechanism in Gram-negative bacteria, primarily involving the secretion of N-acyl homoserine lactone (AHL) to facilitate population density sensing. However, the existence of QS in blue-green algae, a subset of photoautotrophic Gram-negative bacteria forming high-density communities in water blooms, remains elusive. This study delves into the unexplored realm of QS in Microcystis aeruginosa (M. aeruginosa) by investigating AHL-related regulatory mechanisms and their impact on various physiological processes. Utilizing high-performance liquid chromatography coupled with mass spectrometry (HPLC-MS) and biosensors, a hitherto unknown long-chain AHL exhibiting a mass-to-charge ratio of 318 was identified in sterile M. aeruginosa cultures. Our investigation focused on discerning correlations between AHL activity fluctuations and key parameters such as microcystin (MC-LR) production, algal density, photosynthesis, buoyancy, and aggregation. Furthermore, the AHL extract was introduced during the logarithmic stage of M. aeruginosa cultures to observe the response in physiological processes. The results revealed that AHL, functioning as an autoinducer (AI), positively influenced algal growth and photosynthesis, as evidenced by the upregulated photosynthetic conversion efficiency of PSI and chlorophyll synthesis gene (psbA). AI also played a crucial role in altering surface characteristics through the synthesis of polysaccharides and proteins in EPS, subsequently promoting cell aggregation. Concomitantly, AI upregulated mcyD, enhancing the synthesis of MC-LR. Notably, our investigation pinpointed the initiation of QS in Microcystis at a density of approximately 1.22 × 10^7 cells/mL. This groundbreaking evidence underscores the regulatory role of AI in governing the physiological processes of growth, aggregation, buoyancy, and MC-LR production by activating pertinent gene expressions. This study significantly expands the understanding of QS in AHL, providing crucial insights into the regulatory networks operating in blue-green algae.

Optorheological Characteristics of Photosynthetic Bacterium Suspension

Understanding the rheological behavior of materials is of great importance in science. Here, we report a microscopic foundation for optorheology by manipulating the rheological feature through light. A new phenomenon is observed in the photosynthetic bacterial suspension, that the fluid viscosity changes by light-induced electrons. Type IV pili of photosynthetic bacteria is found, and it allows the electron to transport through the exterior of cells and changes the surface potential of cells, which causes an adjustment in the spatial arrangement of cells in the suspension. When an external electric field is applied, the electric dipole of the cells is induced and their dispersion is changed. The rheological properties are measured to evaluate the internal structure of the suspension depending on the light. The photoelectrons enhance the dispersion of the photosynthetic bacteria in the solution, thus leading to a significant increment in the viscosity. We envision that this discovery will provide new applications to the interface of optics, bioengineering, and rheology.

Evaluation of antibacterial and anti-biofilm properties of kojic acid against five food-related bacteria and related subcellular mechanisms of bacterial inactivation

Although the antimicrobial properties of kojic acid have been recognized, the subcellular mechanism of bacterial inactivation caused by it has never been clearly elucidated. In the present study, the antibacterial and anti-biofilm activity of kojic acid was evaluated against five foodborne pathogens including Listeria monocytogenes, Bacillus subtilis, Staphylococcus aureus, Escherichia coli, and Salmonella typhimurium. The antibacterial activity was determined by minimum inhibitory concentration, minimum bactericidal concentration, and the time-kill assay. Among them, the susceptibility of Escherichia coli was significant with the lowest minimum inhibitory concentration and minimum bactericidal concentration values of 10 and 20 mM, respectively. Subcellular mechanism of bacterial inactivation related to kojic acid was revealed through comprehensive factors including cell morphology, membrane permeability, K+ leakage, zeta potential, intracellular enzyme, and DNA assay. Results demonstrated that bacterial inactivation caused by kojic acid, especially for Gram-negative bacteria, was primarily induced by the pronounced damage to the cell membrane integrity. Leakage of intracellular enzyme to the supernatants implied that the cell membrane permeability was compromised. Consequently, the release of K+ from the cytosol leads to the alterations of the zeta potential of cells, which would disturb the subcellular localization of some proteins and thereby cause the bacterial inactivation. The free −CH2OH group at the C-2 of kojic acid could play more significant role in the antimicrobial performance of kojic acid against Gram-negative bacteria. Moreover, remarkable interaction with DNA was also observed. Kojic acid at sub-minimum inhibitory concentration inhibited biofilm formation by these bacteria.

Responses to oxidative and heavy metal stresses in cyanobacteria: recent advances

Cyanobacteria, the only known prokaryotes that perform oxygen-evolving photosynthesis, are receiving strong attention in basic and applied research. In using solar energy, water, CO₂ and mineral salts to produce a large amount of biomass for the food chain, cyanobacteria constitute the first biological barrier against the entry of toxics into the food chain. In addition, cyanobacteria have the potential for the solar-driven carbon-neutral production of biofuels. However, cyanobacteria are often challenged by toxic reactive oxygen species generated under intense illumination, i.e., when their production of photosynthetic electrons exceeds what they need for the assimilation of inorganic nutrients. Furthermore, in requiring high amounts of various metals for growth, cyanobacteria are also frequently affected by drastic changes in metal availabilities. They are often challenged by heavy metals, which are increasingly spread out in the environment through human activities, and constitute persistent pollutants because they cannot be degraded. Consequently, it is important to analyze the protection against oxidative and metal stresses in cyanobacteria because these ancient organisms have developed most of these processes, a large number of which have been conserved during evolution. This review summarizes what is known regarding these mechanisms, emphasizing on their crosstalk.