Chitosan-grafted-phenolic acid copolymers against Shewanella putrefaciens by disrupting the permeability of cell membrane.

Chitosan (CS) is a kind of high molecular polymer with antibacterial properties. A copolymer with high bacteriostatic activity can be formed by grafting phenolic acid compounds into the chitosan molecular chain, which can inhibit the growth of dominant spoilage bacteria in aquatic products. The study aimed to investigate the antibacterial effect and mechanism of chitosan-grafted-phenolic acid copolymers on Shewanella putrefaciens (S. putrefaciens). CS-grafted-protocatechuic acid (CS-g-PA) and CS-grafted-gallic acid (CS-g-GA) were attained by EDC/NHS coupling reaction. The antibacterial tests indicated that CS-g-PA and CS-g-GA had the same minimum inhibitory concentration (MIC) (1.25 mg/mL) and minimum bactericidal concentration (MBC) (5.0 mg/mL) against S. putrefaciens. According to the change trend of growth curve, the growth of S. putrefaciens was significantly restrained under 2MIC graft copolymers (P < 0.05). Moreover, the increment of alkaline phosphatase (AKPase) activity and electrical conductivity demonstrated that the cell wall and membrane permeability of S. putrefaciens were damaged respectively. In addition, the increase of lactate dehydrogenase (LDHase) activity, protein and nucleic acid absorbance and the decrease of adenosine triphosphatase (ATPase) activity suggested that the cell membrane was incomplete and poor fluidity. The irregular shape of bacteria and the outflow of intercellular contents were also observed from scanning electron microscope (SEM). The above results manifested a great potential of CS-g-PA and CS-g-GA for use as food preservatives to aquatic products.

Characterization and antimicrobial mechanism of CF-14, a new antimicrobial peptide from the epidermal mucus of catfish.

In this study, we identified a novel antibacterial peptide, RIVELTLPRVSVRL-NH2 (named CF-14), derived from the epidermal mucus of catfish and characterized its antimicrobial activity. Analysis of antimicrobial activity and hemolytic activity of CF-14 revealed broad spectrum, high levels of antimicrobial activity and low toxicity to eukaryotic cells. CF-14 remained stable at pH values ranging from 4.0 to 12.0 and remained bioactive when exposed to high temperature. CD analysis indicated that CF-14 forms a random coil in PBS buffer and an α-helical conformation in the membrane-mimetic 2.5% SDS micelle. Additionally, the antibacterial mechanism of CF-14 against Shewanella putrefaciens was investigated. Membrane permeability experiments confirmed that CF-14 could increase cell wall membrane permeability and cause nucleotide leakage. Moreover, observations performed using scanning electron and confocal microscopy indicated that CF-14 could penetrate into the cell membranes of S. putrefaciens and accumulate in bacterial cells, but did not break down cell membranes. Further, electrophoresis analysis demonstrated that CF-14 possesses DNA-binding affinity. The results provide a substantial basis for future application of CF-14, a novel cell-penetrating peptide (CPP) derived from catfish.

ε-Polylysine Inhibits Shewanella putrefaciens with Membrane Disruption and Cell Damage.

ε-Polylysine (ε-PL) was studied for the growth inhibition of Shewanella putrefaciens (S. putrefaciens). The minimal inhibitory concentration (MIC) of ε-PL against S. putrefaciens was measured by the broth dilution method, while the membrane permeability and metabolism of S. putrefaciens were assessed after ε-PL treatment. Additionally, growth curves, the content of alkaline phosphatase (AKP), the electrical conductivity (EC), the UV absorbance and scanning electron microscope (SEM) data were used to study cellular morphology. The impact of ε-PL on cell metabolism was also investigated by different methods, such as enzyme activity (peroxidase [POD], catalase [CAT], succinodehydrogenase [SDH] and malic dehydrogenase [MDH]) and cell metabolic activity. The results showed that the MIC of ε-PL against S. putrefaciens was 1.0 mg/mL. When S. putrefaciens was treated with ε-PL, the growth of the bacteria was inhibited and the AKP content, electrical conductivity and UV absorbance were increased, which demonstrated that ε-PL could damage the cell structure. The enzyme activities of POD, CAT, SDH, and MDH in the bacterial solution with ε-PL were decreased compared to those in the ordinary bacterial solution. As the concentration of ε-PL was increased, the enzyme activity decreased further. The respiratory activity of S. putrefaciens was also inhibited by ε-PL. The results suggest that ε-PL acts on the cell membrane of S. putrefaciens, thereby increasing membrane permeability and inhibiting enzyme activity in relation to respiratory metabolism and cell metabolism. This leads to inhibition of cell growth, and eventually cell death.

Uranium speciation and distribution on the surface of Shewanella putrefaciens in the presence of inorganic phosphate and zero-valent iron under anaerobic conditions.

Shewanella putrefaciens (S. putrefaciens) is one of the main microorganisms in soil bioreactors, which mainly immobilizes uranium through reduction and mineralization processes. However, the effects of elements such as phosphorus and ZVI, which may be present in the actual environment, on the mineralization and reduction processes are still not clearly understood and the environment is mostly in the absence of oxygen. In this study, we ensure that all experiments are performed in an anaerobic glove box, and we elucidate through a combination of macroscopic experimental findings and microscopic characterization that the presence of inorganic phosphates enhances the mineralization of uranyl ions on the surface of S. putrefaciens, while zero-valent iron (ZVI) facilitates the immobilization of uranium by promoting the reduction of uranium by S. putrefaciens. Interestingly, when inorganic phosphates and ZVI co-exist, both the mineralization and reduction of uranium on the bacterial surface are simultaneously enhanced. However, these two substances exhibit a certain degree of antagonism in terms of uranium immobilization by S. putrefaciens. Furthermore, it is found that the influence of pH on the mineralization and reduction of uranyl ions is far more significant than that of inorganic phosphates and ZVI. This study contributes to a better understanding of the environmental fate of uranium in real-world settings and provides valuable theoretical support for the bioremediation and risk assessment of uranium contamination.

Pathogenic strains of Shewanella putrefaciens contain plasmids that are absent in the probiotic strain Pdp11.

Shewanella putrefaciens Pdp11 is a strain described as a probiotic for use in aquaculture. However, S. putrefaciens includes strains reported to be pathogenic or saprophytic to fish. Although the probiotic trait has been related to the presence of a group of genes in its genome, the existence of plasmids that could determine the probiotic or pathogenic character of this bacterium is unknown. In the present work, we searched for plasmids in several strains of S. putrefaciens that differ in their pathogenic and probiotic character. Under the different conditions tested, plasmids were only found in two of the five pathogenic strains, but not in the probiotic strain nor in the two saprophytic strains tested. Using a workflow integrating Sanger and Illumina reads, the complete consensus sequences of the plasmids were obtained. Plasmids differed in one ORF and encoded a putative replication initiator protein of the repB family, as well as proteins related to plasmid stability and a toxin-antitoxin system. Phylogenetic analysis showed some similarity to functional repB proteins of other Shewanella species. The implication of these plasmids in the probiotic or pathogenic nature of S. putrefaciens is discussed.

New insights into the role of calcium in the bioreduction of uranium(VI) under varying pH conditions.

The effect of calcium in the uranium-contaminated groundwater on U(VI)aq bioreduction remains uncertain. Some studies indicated that the presence of calcium may inhibit the bioreduction. However, our calculations show the negative standard molar Gibbs free energy of reduction. The bioreduction of the ternary uranyl-carbonate-calcium complexes seems thermodynamically favorable at specific pH. Sorption and reduction experiments were conducted to gain new insights of calcium into the bioreduction. The results show that the complexes were greatly reduced by Shewanella putrefaciens in the slightly acidic pH ~6.0 and alkaline pH ~7.9 solutions with the relatively high CaCl2 (1.0-6.0 mmol/L) although the reduction was difficult at the nearly neutral pH ~6.9. At pH ~6.9, the removal percentage of U(VI)aq decreased from 97.0% to 24.4% with increasing CaCl2 from 0 to 6.0 mmol/L, in contrast to the increasing percentage from 50.9% to 89.7% at pH ~7.9. The obvious removal of U(VI)aq was ascribed to the bioreduction instead of the biosorption, as evidenced by XPS, HRTEM and UV-vis spectra. The calculations such as [Formula: see text] and [Formula: see text] partially accounted for the reduction mechanisms. Accordingly, the U(VI)aq bioreduction is a promising method to remediate the groundwater even rich in calcium and carbonate.