Alleviation of H2O2 toxicity by extracellular catalases in the phycosphere of Microcystis aeruginosa.

High levels of environmental H2O2 represent a threat to many freshwater bacterial species, including toxic-bloom-forming Microcystis aeruginosa, particularly under high-intensity light conditions. The highest extracellular catalase activity-possessing Pseudoduganella aquatica HC52 was chosen among 36 culturable symbiotic isolates from the phycosphere in freshly collected M. aeruginosa cells. A zymogram for catalase activity revealed the presence of only one extracellular catalase despite the four putative catalase genes (katA1, katA2, katE, and srpA) identified in the newly sequenced genome (∼6.8 Mb) of P. aquatica HC52. Analysis of secreted catalase using liquid chromatography-tandem mass spectrometry was identified as KatA1, which lacks a typical signal peptide, although the underlying mechanism for its secretion is unknown. The expression of secreted KatA1 appeared to be induced in the presence of H2O2. Proteomic analysis also confirmed the presence of KatA1 inside the outer membrane vesicles secreted by P. aquatica HC52 following exposure to H2O2. High light intensities (> 100 µmol m-2 s-1) are known to kill catalase-less axenic M. aeruginosa cells, but the present study found that the presence of P. aquatica cells supported the growth of M. aeruginosa, while the extracellular catalases in supernatant or purified form also sustained the growth of M. aeruginosa under the same conditions. Our results suggest that the extracellular catalase secreted by P. aquatica HC52 enhances the tolerance of M. aeruginosa to H2O2, thus promoting the formation of M. aeruginosa blooms under high light intensities.

Sustainable control of Microcystis aeruginosa, a harmful cyanobacterium, using Selaginella tamariscina extracts.

Eco-friendly reagents derived from plants represent a promising strategy to mitigate the occurrence of toxic cyanobacterial blooms. The use of an amentoflavone-containing Selaginella tamariscina extract (STE) markedly decreased the number of Microcystis aeruginosa cells, thus demonstrating significant anti-cyanobacterial activity. In particular, the Microcystis-killing fraction obtained from pulverized S. tamariscina using hot-water-based extraction at temperatures of 40 °C induced cell disruption in both axenic and xenic M. aeruginosa. Liquid chromatographic analysis was also conducted to measure the concentration of amentoflavone in the STE, thus supporting the potential M. aeruginosa-specific killing effects of STE. Bacterial community analysis revealed that STE treatment led to a reduction in the relative abundance of Microcystis species while also increasing the 16S rRNA gene copy number in both xenic M. aeruginosa NIBR18 and cyanobacterial bloom samples isolated from a freshwater environment. Subsequent testing on bacteria, cyanobacteria, and algae isolated from freshwater revealed that STE was not toxic for other taxa. Furthermore, ecotoxicology assessment involving Aliivibrio fischeri, Daphnia magna, and Danio rerio found that high STE doses immobilized D. magna but did not impact the other organisms, while there was no change in the water quality. Overall, due to its effective Microcystis-killing capability and low ecotoxicity, aqueous STE represents a promising practical alternative for the management of Microcystis blooms.

Antagonistic actions of Paucibacter aquatile B51 and its lasso peptide paucinodin toward cyanobacterial bloom-forming Microcystis aeruginosa PCC7806.

Superior antagonistic activity against axenic Microcystis aeruginosa PCC7806 was observed with Paucibacter sp. B51 isolated from cyanobacterial bloom samples among 43 tested freshwater bacterial species. Complete genome sequencing, analyzing average nucleotide identity and digital DNA-DNA hybridization, designated the B51 strain as Paucibacter aquatile. Electron and fluorescence microscopic image analyses revealed the presence of the B51 strain in the vicinity of M. aeruginosa cells, which might provoke direct inhibition of the photosynthetic activity of the PCC7806 cells, leading to perturbation of cellular metabolisms and consequent cell death. Our speculation was supported by the findings that growth failure of the PCC7806 cells led to low pH conditions with fewer chlorophylls and down-regulation of photosystem genes (e.g., psbD and psaB) during their 48-h co-culture condition. Interestingly, the concentrated ethyl acetate extracts obtained from B51-grown supernatant exhibited a growth-inhibitory effect on PCC7806. The physical separation of both strains by a filter system led to no inhibitory activity of the B51 cells, suggesting that contact-mediated anti-cyanobacterial compounds might also be responsible for hampering the growth of the PCC7806 cells. Bioinformatic tools identified 12 gene clusters that possibly produce secondary metabolites, including a class II lasso peptide in the B51 genome. Further chemical analysis demonstrated anti-cyanobacterial activity from fractionated samples having a rubrivinodin-like lasso peptide, named paucinodin. Taken together, both contact-mediated inhibition of photosynthesis and the lasso peptide secretion of the B51 strain are responsible for the anti-cyanobacterial activity of P. aquatile B51.

Rheinheimera faecalis sp. nov., isolated from Ceratotherium simum feces.

A novel strain YR1T, Gram-stain-negative, rod-shaped, catalase- and oxidase-positive, and aerobic bacterium, was isolated from the feces of Ceratotherium simum. The strain grew at 9-42 °C (optimal temperature, 30 °C), at pH 6.0-10.0 (optimal pH, 7.0), and in the presence of 0-3% (w/v) NaCl (optimal salinity, 0%). Phylogenetic analyses based on 16S rRNA gene sequencing indicated that strain YR1T was most closely related to Rheinheimera soli BD-d46T (98.6%), R. riviphila KYPC3T (98.6%), and R. mangrovi LHK 132T (98.1%). Moreover, the average nucleotide identity, average amino acid identity, and digital DNA-DNA hybridization values between strain YR1T and R. mangrovi LHK 132 T were 88.3%, 92.1%, and 35.3%, respectively, indicating that strain YR1T is a novel species in the genus Rheinheimera. The genome size and genomic DNA G + C content of strain YR1T were 4.5 Mbp and 46.37%, respectively. The major polar lipids were phosphatidylethanolamine and phosphatidylglycerol, while the predominant respiratory quinone was Q-8. Summed feature 3 (C16:1 ω7c and/or C16:1 ω6c), C16: 0, and summed feature 8 (C18:1 ω7c) were the primary cellular fatty acids (> 16%). Based on these genotypic and phenotypic characteristics, strain YR1T was identified as a novel species in the genus Rheinheimera, for which the name Rheinheimera faecalis sp. nov. is proposed, with the type strain is YR1T (= KACC 22402T = JCM 34823T).

First report of brain abscess associated with Pseudozyma species in a patient with astrocytoma.

A yeast-like strain was isolated from the brain abscess of a patient diagnosed with astrocytoma. Morphological and molecular analysis on D1/D2 domain in the 26S rRNA gene and internal transcript spacer region of the strain revealed that the strain belonged to the genus Pseudozyma. To the best of our knowledge, this is the first report on the isolation of a Pseudozyma strain from brain abscess.