Isolation of Pseudonocardia strains associated with the shallow water hydrothermal vent crab Xenograpsus testudinatus from a metal-rich environment: biochemical characterization and enzymatic characterization, molecular identification, antibacterial, antibiofilm and antioxidant activity.

A shallow hydrothermal vent at Kueishantao Island, Taiwan provides a challenging environment and has been less explored for its microbial communities, especially the actinomycetes and their antibacterial and antioxidant activity. Nine actinomycete strains were isolated from the endemic hydrothermal vent crab Xenograpsus testudinatus and were identified as belonging to the otherwise rare actinomycete genus Pseudonocardia sp. Physiochemical results showed that the optimum growth conditions of these nine isolates were at pH 7, 35 °C, and 0.5-2% NaCl. Biochemical characterization showed differences between the strains. These isolates were further characterized at genetic barcoding (16s rRNA sequencing) and phenotypic levels and identified at the species/ strain level as Pseudonocardia alni SCSW01, Pseudonocardia yuanmonensis SCSW02, Pseudonocardia sp. strains SCSW03, SCSW04, SCSW05, SCSW06, BCSW29, ECSW09, and ECSW018. The morphology of the strains was analyzed using an environmental scanning electron microscope (ESEM). The nine isolates showed potential antibacterial activity against gram-positive and gram-negative pathogenic strains. The confocal laser scanning microscopy (CLSM) images show live and dead cells and biofilm/ antibiofilm activity of the actinomycete supernatant and crude extracts against pathogenic bacterial strains. The crude extracts of SCSW02, SCSW06, BCSW29, ECSW09, and ECSW018 showed antibiofilm activity against P. aeruginosa, E. coli, and S. aureus. The antioxidant activity such as DPPH and HO scavenging assay results showed that the nine actinomycetes crude extracts hold more substantial radical scavenging properties than supernatants. Our results marked the first report of Pseudonocardia genera from the vent crab Xenograpsus testudinatus of the HV region at Kueishantao island, Taiwan.