Antimicrobial and antioxidant activities of Methylobacterium phyllosphaerae KS50439 isolated from Antarctic seawater

In this study, 85 marine microorganisms were isolated from Antarctic seawater, and their methanolic extracts were screened for antibacterial and antifungal activities against Escherichia coli DH5α, Staphylococcus aureus, and Candida albicans. Among the isolates, Methylobacterium phyllosphaerae KS50439 exhibited substantial antimicrobial activity. Biochemical characterization using API ZYM and API 20NE kits, together with phylogenetic analysis based on 16S ribosomal RNA gene sequencing, identified KS50439 as a unique strain. Growth curve analysis revealed that stationary-phase methanolic extracts exhibited stronger antibacterial and antifungal activities than extracts from other growth phases. Specifically, the extract showed half-maximal inhibitory concentration (IC₅₀) values of 2.47 ± 0.01 and 2.27 ± 0.03 mg/mL against E. coli DH5α and S. aureus, respectively, with corresponding minimum inhibitory concentration (MIC) values of 8.65 ± 0.07 and 8.54 ± 0.06 mg/mL. For C. albicans, the IC₅₀ and MIC values were 3.13 ± 0.02 and 9.09 ± 0.04 mg/mL, respectively. Antioxidant activity was assessed using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging assays. The scavenging activities were 73.80% (DPPH) and 87.62% (ABTS), with IC₅₀ values of 2.45 ± 0.08 and 0.12 ± 0.01 mg/mL, respectively. These findings demonstrate that extremophilic microorganisms, such as KS50439 adapt to harsh Antarctic environments and produce bioactive metabolites with notable antimicrobial and antioxidant potential. Collectively, these findings identify M. phyllosphaerae KS50439 as a promising source of bioactive compounds for biotechnological applications. Although further compound purification and safety evaluations are required before pharmaceutical or cosmetic applications can be considered, this study highlights the potential of polar microorganisms as valuable reservoirs of bioactive compounds.