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Background: Microorganisms are a known source of antibiotics. The study aimed to identify and screen antibiotic-producing microbes isolated from seawater. Method: Three of the fifty (50) bacteria isolated from seawater showed positive for antibiotic activity. The antimicrobial activity of Pseudomonas guguanensis (KD1) was screened against the ESKAPE pathogens using agar-well diffusion assays. P. guguanensis (KD1) was selected for the fermentation and extraction of antimicrobial compounds using solvent extraction assays. Results: P. guguanensis (KD1) produced the highest antibacterial activity after 36 h of cultivation, inhibiting S. aureus, E. faecium, A. baumannii and E. cloacae. According to sensitization assay, K. pneumoniae was impermeable to all the cell-free supernatants of P. guguanensis (KD1). Using agar-well diffusion assays, ethyl acetate extracts from the supernatant recorded zones of inhibition against S. aureus, E. faecium, and E. cloacae, producing zones of 20.1 ± 0.432, 17.8 ± 0.121 and 16 ± 0.162 mm, respectively. Acetonitrile extract from the supernatant inhibited A. baumannii and S. aureus, forming zones of inhibition 18.2 ± 0.323 mm and 18 ± 0.234. The minimum inhibitory concentration and minimum bactericidal concentration recorded for the ethyl acetate extract and acetonitrile extract ranged from 1.56 to 6.25 mg/mL and 12.5–25 mg/mL, respectively. Conclusions: P. guguanensis (KD1) offers a potential source of antibiotics for infections caused by multidrug-resistant bacteria.

Levofloxacin is a widely used fluoroquinolone in several infectious diseases. The structure–activity relationship of levofloxacin has been studied. However, the effect of changing the carbonyl into thiocarbonyl of levofloxacin has not been investigated up to the date of this report. In this work, levofloxacin structure was slightly modified by making a thionated form (compound 3), which was investigated for its antibacterial activity, biocompatibility, and cytotoxicity, as well as spectroscopic properties. The antibacterial susceptibility testing against five different bacteria showed promising minimum inhibitory concentrations (MICs), particularly against B. spizizenii and E. coli, with an MIC value of 1.9 µM against both bacteria, and 7.8 µM against P. mirabilis. The molecular docking experiment showed similar binding interactions of both levofloxacin and compound 3 with the active site residues of topoisomerase IV. The biocompatibility and cytotoxicity results revealed that compound 3 was more biocompatible with normal cells and more cytotoxic against cancer cells, compared to levofloxacin. Interestingly, compound 3 also showed an excitation profile with a distinctive absorption peak at λmax 404 nm. Overall, our results suggest that the thionation of quinolones may provide a successful approach toward a new generation with enhanced pharmacokinetic and safety profiles and overall activity as potential antibacterial agents.

Abstract Background The experimental materials were a 60-year-old tea tree (Camellia sinensis cv. Shu Cha Zao; SCZ) (the mother plant) and 1-year-old and 20-year-old plants of SCZ that originated as mother plant cuttings. The aim of this study was to use high-throughput sequencing to study the spatial and dynamic distribution of endophytic fungi in different leaf niches (upper leaves, middle leaves, lower leaves) and rhizosphere soil on tea plants of different ages in the same garden. Results Ascomycota (83.77%), Basidiomycota (11.71%), and Zygomycota (3.45%) were the dominant fungal phyla in all samples. Cladosporium (12.73%), Zymoseptoria (9.18%), and Strelitziana (13.11%) were the dominant genera in the leaf. Alpha diversity analysis revealed that endophytic communities in leaves differed from those in rhizosphere soil and different leaf niches had similar fungal diversity. Shannon’s indices and NMDS analysis indicated significant differences in fungal diversity and composition among the SCZ trees of different ages (p ≤ 0.01). The abundance of Cladosporium and Zymoseptoria decreased with increasing SCZ age, whereas the abundance of Strelitziana increased. Conclusions The results illustrate variation in endophytic fungi among different niches on tea plants of different ages. The distribution of endophytic fungi in leaves of C. sinensis shows spatiotemporal variation.