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The present study aimed to screen plants for bioactive compounds with potential antibacterial activities. In our efforts to evaluate plants from Borneo, we isolated and elucidated the structures of four natural products from the bioactive fraction of a chloroform extract of Goniothalamus longistipetes using various chromatographic and spectroscopic techniques. The bioactive compounds were identified as a known styryllactone, (+)-altholactone ((2S,3R,3aS,7aS)-3-hydroxy-2-phenyl-2,3,3a,7a-tetrahydrobenzo-5(4H)-5-one) (1), a new styryllactone, (2S,3R,3aS,7aS)-3-hydroxy-2-phenyl-2,3,3a,7a-tetrahydrobenzo-5(4H)-5-one) (2) as well as a new alkaloid, 2,6-dimethoxyisonicotinaldehyde (3) and a new alkenyl-5-hydroxyl-phenyl benzoic acid (4). 1 and 4 showed broad-spectrum anti-bacterial activities against Gram-positive and Gram-negative bacteria as well as acid-fast model selected for this study. Compound 2 only demonstrated activities against Gram-positive bacteria whilst 3 displayed selective inhibitory activities against Gram-positive bacterial strains. Additionally, their mechanisms of anti-bacterial action were also investigated. Using Mycobacterium smegmatis as a fast-growing model of tubercle bacilli, compounds 1, 2 and 4 demonstrated inhibitory activities against whole-cell drug efflux and biofilm formation; two key intrinsic mechanisms of antibiotic resistance. Interestingly, the amphiphilic compound 4 exhibited inhibitory activity against the conjugation of plasmid pKM101 in Escherichia coli using a plate conjugation assay. Plasmid conjugation is a mechanism by which Gram-positive and Gram-negative-bacteria acquire drug resistance and virulence. These results indicated that bioactive compounds isolated from Goniothalamus longistipetes can be potential candidates as ‘hits’ for further optimisation. Full article

The antimicrobial activity of altholactone, a naturally extracted styryllactone isolated from Goniothalamus malayanus, was determined against Gram positive (S. aureus ATTC 25923, S. aureus ATTC 25392, and E. faecalis ATTC 29212) and Gram negative (E. coli ATTC 35218, S. typhi ATTC 14023 and P. aeruginosa ATCC 27853) reference bacteria and against the fungus C. albicans ATTC 10231. Different concentrations of altholactone (0, 12, 25, and 50 μg/mL) were used. Results revealed that altholactone inhibited the growth of all tested microbes except P. aeruginosa ATCC 27853 in a dose-dependant manner, with the highest cytotoxic effects occuring at 50 μg/mL. The average of the inhibition zones of the different concentrations was between 0–30 mm. Furthermore, altholactone-induced antimicrobial activity against the more sensitive microbes was assessed by measuring the minimal inhibitory concentration (MIC). Results indicated that Gram positive (S. aureus ATTC 25923, S. aureus ATTC 25392, and E. faecalis ATTC 29212) cells were more sensitive to altholactone than Gram negative ones (E. coli ATTC 35218, S. typhi ATTC 14023). C. albicans showed moderate sensitivity. These results indicate that altholactone might be a potential antimicrobial agent, particularly in ciprofloxacin-refractory S. aureus and E. faecalis infections. Further investigations are required to illustrate the mechanism(s) by which altholactone produces its antimicrobial effects. Full article