Abstract
Streptococcus zoonotic bacteria cause serious problems in aquaculture with clinical effects on humans. A structure-antibacterial activity relationships analysis of 22 isoflavones isolated from M. tricuspidata (leaves, ripe fruits, and unripe fruits) against S. iniae revealed that prenylation of the isoflavone skeleton was an important key for their antibacterial activities (minimum inhibitory concentrations: 1.95–500 μg/mL). Through principal component analysis, characteristic prenylated isoflavones such as 6,8-diprenlygenistein (4) were identified as pivotal compounds that largely determine each part’s antibacterial activities. M. tiricuspidata ripe fruits (MTF), which showed the highest antibacterial activity among the parts tested, were optimized for high antibacterial activity and low cytotoxicity on fathead minnow cells using Box–Behnken design. Optimized extraction conditions were deduced to be 50%/80 °C/7.5 h for ethanol concentration/extraction temperature/time, and OE-MTF showed contents of 6,8-diprenlygenistein (4), 2.09% with a MIC of 40 µg/mL. These results suggest that OE-MTF and its active isoflavones have promising potential as eco-friendly antibacterial agents against streptococcosis in aquaculture.
Highlights
Global fish production peaked at about 179 million tons in 2018, with aquaculture representing 46% of total fish production
In the present study, we investigated the antibacterial activity of isoflavones isolated from the leaves, ripe fruits, and unripe fruits of M. tricuspidata against fish pathogenic Streptococcus and their structure–activity relationships (SAR)
Three-factor Box–Behnken design (BBD) was applied in this study to investigate individual and interactive effects of ethanol concentration (X1), extraction temperature (X2), and extraction time (X3) as variables on antibacterial activity against S. iniae DSJ19 and cytotoxicity to fathead minnow (FHM) cells
Summary
Global fish production peaked at about 179 million tons in 2018, with aquaculture representing 46% of total fish production. It causes serious problems in aquaculture, and has clinical effects on humans. It has been reported that prenylated isoflavonoids are major compounds in M. tricuspidata [11,14,17] These prenylated isoflavonoids have strong antibacterial activities against human pathogenic Gram-positive bacteria such as Listeria monocytogenes and methicillin-resistant Staphylococcus aureus (MRSA) [18,19]. In the present study, we investigated the antibacterial activity of isoflavones isolated from the leaves, ripe fruits, and unripe fruits of M. tricuspidata against fish pathogenic Streptococcus and their structure–activity relationships (SAR). To obtain effective and non-toxic antibacterial extracts from M. tricuspidata ripe fruits, extraction conditions such as ethanol concentration, extraction temperature, and extraction time were optimized to achieve high antibacterial activities against S. iniae while achieving low cytotoxicity to fathead minnow cells using a three-factor–three-level Box–Behnken design (BBD)
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