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Aims The aims of this study were to identify the structure of antibacterial lipopeptide (LP) produced by Bacillus amyloliquefaciens strain FJAT‐2349, to analyse the effects of the culture medium and temperature on LP production and to assess the biocontrol efficiency of the LPs against tomato bacterial wilt. Methods and Results Lipopeptides were extracted by acid precipitation and resolved in methanol and their structure was identified through LC‐QTOF‐MS/MS method. The antibacterial activities of the LPs were evaluated through inhibition zone experiments. The biocontrol efficiency of the LPs against tomato bacterial wilt was examined by a pot test. The LPs were composed of iturin (C14–C17 iturin A), fengycin (C14/C16 fengycin A, C14 fengycin B2, C16 fengycin A2/B2, C16–C17 fengycin B, C15 fengycin A derivatives and C15 fengycin B derivatives) and surfactin (C12–C16 surfactin A). Moreover, the composition of the LPs was significantly influenced by the culture medium and temperature; the contents of iturin, fengycin and surfactin varied within the range from 0·41–5·89, 4·54–181·67 and 2·05–19·65 mg l−1 in the different culture media and from 0·39–11·04, 1·45–215·14 and 7·79–24·43 mg l−1 under different culture temperatures respectively. The results indicated that the contents of the LP mixture, fengycin and surfactin secreted from FJAT‐2349 all decreased along with an increasing culture temperature. The fermentation supernatants and LP extracts had the strongest inhibition activities of Ralstonia solanacearum when strain FJAT‐2349 was incubated at 25°C using potato dextrose broth as the culture medium among all the assayed culture conditions. The purified fengycin was found to be the active antibacterial compound against R. solanacearum, but the purified surfactin was not. The pot experiments demonstrated that the LPs secreted from the strain FJAT‐2349 could effectively control the tomato bacterial wilt with a biocontrol efficiency of 97·6%. Conclusions The LPs secreted from strain FJAT‐2349 could serve as potential biocontrol agents against tomato bacterial wilt. Significance and Impact of the Study The LPs exhibited good potential applications in the biocontrol of tomato bacterial wilt.

Background: There is an urgent need to discover biocontrol agents to control bacterial wilt. This study reports on a new lipopeptide-producing biocontrol strain FJAT-46737 and explores its lipopeptidic compounds, and this study investigates the antagonistic effects of these compounds. Results: Based on a whole genome sequence analysis, the new strain FJAT-46737 was identified as Bacillus velezensis, and seven gene clusters responsible for the synthesis of bioactive secondary metabolites in FJAT-46737 were predicted. The antimicrobial results demonstrated that FJAT-46737 exhibited broad-spectrum antimicrobial activities in vitro against three bacteria and three fungi. Pot experiments showed that the control efficiencies for tomato bacterial wilt of the whole cultures, the 2-fold diluted supernatants and the crude lipopeptide of FJAT-46737 were 66.2%, 82.0%, and 96.2%, respectively. The above results suggested that one of the antagonistic mechanisms of FJAT-46737 was the secretion of lipopeptides consisting of iturins, fengycins and surfactins. The crude lipopeptides had significant antagonistic activities against several pathogens (including Ralstonia solanacearum, Escherichia coli and Fusarium oxysporum) and fengycins were the major antibacterial components of the lipopeptides against R. solanacearum in vitro. Furthermore, the rich organic nitrogen sources (especially yeast extracts) in the media promoted the production of fengycin and surfactin by FJAT-46737. The secretion of these two lipopeptides was related to temperature fluctuations, with the fengycin content decreasing by 96.6% and the surfactins content increasing by 59.9% from 20°C to 40°C. The optimal temperature for lipopeptide production by FJAT-46737 varied between 20°C and 25°C. Conclusions: The B. velezensis strain FJAT-46737 and its secreted lipopeptides could be used as new sources of potential biocontrol agents against several plant pathogens, and especially the bacterial wilt pathogen R. solanacearum.

The control of food browning and growth of disease-causing microorganisms is critical to maintaining the quality and safety of food. Tyrosinase is the key enzyme in food browning. The inhibitory effect of methyl trans-cinnamate on the activity of tyrosinase has been investigated. Methyl trans-cinnamate can strongly inhibit both monophenolase and diphenolase activity of mushroom tyrosinase. When the concentration of methyl trans-cinnamate reached 2.5 mM, the lag time was lengthened from 32 to 160 s and the steady-state activity was lost about 65%. The IC(50) value was 1.25 mM. For the diphenolase activity, the inhibition of methyl trans-cinnamate displayed a reversible and noncompetitive mechanism. The IC(50) value was 1.62 mM, and the inhibition constant (K(I)) was determined to be 1.60 mM. Moreover, the antibacterial activity against Escherichia coli, Bacillus subtilis and Staphyloccocus aureus and antifungal activity against Candida albicans were tested. The results showed that methyl trans-cinnamate possessed an antimicrobial ability.

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Tyrosinase (EC 1.14.18.1) is a key enzyme in pigment biosynthesis of organisms. trans-Cinnamaldehyde thiosemicarbazone, a derivative of benzaldehyde thiosemicarbazone, was synthesized as an inhibitor of tyrosinase. The inhibitory effects of this compound on the activity of mushroom tyrosinase were investigated. The results showed that trans-cinnamaldehyde thiosemicarbazone could potently inhibit both monophenolase activity and diphenolase activity of tyrosinase. For monophenolase activity, trans-cinnamaldehyde thiosemicarbazone could not only lengthen the lag time but also decrease the steady-state rate. For diphenolase activity, the IC(50) value was determined to be 5.72 microM. Kinetic analyses showed that trans-cinnamaldehyde thiosemicarbazone was a reversible and mixed type inhibitor on this enzyme. The inhibition constants (K(I) and K(IS)) were determined to be 4.45 and 8.85 muM, respectively. Furthermore, the antibacterial activity against Bacillus subtilis, Escherichia coli, Staphyloccocus aureus, Pseudomonas aeruginosa, Klebsiella pneumonia, and Agrobacterium tumefaciens was investigated. The results showed that trans-cinnamaldehyde thiosemicarbazone was more effective against B. subtilis and S. aureus with the same minimum inhibitory concentration (MIC) of 50 microg/mL and with the same minimum bactericidal concentration (MBC) of 50 microg/mL.

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Cry2Ab, a pore-forming toxin derived from Bacillus thuringiensis, is widely used as a bio-insecticide to control lepidopteran pests around the world. A previous study revealed that proteolytic activation of Cry2Ab by Plutella xylostella midgut juice was essential for its insecticidal activity against P. xylostella, although the exact molecular mechanism remained unknown. Here, we demonstrated for the first time that proteolysis of Cry2Ab uncovered an active region (the helices α4 and α5 in Domain I), which was required for the mode of action of Cry2Ab. Either the masking or the removal of helices α4 and α5 mediated the pesticidal activity of Cry2Ab. The exposure of helices α4 and α5 did not facilitate the binding of Cry2Ab to P. xylostella midgut receptors but did induce Cry2Ab monomer to aggregate and assemble a 250-kDa prepore oligomer. Site-directed mutagenesis assay was performed to generate Cry2Ab mutants site directed on the helices α4 and α5, and bioassays suggested that some Cry2Ab variants that could not form oligomers had significantly lowered their toxicities against P. xylostella. Taken together, our data highlight the importance of helices α4 and α5 in the mode of action of Cry2Ab and could lead to more detailed studies on the insecticidal activity of Cry2Ab.