2.1 Cultures
The bacteria strain was isolated in October 2017 from a suppressive soil sample in Shandong province, China, and subsequently stored at the Shandong Agricultural Microbiology Collection Center (AMCC) under the registration number AMCC 1040 (Wang et al. 2021b).
Representative specimens of Meloidogyne incognita were used as targets for nematicidal activity bioassays. Egg masses of Meloidogyne incognita were collected from infected tomato roots after 30 days of incubation in a greenhouse at Shandong Agricultural University. Before use, they were surface sterilized with 1.5% NaClO solution, washed with distilled water, and then transferred to Baermann funnel at 25℃ to obtain second-stage juveniles (J2s) (Eloh et al. 2015).
2.2 Nematicidal activity bioassay
The bioassay of nematicidal has been described in our previous studies (Wang et al. 2021b). In summary, 100–150 Meloidogyne incognita J2s were mixed with in 0.4 mL of distilled H2O with 0.1 mL of the culture supernatant in 24 microporous plates and kept in dark at 28°C. To prepare a cell-free culture supernatant, cultures were centrifuged at 12,000 rpm for 5 min at 4°C and then filtered through 0.22 µm Millipore filters. BPY medium (Beef extract 5.0 g, Peptone 10.0 g, Yeast extract 5.0 g, Glucose 10.0 g, NaCl 5.0 g, ddH2O 1000 mL, pH 7.0) was used as a negative control. The nematodes were considered dead if their bodies were linear or insensitive to mechanical contact. The experiment was performed twice with three replicates per each treatment. The corrected mortality rates were calculated using the following formula: Mortality (%) = [(mortality percentage in treatment − mortality percentage in untreated control) / (100 − mortality percentage in untreated control)] × 100 (Bi et al. 2018).
2.3 Partial Characterization of Active Metabolites
A good understanding of the basic properties of nematicidal substances can be of great help in developing more effective analytical strategies. The active substances were characterized in terms of: dynamic nematicidal activity bioassay, thermal and pH stability, solubility, generation stability, adsorption/ion exchange properties and volatility. All the experiments were performed twice with three replicates per treatment.
2.3.1 Dynamic nematicidal activity bioassay
Two experiments were designed to assess the dynamic nematicidal activity: (1) The virulence capacity varied with the time of incubation of Bacillus altitudinis. The incubation process was divided into two stages. To prepare the seed culture, the strains were activated on solid BPY medium at 37°C for about 18 h. Individual colonies were then selected and inoculated into a 250 mL erlenmeyer flask containing 50 mL of liquid BPY medium, followed by incubation at 37℃ for 12 h with constant shaking at 200 rpm. In the second stage, 100 mL of BPY medium was placed into a 500 mL Erlenmeyer flask and inoculated with 1 mL seed culture. The culture conditions were the same as described above, and samples were taken every two hours for bioassay of nematicidal activity until juvenile mortality were stable at 100%. (2) Minimum time to reach maximum juvenile mortality. In this section, mortality of J2 was counted every 10 min using 48 h of fermentation broth.
2.3.2 Heat, pH and generation stability
To determine thermal stability, the culture supernatant was heated at different temperatures (50, 60, 70, 80, 90, 100, 120℃) for 30 mins and subsequently tested for the nematicidal activity as its temperature dropped to ambient.
The culture supernatant was adjusted to pH 2 to 12 and subsequently incubated at 28°C for 30 min, then the pH was adjusted back to its original value (approximately 5.06) and tested for nematicidal activity.
In another experiment, a single colony was incubated continuously at monthly intervals for a total of 10 times, and each generation was tested for nematicidal activity to determine the stability of generation.
2.3.3 Solubility
To determine the solubility of the active metabolites, the culture filtrate was fully extracted with organic solvents including n-butanol, ethyl acetate, cyclohexane, chloroform. The organic solvents were mixed fermentation supernatant liquid (100 mL) in equal proportion, transferred to a separatory funnel, and left for 12 h. The organic and aqueous phase were separated, and the organic phase was combined after repeated extraction three times (300 mL in total). The residual aqueous phase was removed by adding an appropriate amount of anhydrous Na2SO4 to the organic phase, and the organic extracts were dried over anhydrous sodium sulfate and then centrifuged at 100 rpm, 40℃, -0.1 Mpa on a rotary vacuum evaporator. The resulting yellow residue was dissolved in 5 mL methanol and transferred to a 10 mL glass bottle then placed in a vacuum desiccator at -0.1 Mpa. After complete removal of methanol, the gummy mass were redissolved with 0.5% DMSO and subsequently tested for nematicidal activity using 0.5% DMSO as negative control (Sharma et al. 2014).
2.3.4 Adsorption/ion exchange properties
Four amberlite resins were used in the adsorption/ ion exchange experiments, including XAD 16, XAD 1180, FPC 3500, and FPC 23H. The resins were soaked in equal volume of methanol for 24 h according to the manufacturer's requirements and then loaded onto a column (the bed volume was about 100 mL, 30 cm × Φ 2 cm). The column was rinsed thoroughly with distilled water until the eluate was clear and free of methanol order. The column was then soaked in 5% NaOH for 6 h and rinsed with distilled water until neutral, finally, soaked in 5% HCl for 6 h, rinsed with distilled water until neutral (Wang et al. 2019). 200 mL culture supernatant was passed through the column at a flow rate of 0.5 BV/h, followed by elution with 200 mL of water. In the next sections, the column was eluted with 200 mL of 50% methanol and 100% methanol sequentially for the adsorbent resin and 200 mL of 0.1 mol/L and 0.5 mol/L HCl sequentially for the ion exchange resin with absolute desalting. The collection rate was 50 mL/tube, 16 fractions in total in each resin. The flow-through fluid of XAD 16, XAD 1180 were tested for the nematicidal activity directly and the methanol part was treated in the same way as described above. Regarding FPC 3500 and FPC 23H, the pH was adjusted back to the original value (4.71–5.06) and tested for nematicidal activity.
2.3.5 Volatility
Further freeze-drying experiments were used to investigate the volatility of substances. 100 mL of culture supernatant was completely pre-frozen at -80℃ for 12 h, and subsequently freeze-dried through lyophilization to obtain yellow viscous solid. The yellow viscous solid was redissolved with 100 mL ddH2O and subsequently tested for nematicidal activity. In another experiment, 100 mL of culture supernatant was evaporated on a rotary vacuum evaporator at 100 rpm, 60℃, -0.1 Mpa, and the yellow solid in the distillation flask was re-dissolved with 100 mL ddH2O, while the liquid in the condensation flask was collected and then tested nematicidal activity separately.
2.4 Analysis of VOCs by using HS-SPME-GC/MS
Bacillus altitudinis AMCC 1040 was cultured for 48 h. Culture supernatants were prepared as described above, and volatiles were collected and analyzed via HS-SPME-GC/MS (Gu et al. 2007). The fiber (50/30 µm DVB/CAR/PDMS) used for headspace solid-phase microextraction was first preconditioned at 250°C for 30 min (Murungi et al. 2018; Estupiñan-López et al. 2018). To adsorb the volatiles, the pre-cleaned fiber was inserted into a 50 mL headspace vials containing 25 mL of fermentation broth and soaked at 95℃ for 1 hr. Subsequently, the fiber was manually inserted into the injector port and desorbed at 250°C for 5 min (Zhai et al. 2018). A GC–MS QP 2010 Ultra (Shimadzu, Japan) gas chromatograph–mass spectrometer, equipped with Rtx-5MS (60m×0.32mm×0.25µm) column was used for chromatographic separation. The injector operated in splitless mode. The injector, interphase and ion detector temperatures was 230℃, 250℃, and 220℃, respectively. Grade 5.0 helium was used as the carrier gas at a flow rate of 3 mL/min. The column was held at 50℃ for 2 min, then ramped up to 120℃ at 2°C/min and finally to 200℃ at 4°C/min and held for 1 min. The EI ion source was acquired at 70 eV with an acquisition range of m/z 35 and 500. according to the mass spectrometry and database NIST 08, NIST 08s and mass spectra from the FNSC 1.3 library were compared to determine the identification of VOCs.
2.5 Nematicidal activity of VOCs produced by Bacillus altitudinis AMCC1040
The nematicidal activity of 2,3-butanedione, acetic acid, acetoin, 2-isopropoxy ethylamine, 2,3-butanediol, 3-methyl-butanoic acid, 2-methyl-butanoic acid and octanoic acid was evaluated for mortality of J2s. All reagents were analytical pure and purchased from Aladdin. Since octanoic acid is slightly soluble in water, a master solution of 0.2 µL/mL was prepared by adding 2 µL of octanoic acid (Aladdin, 99% purity) to 9998 µL distilled water. Stock solutions (1 µL/mL) of other compounds were prepared in sterile distilled water. The solutions should be prepared at the time of use and should not be left for too long after preparation. In vitro experiments were performed as described in 2.2. The working concentration range of each compound is from 0.01 µL/mL to 1 µL/mL to detect the LC100/12h of each compound.