Procedure: The pH of solution II was adjusted to 4.5 by gradually adding the solution I and the volume of the mixture were finally adjusted to 100 ml by adding ddH2O.
Borate Buffer (0.2 M, pH 9.8):
Solution I: 0.2 M Boric acid (M.W. 61.83); 1.36 g of boric acid was added to 100 ml of ddH2O.
Solution II: 0.2 M sodium tetraborate (M.W. 381.37); 7.62 g of sodium tetraborate (Borax) was added to 100 ml of ddH2O.
Procedure: The pH of the solution II was adjusted to 9.8 by gradually adding the solution I and the final volume was adjusted to 100 ml by adding ddH2O.
Sabouraud dextrose yeast extract (SDY) broth, pH 5.5:
Sabouraud dextrose yeast extract (SDY) broth contained (%, w/v): dextrose, 4
%; peptone, 1 and yeast extract, 1. The SDY agar at 6.5 % (w/v) was prepared in ddH2O.
Czapek dox (CZ) broth, pH 5.5
Medium M1: Medium M1 contained (%, w/v): sucrose, 3; NaNO3, 0.3 %;
K2HPO4, 0.1; MgSO4, 0.05 %; KCl, 0.05 and FeSO4, 0.001 %
Medium M2: The composition was same as M1, but was supplemented with L-lysine (0.01 to 0.10 %, w/v). Lysine was filter sterilized (0.2 µm) and added to the medium after sterilization.
Potato dextrose broth (PDB)/agar, pH 5.5
Potato dextrose broth (PDB) contained (%, w/v): potato infusion, 0.4 and dextrose, 2. Potato dextrose agar (PDA) at 2.4 % (w/v) was prepared in ddH2O.
Oatmeal media
Oatmeal powder was used in various concentrations (2-10 %, w/v) for screening and optimization experiments.
2.2.3. Sterilization and aseptic techniques
All the culture media were sterilized by autoclaving at a steam pressure of 10.3 kPa (15 lb/in2), and a temperature of 121°C, for 20 min. All inoculum preparations and culture transfers were carried out under aseptic conditions using laminar air flow chambers.
2.2.4. Dry cell weight method for growth profile studies
Biomass was estimated by quantifying dry cell weight (DCW) from the duplicate 250 ml Erlenmeyer flasks containing 6 % (w/v) complex oatmeal at every 24 h of incubation at 28 °C and 180 rpm. The biomass was filtered through pre- weighted Whatman No. 1 filter paper and washed with distilled water. The filtered biomass was dried overnight at 100 °C and finally weighed to estimate the growth pattern. The DCW of the biomass was measured against every 24 h of post inoculation incubation. The slope of the exponential phase was measured which represents the specific growth rate (µ) of the culture ant the doubling time (Td) for the culture was calculated using the following formula.
= ( )
µ ( . 2.1)
2.2.5. Mannosidase inhibition assay for the quantification of swainsonine
The swainsonine concentration in the culture supernatant was determined using α-mannosidase inhibition assay (Sim and Perry, 1995). The assay relies on the potent and specific inhibition of jack bean α-mannosidase by the low concentrations of swainsonine. The broth samples (1ml) were withdrawn at indicated time intervals and centrifuged at 16,000g for 20 min at 4 °C and then the supernatants were filtered using Whatman No. 1 filter paper to separate the cells. The reaction mixture was miniaturized to 50 µl volume to carry out the experiment in 96-well microtitre plates.
The reaction mixture was constituted with 19.71µl citrate buffer (0.05 M, pH 4.5), 5µl of reduced glutathione (0.1mM), 5 µl ZnCl2 (0.1mM), 5.29 µl of α-mannosidase enzyme (0.03 U/ml) and 10 µl of culture supernatants. All the components were added sequentially and incubated for 10 min at room temperature. Then 5 µl of substrate, p- nitrophenyl-α-D-mannopyranoside (3mM) was added and the reaction was further incubated for 4 min. The enzymatic reaction was finally stopped by adding 100 µl Borate buffer (0.2M, pH 9.8). The release of p-nitrophenol from the substrate by mannosidase activity was colorimetrically measured at absorption wavelength of 405 nm in ELISA plate reader (Tecan, Infinite 200, USA). A calibration curve was obtained by plotting the ratio of enzyme activity in the absence to the presence of swainsonine (VC/VI) against the inhibitor concentrations [I], ranging from 0.1 to 0.5 µg/ml (Fig. 2.1). Swainsonine concentration was measured by the correlation of absorbance with the corresponding calibration curve equation. The inhibitor concentrations lying outside the sensitivity range of the assay were determined after appropriate dilutions.
To ensure that the α-mannosidase inhibition by a sample was caused due to
activity, a control enzyme, α-fucosidase was incorporated into the procedure. If a reaction mixture is found to be inhibit both the classes of glycosidases (mannosidases and fucosidases), then the process would be inferred as a non-specific kind of inhibition.
2.2.6. Screening of Metarhizium strains and growth profile studies
Strains of M. anisopliae were initially screened for their growth and sporulation characteristics on oatmeal broth media and the ten selected strains were finally evaluated for swainsonine production in 2 % (w/v) oatmeal broth (Justo et al., 2007).
The culture inoculum of 4×108 spores/ml was inoculated in 250 ml production media in duplicate Erlenmeyer flasks. The cultures were incubated at 180 rpm and 28 °C for 168 h. Samples were removed after every 24 h and assayed for swainsonine production as described in the Section 2.2.5. The growth profile of finally screened Metarhizium strain was evaluated in optimal oatmeal concentration using DCW method as described in Section 2.2.4.
2.2.7. One factor at a time approach screening
One factor at a time (OFAT) approach was used for optimizing the swainsonine production from M. anisopliae ARSEF-1724 (UM8). Apart from the earlier known physical parameters for growth conditions, all other chemical parameters were optimized individually at a time keeping the other parameters constant. The effects of different oatmeal concentrations (2-10 %, w/v), growth media (SDB, CZ M1 and M2), carbon sources (glucose, fructose, maltose, sucrose and starch), glucose concentrations (0.5-3.5 %, w/v), 6 % (w/v) complex nitrogen sources (oatmeal, peptone, beef extract and yeast extract), 0.5 % (w/v) of simple nitrogen sources (urea,
ammonium sulphate, ammonium chloride and potassium nitrate) and pH (3-7) were screened for optimal swainsonine production from M. anisopliae fermentation.
2.2.8. Plackett-Burman screening
Overall seven parameters were screened using Plackett Burman-design of experiment (PB-DOE) (Plackett and Burman 1946) in the “Design Expert software DX7” for their cumulative effects towards fermentative production of swainsonine from M. anisopliae. Based on PB factorial design, each factor was examined in two levels: -1 for low level and +1 for high level (Plackett and Burman, 1946). Table 2.2 shows levels of each factor used in the experimental design and the design matrix.
These factors used in PB with their high and low range values were selected based upon the OFAT screening experiments (described in Section 2.2.7) and reports. The parameters for PB screening were (A) glucose, (B) lysine, (C) oatmeal extract, (D) MgSO4.7H2O, (E) CaCl2, (F) pH and (G) inoculum size. The design of experiment (DOE) study ranges and significant values for each parameter are shown in Table 2.2.
The level of significance for each variable was determined using the lack of fit test (F- test) with at p < 0.05.