Optimization, Composition, and Antioxidant Activities of Exo- and Intracellular Polysaccharides in Submerged Culture of Cordyceps gracilis (Grev.) Durieu & Mont.

Under present experiments, EPS and IPS production, monosaccharide composition, and antioxidant activities of C. gracilis were studied for the first time under submerged culture conditions. Effect of different factors on polysaccharides production was studied by orthogonal experiments using one-factor-at-a-time method. Incubation of culture in the medium with capacity 200 mL (675.12 ± 5.01 and 385.20 ± 5.01 mg/L), rotation speed 150 rpm (324.62 ± 3.32 and 254.62 ± 4.62 mg/L), 6-day culture incubation time (445.24 ± 1.11, 216.60 ± 1.71 mg/L), pH 6.0 (374.81 ± 2.52 and 219.45 ± 2.59 mg/L), and temperature 23°C (405.24 ± 1.11 and 215.60 ± 1.71 mg/L) produced higher EPS and IPS, respectively. Maximum EPS and IPS production was observed in the medium supplemented with glucose as a carbon source (464.82 ± 2.12 and 264.42 ± 2.62 mg/L) and yeast extract as a nitrogen source (465.21 ± 3.11 and 245.17 ± 3.24 mg/L), respectively. Carbon to nitrogen ratio for maximum EPS and IPS production was observed as 10 : 1 (395.29 ± 2.15 and 235.62 ± 1.40 mg/L), respectively. Glucose was found to be the major monosaccharide (62.15 ± 7.33%). Both EPS and IPS of C. gracilis showed significant DPPH radical scavenging activity, ABTS radical scavenging activity, reducing power, and iron chelating activity.

1. Introduction

Cordyceps Fr. is a widespread genus with 400 species worldwide [1-3]. Many species of this genus have been reported as the source of disease combating natural products with tremendous biological activities and considered as a rare and exotic medicinal fungus for centuries. Fruit bodies and mycelial extracts of this fungus have been found to exhibit different pharmacological activities [4]. Cordyceps sinensis (Berk.) Sacc. has been used extensively to cure various cancerous diseases and known to have different immunomodulatory activities [5-10]. Polysaccharides present in Cordyceps mycelium constitute the main bioactive agents and their range in Cordyceps species varies within 3–8% of the total weight [11-13]. Recent studies on the composition of anamorphic Cordyceps species have revealed higher percentage of cordycepin and other biological active compounds [14]. Previous studies have shown that Cordyceps species possesses liver protective effects [15], antioxidative activities [16], enhances the T-cell and macrophages activity [17], reduce the level of c-Myc, c-Fos, and VEGF levels in the lungs and liver by exopolysaccharide fraction [18], and reduce the level of cholesterol and triglyceride [19]. Besides these, Cordyceps contains some uncommon cyclic dipeptides, including cyclo-[Gly-Pro], cyclo-[Leu-Pro], cyclo-[Val-Pro], cyclo-[Ala-Leu], cyclo-[Ala-Val], and cyclo-[Thr-Leu] and small amounts of polyamines, such as 1,3-diamino propane, cadaverine, spermidine, spermine, and putrescine [20].

Polysaccharides extracted from Cordyceps species were known to exhibit multiple pharmacological activities including antitumor, anti-inflammatory, immunopotentiation, hypoglycemic, and hypocholesterolemic effects, protection of neuronal cells against the free radical-induced cellular toxicity, steroidogenesis, and antioxidant activities [21-27]. Polysaccharides isolated from the mycelia of Cordyceps fungi have been known to possess antioxidant properties which provide health benefits in preventing damage due to free radicals produced by biological degeneration [28-32].

Commercial cultivation through submerged culture is now becoming quite useful because of higher mycelial yield with fewer chances of contamination [33, 34]. EPS as well as IPS extracted from submerged culture possess the same biological activities [35]. However, the productivity of polysaccharides has been found to vary with environmental conditions and medium composition, including carbon source, nitrogen source, and pH [36]. In view of this, present studies were conducted to optimize the EPS and IPS production by one-factor-at-a-time method and orthogonal matrix method, monosaccharide composition and to evaluate the antioxidant activities of EPS and IPS in submerged culture of C. gracilis.

2. Materials and Methods

2.1. Culturing and Optimization Studies on Polysaccharides Production

Cordyceps gracilis was collected from Northwest India and was deposited at Herbarium, Department of Botany, Punjabi University, Patiala (PUN 6964). Isolation was done on potato dextrose agar (PDA) slants. The slants were incubated at 25°C for 10 days. Subculturing was done in every 45-day interval to maintain them viable. Submerged culturing was done in a standard basal medium (saccharine 30.0 g/L, yeast powder 5.0 g/L, peptone 5.0 g/L, MgSO4·7H2O 1.0 g/L, and KH2PO4 0.5 g/L) [37, 38]. Effect of medium capacity (50, 100, 150, 200, and 250 mL), rotation speed (50, 100, 125, 150, 175, and 200 rpm) of culture medium, incubation time (2–10 days), pH (3.0–8.0), temperature (20, 23, 25, 27, 30, and 33°C), carbon sources (glucose, galactose, sucrose, mannitol, maltose, and fructose), nitrogen sources (yeast extract, peptone, NaNO2 (NH4)2SO4, and L-arginine HCL), and C/N (1 : 5, 1 : 10, 1 : 20, 1 : 30, and 1 : 40) ratio on EPS and IPS production was studied by orthogonal experiments using one-factor-at-a-time method.

2.2. Extraction of EPS and IPS

Exopolysaccharides (EPS) were extracted by the standard method with minor modifications [39]. Briefly, mycelial biomass in the medium was centrifuged at 10,000 ×g for 12 min. The supernatant obtained was mixed with three volumes of pure ethanol and left for 24 hours at 4°C. The resulting precipitate was then separated by centrifugation at 8000 ×g for 10 min. The precipitate (EPS) was washed with ultrapure water and subsequently lyophilized for quantitative assessment and analysis.

For intracellular polysaccharides, mycelial biomass was subjected to extraction with boiling water for an hour and the mixture was filtered through Whatman number 1 filter paper. The filtrate was allowed to precipitate with four volumes of 95% (v/v) ethanol and left overnight at 4°C. The polysaccharides thus precipitated were separated by centrifugation at 8000 ×g for 10 min. The precipitates (IPS) were washed with ultrapure water and subsequently lyophilized for quantitative assessment and analysis [40].

2.3. Polysaccharides Composition

Monosaccharide composition of polysaccharides was determined by high performance liquid chromatography coupled to an evaporative light scattering detector [41]. Polysaccharide fraction (0.1 g) was extracted with 2.5 mL of 70% aqueous methanol followed by 1.5 mL of 70% aqueous methanol and then 1 mL of 70% aqueous methanol. This extract was centrifuged at 4000 rpm at 4°C for 10 min. Supernatant was collected and volume made up to 5 mL with 70% methanol. The extract was passed through Millipore filter (0.45 µm) prior to injection on the HPLC.

2.4. DPPH Radical Scavenging Activity

The DPPH scavenging activity was measured by the standard method described by Vamanu [42]. Briefly, DPPH (200 μm) solution at different concentrations (2–10 mg/mL) was added to 0.05 mL of the samples dissolved in ethanol. An equal amount of ethanol was added to the control. Ascorbic acid was used as the control. The absorbance was read after 20 min at 517 nm and the inhibition was calculated using the formulawhere A 0 was the absorbance of the control and A was the absorbance in the presence of the sample.

EC50 values were calculated using different sample concentrations (2–10 mg/mL) to obtain antiradical curves. Antiradical curves were plotted referring to concentration on the x-axis and relative scavenging capacity on the y-axis. EC50 values were calculated using the following equation:where  log⁡ is the center of the curve.

2.5. ABTS Radical Scavenging Assay

ABTS radical scavenging activity was measured by the method described by Li et al. [43]. For this, 10 μL of the sample was added to 4 mL of the diluted ABTS∙+ solution (prepared by adding 7 mM of the ABTS stock solution to 2.45 mM potassium persulfate, kept in the dark, at room temperature, for 12–16 h before use). The solution was then diluted with 5 mM phosphate-buffered saline (pH 7.4) to an absorbance at 730 nm of 0.70 ± 0.02 [43]. After the addition of 10 μL of the sample to 4 mL of the diluted ABTS∙+ solution, the absorbance was measured at 30 min. Ascorbic acid was used as control. The ABTS radical scavenging activity was calculated asDifferent sample concentrations (2–10 mg/mL) were used to obtain EC50 values. Standard curve was prepared with the sample concentration on the x-axis and relative scavenging capacity on the y-axis. EC50 values were calculated using the following equation:where  log⁡ is the center of the curve.

2.6. Reducing Power

Reducing power was estimated by the standard method given by Papuc et al. [44]. Briefly, 200 μL of the samples was mixed with sodium phosphate buffer (pH 6.6), 1 mM FeSO4, and 1% potassium ferricyanide and incubated for 20 min at 50°C; after that trichloroacetic acid was added and the mixtures were centrifuged. Supernatant (2.5 mL) was mixed with an equal volume of water and 0.5 mL 0.1% FeCl3. The absorbance was measured at 700 nm. Reducing power of the samples was calculated using EC50 values with different sample concentrations (2–10 mg/mL). Standard curve was made referring to concentration on the x-axis and relative scavenging capacity on the y-axis. EC50 values were calculated using the following equation:where  log⁡ is the center of the curve.

2.7. Ferrous Ion Chelating Assay

For this, 1 mL of the sample (2–10 mg/mL) was mixed with 3.7 mL of ultrapure water, following which the mixture was reacted with ferrous chloride (2 mmol/L, 0.1 mL) and ferrozine (5 mmol/L, 0.2 mL) for 20 min. And the absorbance was read at 562 nm. EDTA was used as positive control. The chelating activity on the ferrous ion was calculated using the following formula: chelating activity (%) = [(A − A )/A ] × 100, where A is the absorbance of the blank and A is the absorbance in the presence of the extract [45]. Chelating activity of samples was calculated using EC50 values with different sample concentrations (2–10 mg/mL) by standard curve referring to concentration on the x-axis and chelating activity on the y-axis. EC50 values were calculated using the following equation:where  log⁡ is the center of the curve.

2.8. Experimental Design

Different factors considered for the orthogonal design were carbon sources, nitrogen sources, and ratio of carbon to nitrogen sources, temperature, pH value, medium capacities, rotation speed, and culture time. According to the results of the single factor experiment, the orthogonal L9 (34) was used for optimal culture conditions in submerged cultures.

2.9. Statistical Methods

All results are expressed as mean ± standard error. P < 0.05 was considered significant, and SPSS software (SPSS Inc., Chicago, IL, USA) was used to calculate differences. All experiments were performed at least 3 times and with 3 replicates. The results were analyzed using one-way analysis of variance (ANOVA). Tests of significant differences were determined by Tukey-HSD at P < 0.05.

3. Results and Discussion

3.1. Optimization of Submerged Culture Conditions and Polysaccharide Composition

To find the suitable medium capacity for maximum EPS and IPS production, Cordyceps gracilis was grown in flasks (500 mL) containing liquid media with different capacities. The maximum EPS (675.12 ± 4.01 mg/L) and IPS (385.20 ± 3.00 mg/L) production was observed in 200 mL of the medium, while the least values for EPS (302.17 ± 4.46 mg/L) and IPS (122.13 ± 5.46 mg/L) production were obtained in 50 mL of the medium. However, no significant difference P < 0.05 was observed in EPS and IPS production in the medium with 100–150 mL capacities. Variation in rotation speed showed a direct relation with EPS and IPS production. Results obtained for the effect of the rotation speed on polysaccharide production showed maximum EPS (324.62 ± 3.32 mg/L) and IPS (254.62 ± 4.62 mg/L) production in culture medium with rotation speed 150 rpm. Similar results were obtained for medicinal fungus C. ophioglossoides (Ehrh.) Link and other ascomycetes. The reason for this is low oxygen requirement by the culture at 150 rpm rotation [27, 37]. The results obtained under present experiments on the effect of rotation speed on polysaccharide production were the same as those obtained in C. ophioglossoides [37]. Culture incubation time and pH range showed significant effect on EPS and IPS production. C. gracilis incubated for 6 days and pH 6.0 showed maximum EPS and IPS production (Table 1). In the species, namely, C. ophioglossoides and C. sinensis, incubation period of 5-6 days and slightly acidic pH 5.0–6.0 promoted maximum IPS production [37, 46]. C. gracilis culture showed maximum EPS (405.24 ± 1.11 mg/L) and IPS (215.60 ± 1.71 mg/L) production at 23°C. This temperature for C. sinensis was observed as 20°C and, for C. ophioglossoides, it was found to be 25°C [37, 47].

Table 1

Effect of different factors on EPS and IPS yield in submerged culture of C. gracilis.

Sources	EPS (mg/L)	IPS (mg/L)
Medium capacity/mL
50	302.17 ± 4.46a	122.13 ± 5.46a
100	450.10 ± 5.46a	203.13 ± 4.48a
150	449.12 ± 4.62a	204.62 ± 4.62a
200	675.12 ± 4.01b	385.20 ± 3.00b
250	403.11 ± 5.46a	199.13 ± 5.46a
Rotation speed (rpm)
50	213.17 ± 2.46a	121.12 ± 1.62a
100	244.62 ± 2.12a	152.13 ± 5.46a
125	255.20 ± 2.51a	185.20 ± 5.01a
150	324.62 ± 3.32b	254.62 ± 4.62b
175	283.13 ± 2.49a	198.13 ± 5.46a
200	187.20 ± 2.01a	125.20 ± 5.01a
Incubation time/d
2	253.10 ± 3.16a	123.12 ± 3.42a
3	265.22 ± 2.21a	145.17 ± 3.01abc
4	314.12 ± 1.32a	164.69 ± 2.02abc
5	353.19 ± 4.16ad	173.63 ± 2.46b
6	445.24 ± 1.11ab	216.60 ± 1.71bc
7	304.82 ± 2.12a	164.42 ± 2.62abc
8	253.71 ± 1.43a	157.49 ± 1.46abc
9	245.24 ± 4.11ac	149.22 ± 1.91abc
10	204.52 ± 1.63acd	134.02 ± 2.12abc
pH
3.0	324.11 ± 1.15a	154.60 ± 2.12a
4.0	343.14 ± 1.19a	173.61 ± 2.16ab
5.0	355.29 ± 2.15a	185.62 ± 1.49ab
6.0	374.81 ± 2.52ab	219.45 ± 2.59b
7.0	353.79 ± 2.44a	197.59 ± 5.16ab
8.0	247.19 ± 2.11ac	169.12 ± 4.93ab
Temperature
20	362.19 ± 2.16a	172.63 ± 2.33a
22	405.24 ± 1.11b	215.60 ± 1.71ab
25	394.82 ± 2.55c	162.42 ± 2.62a
27	253.71 ± 1.46d	159.49 ± 1.46a
30	209.24 ± 4.91e	147.22 ± 1.91a
35	103.19 ± 4.10f	63.10 ± 2.46ac

Values are expressed as mean ± SE and the same alphabets in the same column are not statistically significant according to Tukey's test for multiple comparisons with P < 0.05 for different conditions as mentioned in the table.

Six different carbon sources were studied to find the suitable medium source for the production of EPS and IPS in Cordyceps gracilis. Although all the tested carbon sources yielded EPS and IPS, maximum EPS (464.82 ± 2.12 mg/L) and IPS (264.42 ± 2.62 mg/L) production took place in the medium supplemented with glucose as carbon source. Glucose was also found to be the most favorable carbon source for polysaccharide production in many of the Cordyceps species [37, 48, 49]. To find the best nitrogen source, six different nitrogen sources were selected. Amongst them, yeast extract provided maximum EPS (465.21 ± 3.11 mg/L) and IPS (245.17 ± 3.24 mg/L) production. The results obtained under present studies are similar to those obtained from other researches [47]. C/N ratio 10 : 1 promoted maximum EPS (395.29 ± 2.15 mg/L) and IPS (235.62 ± 1.40 mg/L) production for C. gracilis (Table 2). Present results are in conformity with previous reports on C. ophioglossoides, as C/N ratio 10 : 1 provided maximum IPS (653.79 ± 5.24 mg/L) production [37].

Table 2

Effect of different factors on EPS and IPS yield in submerged culture of C. gracilis.

Factors	EPS (mg/L)	IPS (mg/L)
Carbon sources
Mannitol	319.12 ± 1.82a	164.69 ± 2.02a
Galactose	353.19 ± 3.19a	173.63 ± 2.46a
Sucrose	305.24 ± 1.11a	216.60 ± 1.71a
Glucose	464.82 ± 2.12ab	264.42 ± 2.62ab
Maltose	253.71 ± 1.43a	157.49 ± 1.46a
Fructose	245.24 ± 4.11ac	149.22 ± 1.91ac
Nitrogen source
Yeast extract	465.21 ± 3.11a	245.17 ± 3.24a
Peptone	324.12 ± 1.32a	164.69 ± 2.69a
NaNO2	345.24 ± 2.17a	146.60 ± 1.71a
(NH4)2SO4	304.82 ± 2.12a	104.42 ± 2.62a
L-arginine HCL	263.71 ± 3.43a	107.49 ± 3.46a
DL-ascorbic acid	255.24 ± 4.11b	99.22 ± 1.91b
C/N ratio
40 : 1	117.70 ± 1.07a	97.79 ± 1.59a
30 : 1	324.11 ± 1.71a	154.60 ± 2.32a
20 : 1	343.14 ± 1.19a	173.61 ± 2.16a
10 : 1	395.29 ± 2.15b	235.62 ± 1.40b
5 : 1	374.81 ± 2.52a	219.45 ± 2.59a
1 : 1	353.79 ± 2.44a	197.59 ± 5.16a

Values are expressed as mean ± SE and the same alphabets in the same column are not statistically significant according to Tukey's test for multiple comparisons with P < 0.05 for different conditions as mentioned in the table.

Monosaccharide composition of polysaccharides showed glucose in major percentage (62.15 ± 7.33%) followed by rhamnose (36.81 ± 4.57%) and galactose in minor quantities (0.10 ± 0.0%) (Table 3). Similar results were obtained for polysaccharide composition of C. militaris (L.) Link and other medicinal basidiomycetes, in which glucose was found to be the major monosaccharide along with sucrose and galactose [42, 50].

Table 3

Monosaccharide composition (mean ± SE) of polysaccharides in C. gracilis.

Monosaccharides	(%)
Xylose	21.14 ± 5.19
Glucose	62.15 ± 7.33
Rhamnose	36.81 ± 4.57
Mannose	13.76 ± 2.44
Galactose	0.10 ± 0.0

Results obtained for influence of different factors on yield of EPS and IPS showed a significant positive effect. Results revealed the effect on EPS and IPS production in the order temperature > incubation time > pH > rotation speed > medium capacity (Table 4).

Table 4

Results obtained for orthogonal design.

Experimental group	Temperature (°C)	pH	Rotation speed/min	Culture time/d	EPS (mg/L)	IPS (mg/L)
1	1	1	1	1	316.83 ± 2.35	276.83 ± 1.65
2	1	2	2	2	465.17 ± 12.16	315.10 ± 12.16
3	1	3	3	3	384.25 ± 16.10	324.20 ± 16.10
4	2	1	2	3	618.49 ± 34.15	398.19 ± 34.15
5	2	2	3	1	598.69 ± 14.10	408.60 ± 14.10
6	2	3	1	2	648.69 ± 9.18	441.61 ± 7.58
7	3	1	3	2	248.11 ± 44.47	198.20 ± 14.07
8	3	2	1	3	362.11 ± 3.75	252.18 ± 7.75
9	3	3	2	1	206.76 ± 58.91	176.76 ± 18.01
K 1	11.26	9.28	15.26	14.89
K 2	24.10	14.60	13.90	15.10
K 3	11.15	9.05	14.15	14.15
R	12.17	4.67	0.82	2.69
K 1 *	451.56	351.84	424.43	327.16
K 2 *	678.59	576.19	622.09	478.59
K 3 *	302.11	342.17	362.71	444.11
R *	389.38	84.08	25.38	92.18

K 1 = ∑ EPS at culture factor level 1/3. K 1 * = ∑ IPS yield at culture factor level 1/3.

3.2. Antioxidant Activities of EPS and IPS

The DPPH free radical scavenging activity of EPS and IPS extracted from the mycelium of C. gracilis showed positive and direct correlation with the concentration of the sample (Figure 1(c)). Both EPS and IPS extracted from C. gracilis showed high DPPH scavenging activity. However, EPS showed higher DPPH scavenging activity than IPS. The results are supported by EC50 values, which were found to be 7.32 ± 0.09 mg/mL for EPS and 6.79 ± 0.04 mg/mL for IPS (Table 5). These results are in conformity with previous reports on other medicinally important species like C. militaris and C. sinensis [51, 52]. The inhibition percentage of the ABTS radical by EPS and IPS of C. gracilis was found to be directly dependent upon the concentration of the sample. The scavenging effect of all the extracts increased with increasing concentration as shown in the figure. At a concentration of 10.0 mg/mL, the percentage inhibition of EPS and IPS was found to be the same. High concentrations of the EPS and IPS are able to quench the free radicals in the system. The results indicated that the EPS and IPS of C. gracilis possessed significant scavenging power for the ABTS radicals (Figure 1(d)).

Figure 1

(a) Wild fruit body of C. gracilis on Hymenoptera insect. (b) Four-day-old mycelium on potato dextrose agar (PDA) medium. (c) DPPH scavenging activity of EPS and IPS. (d) ABTS radical scavenging activity of EPS and IPS. (e) Reducing power of EPS and IPS. (f) Iron chelating activity of EPS and IPS.

Table 5

EC50values of EPS and IPS.

EC50	EPS (mg/mL)	IPS (mg/mL)
DPPH radical scavenging activity	7.32 ± 0.09	6.79 ± 0.04
ABTS radical scavenging activity	7.08 ± 0.32	5.11 ± 0.22
Reducing power	6.96 ± 0.22	5.33 ± 0.27
Iron chelating activity	1.64 ± 0.41	1.49 ± 0.21

The results obtained for reducing power abilities of EPS and IPS in submerged culture of C. gracilis showed that both types of polysaccharides possessed the high reducing ability. The reducing powers of EPS and IPS increased as the concentration increased (Figure 1(e)). However, the reducing power of EPS was found to be higher than the reducing power of IPS. At a concentration 10 mg/mL, maximum difference in the reducing power of EPS (1.23 ± 0.02 mg/mL) and IPS (0.73 ± 0.00 mg/mL) was observed. The reducing power of polysaccharides is due to the presence of reductones and present results showed that EPS and IPS of C. gracilis contained reductones which react with certain precursors of peroxides to prevent peroxide formation. The iron chelating ability of the EPS and IPS was found to be related with the concentration of sample. At initial concentration 2 mg/mL, there was not much difference between the iron chelating ability of both EPS (60%) and IPS (58%). However, at higher sample concentrations, EPS showed higher iron chelating ability than IPS (Figure 1(f)). Both EPS (80%) and IPS (60%) showed maximum iron chelating activity at a concentration 10 mg/mL. The results are further supported by EC50 values (Table 5). EPS and IPS extracted from submerged culture of C. gracilis have shown significant antioxidant activities as reported in C. sinensis [53] and C. militaris [54]. Since antioxidants are well known for playing important roles in the human metabolic system and for protecting against cardiovascular and neurodegenerative diseases, present investigations showed the medicinal value of the submerged culture of C. gracilis [55, 56].

4. Conclusion

Results obtained under present investigations showed that Cordyceps gracilis is a medicinal fungus and it contained both EPS and IPS in appreciable amount. In vitro evaluation of antioxidant activities of EPS and IPS showed significant higher antioxidant activities. However, submerged cultivation of this fungus required several factors for the production of EPS and IPS. Factors such as temperature, rotation speed, pH, incubation time, carbon, nitrogen, and carbon to nitrogen ratio showed significant effect on the production of EPS and IPS. Due to significantly higher DPPH radical scavenging activity, ABTS radical scavenging activity, reducing power, and iron chelating abilities, this fungus is useful for its pharmaceutical applications. Present findings will open the scope for its large-scale industrial fermentations for commercial uses like other commercially explored Cordyceps species.