Introduction

The word "Actinomycetes" is derived from the Greek “atkis” (ray) and “mykes” (fungus), that is, it has characteristics of bacteria and fungi [1]. These microorganisms produce aerobic spores, they are gram-positive bacteria; with high guanine-cytosine content (57–75%) in its genome, and belong to the order Actinomycetales characterized by the growth of aerial mycelium and substrate [2, 3]. Actinomycetes are present in various ecological habitats, are common organisms in soils [4, 5], especially in dry soils, slightly acidic soils, rich in organic matter and represent a high proportion of total microbial biomass [6]. These microorganisms are characterized by generating a great diversity of bioactive metabolites that are highly appreciated in the biotechnology industry [7, 8]. Of the approximately 18,000 known bacterial compounds, more than 10,000 were isolated from bacteria of the genus Actinomyces and Streptomyces [9].

Soil actinomycetes have been a myriad source of secondary metabolites, including antibiotics, anti-cancer, immunosuppressants, pigments, herbicides and enzymes [10, 11], however the frequency of discovery of novel compounds is decreasing in these years and the findings seem to imply that the easily accessible microorganisms in the soil have been depleted and that there is a need to opt for unexplored or under-explored sources [12, 13]. It is believed that ecological and environmental conditions influence the metabolic capacity of bacteria for the production of secondary metabolites [14]. In this context, the exploration of new habitats is one of the most promising ways to isolate new actinomycete species that will probably lead to the discovery of potentially beneficial new metabolites. It has been described that extreme habitats induce microorganisms to produce more stable enzymes, representing an economic advantage for biotechnological processes [15]. Furthermore, the interaction between different species or genera of microorganisms results in large amounts of metabolites being secreted that fulfill different functions, one of which is to participate in their adaptation [16]. For example, it has been described that microorganisms that grow in alkaline soils tend to fix phosphorus more efficiently [17]. Therefore, it is of interest to study microorganisms that are capable of growing in extreme habitats, which would increase the probability of finding no-common metabolites such as enzymes. More recently, there has been a growing interest in exploring arid and semi-arid environments, where bacteria are exposed to drying conditions, high salinity and UV radiation [18].

The actinomycetes of arid and semi-arid regions have been studied for several authors, for example: Aouar et al. [19], identified 72 actinomycetes from semi-arid soils associated with the rhizosphere of cereals, trees and vegetables in Algeria in order to evaluate its capacity for biological control in phytopathogenic fungi through the secretion of antifungal compounds and chitinolytic enzymes. Korayem et al. [20], isolated 50 actinomycetes of the genus Streptomyces from arid soils in Egypt and determined lipolytic activity and biosurfactant production. Yeager et al. [21], examined the potential of 112 strains of actinomycetes isolated from a semi-arid prairie belonging to Colorado, Utah, they detected enzymatic activities of endocellulase, xylanase, pectinase and chitinase using hydrated herbal extracts as substrate.

The MV region represents a particular ecosystem located southeast of the Hidalgo state, Mexico. This area is the largest block of land in the world where untreated sewage has been used for agricultural irrigation for more than a century [22]. The climate is semi-arid with an average annual temperature of 15–17 °C and an average annual rainfall of 400–600 mm [23]. In addition, it is considered one of the most polluted regions in Mexico due to the presence of chemical, cement, refinery and thermoelectric industries [24]. The reports on the analysis of microbial communities in this region are limited with the exception of two works previously carried out by pyrosequencing, the first is that of Broszat et al. [25] determined the influence of the use of residual water on the bacterial communities of agricultural soils of the VM, they found that over the years certain bacterial communities were benefited (Actinobacteria 27.4%, Proteobacteria 14.6% and Acidobacteria 14%), probably due to the supply of organic matter through wastewater. And the second, reported by Lüneberg et al. [26], where the impact on the composition of bacterial communities of irrigated MV soils with different qualities of water (fresh water, untreated wastewater stored in dams and water untreated residual) was evaluated. These authors suggest that the diversity of soils is due to the specific adaptations of each lineage. The bacterial taxa associated with the soil irrigated by wastewater are adapted to higher concentrations of sodium ions, presence of surfactants and heavy metals.

Studies of the microbial communities present in MV soils have been carried out, but there are no studies focused on the isolation of Streptomyces species or of the enzymes production. In this study, isolated Streptomyces species from two semi-arid agricultural soils of the MV were characterized and evaluated to determine the production of endogluconase (CMCase) and xylanase (Xyl) enzymes in solid-state cultivation. The strains that presented the highest enzymatic activities were molecularly identified.

Materials and Methods

Study Area

Composite samples of rhizosphere of agricultural soils were collected in the semi-arid region of the MV, Hidalgo, Mexico. The first site was a barley crop with wheat rotation in Cinta Larga, Mixquiahuala (20° 13′ 46.9″ N, 99° 05′ 21.23″ W,) and the second a corn crop with oat rotation in Francisco I. Madero, Hidalgo, México (20° 13′ 20.45″ N, 99° 05′ 28.82″ W), the specifications of Mexican Standard [27] were used for sampling, soil samples were taken at a depth of 10–20 cm. The samples were transported to the laboratory in sealed plastic bags at 4 °C, for later analysis.

Streptomyces Species Isolation

The method used for the isolation of Streptomyces species was that of serial dilutions [28]. One gram of dry soil from each sample was vigorously stirred in 9 mL of sterile distilled water and preheated at 50 °C for 30 min [29]. This solution (101) was diluted five times (105) using sterile distilled water. An aliquot of 50 μL was taken which were distributed on plates of two culture media: Agar yeast extract-malt extract ISP2 (Yeast extract 4 g/L (Dibico), extract 10 g/L malt (Dibico), 4 g/L dextrose (Sigma) and 20 g/L bacteriological agar at pH 7.3) and ISP3 oat agar (20 g/L oat and 18 g/L bacteriological agar (Dibico) at pH 7.2) supplemented with nystatin (50 µg/mL) and nalidixic acid (25 µg/mL) to inhibit fungal and bacterial contamination respectively. Plates were incubated at 28 °C for 28 days. The colonies were purified by consecutive reseed on ISP2 agar and preserved in a spore solution with 50% glycerol at − 80 °C.

Morphological Characterization of Isolated Streptomyces species

Phenotypic identification of purified isolates was based on culture and morphological characteristics using standard methods described in the International Streptomyces Project [30]. The morphological determinations included aerial mycelium and substrate coloration, soluble pigment production, colony texture and Gram staining.

Qualitative Determination of Enzymatic Activity

The ability of the isolated strains to produce Endoglucanases (CMCase) and Xylanase (Xyl) activity during their growth on plate was determined. For the detection of CMCase activity Agar plates [31] were used with the following composition: carboxymethylcellulose (CMC) 2.0 g/L, (NH4)2SO4 0.5 g/L, CaCl2 0.1 g/L, K2HPO4 1 g/L, MgSO4 0.2 g L, KCl 0.5 g/L, yeast extract (Dibico) 0.5 g/L and bacteriological agar (Dibico)18 g/L at pH 7.0. After the incubation period (10 days at 28 °C), the plates were stained with lugol-iodine solution for 10 min, the solution was removed and rested for 1 h. The plates with cellulolytic activity showed clear areas around the colony [32]. Whereas, for the detection of Xyl activity, xylan agar [33] was used with the following composition: birch xylan 2 g L, (NH4)2SO4 0.5 g/L, CaCl2 0.1 g/L, K2HPO4 1 g/L, MgSO4 0.2 g/L, KCl 0.5 g/L, yeast extract (Dibico) 0.5 g/L, peptone 0.0015 g/L and bacteriological agar (Dibico) 20 g/L at pH 7.0. The plates were incubated 10 days at 28 °C and subsequently stained with Congo red solution (0.2%) for 10 min and consecutive washes with 1 M NaCl solution. The plates with positive Xyl activity showed yellow areas against red bottoms [34]. The results were reported indicating the radius of the hydrolysis zones and were classified as: ( +) weak producer (0.8–1.4 cm); (++) moderate producer (1.5–2.4 mm) and (+++) strong producer (2.5–3.0 mm) [35]. All chemical reagents from tha basal medium were purchased from Sigma-Aldrich.

Raw Material

The barley straw was washed with tap water, dried at 60 °C for 12 h in a convection oven (Thermo Scientific), then cut in lengths of 5 cm, ground in a commercial grain mill (Thomas Scientific) and passed through sieves Inox mesh size 8 (2380 µm) and 12 (1680 µm). Finally, the ground straw was stored in airtight containers.

Inoculum Preparation

Each Streptomyces specie was grown on ISP2 agar plates until sporulation at 28 °C. For the spore harvest, 5 mL of sterile distilled water was added per plate and with the help of a sterile disposable spatula the mycelium spores were detached. The resulting solution was stored under refrigeration (4 °C) and the spore count was performed with a Neubauer chamber (Marienfeld).

Solid-State Cultivation (SSC)

The SSC was carried out in 250 mL Erlenmeyer flasks, 1 g of barley straw was added to each flask and both autoclaved at 121 °C for 15 min. Particle sizes of 1680 and 2380 µm were evaluated in independent experiments. Under aseptic conditions, 1 × 107 spores/gdm (grams of dry mater) of each Streptomyces specie was added to each flask and incubated at 28 °C, the humidity was adjusted to 85% with sterile distilled water. A 10-day kinetics (240 h) was carried out for the quantification of the enzymatic activities of CMCase and Xyl in liquid, samples were taken in triplicate every 24 h from which the crude enzyme extract was obtained. To obtain the crude extract, 20 mL of acetate buffer (100 mM, pH 5.3) was added to each flask and stirred at room temperature for 20 min at 180 rpm. The extract was centrifuged for 10 min at 10,000 rpm at 4 °C and was received in tubes (50 mL) and kept at 4 °C until further analysis.

Quantitative Determination of Enzymatic Activity

CMCase and Xyl activity was estimated from the release of reducing sugars by the DNS method [36]. Enzymatic extracts were incubated for 20 min at 40 °C in a reaction mixture with CMC or birch xylan (0.2% w/v in 100 mM acetates buffer, pH 5.3) according to Loera and Cordova [37]. The standard curve was performed with glucose or xylose at a concentration of 2 mg/mL. One unit of enzymatic activity was defined as the amount of enzyme needed to release 1 µmoL of glucose or xylose per minute under the conditions tested. The protein concentration in the supernatants was determined according to the method of Bradford [38], using bovine serum albumin as standard. All analysis was performed in triplicate and the average rates were calculated to represent enzyme activity.

Molecular Identification

The Streptomyces species were identified based on the sequencing of the 16S rDNA gene. For DNA extraction, biomass was obtained from culturing the strains in ISP2 liquid medium at 28 °C for 48 h and 150 rpm. Total genomic DNA was extracted according to the CTAB procedure [39] (Wilson, 1997). The 16S rDNA gene was amplified by PCR using primers 8F (5′-AGAGTTTGATCCTGGCTCAG-3′) and 1492R (5′-GGTTACCTTGTTACGACTT-3′) in a thermocycler (SimpliAmp, Applied Biosystems, California, USA). The PCR amplification of 10 ng of genomic DNA was carried out in a volume of 25 μL of reaction, the composition of the reaction mixture was: 1 × Taq Buffer (10 mM Tris–HCl, 50 mM KCl, pH 8.3), MgCl2 2.5 mM, 200 μM dNTP’s, 0.4 μM of each primer and 1 U Taq DNA polymerase. The PCR program consisted of an initial denaturation of 94 °C for 3 min, 25 cycles of denaturation (94 °C for 30 s), alignment (53 °C for 30 s), extension (72 °C for 1 min 40 s) and final extension at 72 °C for 10 min. In both cases, the integrity of the DNA as well as the size of the amplified bands were determined by 1% agarose gel horizontal electrophoresis that were revealed with ethidium bromide (0.5 μg/mL) and visualized with ultraviolet light (254 nm). The PCR products were purified with the Wizard® SV Gel and PCR Clean-Up System kit (Promega, USA), taking into account the instructions suggested by the supplier and sequenced in a 16-capillary automatic sequencer (Applied Biosystems, model 3130xl) at the facilities of the Synthesis and Sequencing Unit of the Biotechnology Institute of the UNAM (Morelos, Mexico).

The ribosomal gen16s sequences were manually edited based on the quality of the electropherograms using the Chromas v2.6.6 software (https://technelysium.com.au). The degree of similarity with other closest sequences deposited in the GenBank (https://www.nlm.nih.gov/) was determined by BLAST (Basic Local Aligment Search Tool) program. The sequences were aligned with reference actinomycete strains in the CLUSTAL × 2.1 program (https://www.clustal.org/clustal2/) [40] and edited in BioEdit Sequence Alignment Editor [41]. All the sequences were deposited in the GenBank. The phylogenetic tree was constructed using the Bayesian inference method by means of BEAST v1.8.4 software and the General Time-Reversible Nucleotide Substitution Model (GTR) [42] obtained through the jModelTest v2.1.10 program [43] based on the Akaike Information Criterion (AIC), with a bootstrap value of 1000 replicas. The graphic representation and edition of the phylogenetic tree was done in FigTree version 1.4.3 (https://tree.bio.ed.ac.uk/software/figtree/). The genera Nocardia asteroides ATCC 19247 and Micromonospora echinospora ATCC 15837 were chosen because of their close phylogenetic relationship with the genus Streptomyces, while Bacillus circulans subsp. ATCC 4513 circulans was used as an external group.

Statistical Analysis

For the analysis of variance (ANOVA) and comparison of means by the Tukey method OriginPro 8 software (OriginLab Corporation, Northampton, Massachusetts, USA) and Minitab 17 (Minitab Inc., State College, PA, USA) respectively were used.

Results and Discussions

Isolation of Streptomyces Species

In this first stage 73 possible Streptomyces species were isolated from the two-soil rhizosphere of the MV, based on morphological and growth characteristics. Of the total isolates, 42.47% were obtained in ISP2 agar and 57.53% in ISP3 agar. The most suitable medium for the isolation of Streptomyces species in this study was ISP3, which according to the International Streptomyces Project is recognized as one of the most effective medium of obtaining high proportions of actinomycetes [30].

There is a great variation in the number of actinomycetes recovered from the rhizosphere, studies such as that of Hozzein et al. [44], report the isolation of 27 actinomycetes from soils in the Jouf region, Saudi Arabia using yeast extract agar and glycerol, while Salehghamari et al. [45] they isolated 69 actinomycetes in Alborz soil, Iran, on yeast extract agar plates-malt extract and casein starch agar. The differences found in the number of isolates are related to the different media and generally to the nutrient content in each of them and to the requirements on the part of the microorganisms, however, the actinomycetes present nutritional versatility which makes them be present in a large number of environments [46]. The diversity of the microorganisms present in the rhizosphere is correlated with the level of organic matter in soils [47]. The MV, receiving untreated wastewater effluents for irrigation, allows the incorporation of water, nitrogen, phosphorus, nutrients and low molecular weight organic compounds that can be used by soil biota [48, 49], it has been described that prolonged use of wastewater has a great effect on soil microbial communities [50]. Lüneberg et al. [26], reported 45 bacterial species in soils of the MV, being the predominant the actinobacteria with 30% of the total microbial communities. However, different results have been reported indicating that wastewater contains high concentrations of salt that increase soil salinization, with the consequent decrease in microbial biomass and enzymatic activities [51].

Qualitative Enzymatic Determination

All strains were evaluated for their ability to produce CMCase and Xyl extracellular enzymes using specific colorimetric staining methods for each of the enzymatic activities sought. The interactions between dye-polysaccharide provide a visual indication of the hydrolysis of the substrate reflected in clear areas or halos around the growth colony (Fig. 1) [52].

Fig. 1
figure 1

Hydrolysis halo formed around an actinomycete colony. A 0.2% CMC agar stained with lugol-iodine solution for Endoglucanase activity; B 0.2% xylan agar stained with Congo red solution and 1 M NaCl washes for xylanase activity

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Of the 73 isolates, 72 strains (98.63%) showed positive activity for CMCase production in CMC agar plates stained with lugol-iodine solution (Fig. 2). Several studies report that lugol-iodine solution staining is an easy and fast method to detect microorganisms producing lignocellulolytic enzymes mainly identifying as endoglucanases, in addition to generating a strong increase in the color contrast of the hydrolyzed area and the non-hydrolyzed portion [53, 54]. On the other hand, 68 isolates (93.15%) were positive for Xyl using as birch xylan substrate stained with Congo red solution (Fig. 2). Congo red interacts with 1,4-β-d-glucans, 1,3-β- d-glucans and 1,4-β- d-xylanes [55], which makes it the dye more using in the detection of Xyl in plate [56].

Fig. 2
figure 2

Production of CMCase and Xyl of Streptomyces species (72 strain) grown on plate with 0.2% CMC and 0.2% birch xylan as substrates for 10 days at 28 °C. NP no activity, + weak producer (halo 0.8–1.4 mm), ++ moderate producer (halo 1.5–2.4 mm), +++ strong producer (halo 2.5–3.0 mm)

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Of the strains studied, 72 showed to be strong CMCase producers (+++) and only one did not show this activity. Regarding the xylanolytic activity, 35 strains showed to be strong producers of xylanase (+++), 28 strains were moderate producers of xylanase (++), 5 strains were weak producers of xylanase (+) and five did not show this activity (Fig. 2) according to the scale of Porsuk et al. [32]. For the next stage that was the SSC, 38 strains were chosen, which were the ones that presented, on average, the highest halo of enzymatic activity for both CMCase and Xyl.

Quantitative Enzyme Activity of Crude Enzyme Extracts

SSC is a growth process in which microorganisms are able to grow in solid media that have relatively low water activity, which is trapped between the structure of the substrate in the form of free water.[57]. The microorganisms more viable to grow in solid medium are filamentous fungi, although actinomycetes such as Streptomyces sp. they are also able to grow in SSC systems, since they have characteristics such as abundant solid waste colonization, production of a wide range of hydrolytic enzymes and high resistance to extreme conditions [58].

SSC technology used by its importance for the generation of products with high added value, for example, enzymes from agro-industrial by-products [59]. The CMCase and Xyl production by the 38 Streptomyces species in SSC over a period of 240 h is observed in Fig. 3. The values indicated in the graph belong to the maximum enzymatic synthesis on barley straw with a particle size of 1680 µm. The growth of microorganisms on the substrate was evident this suggests that barley straw is used as a substrate promoting the expression of CMCase and Xyl during the growth of the microorganisms used.

Fig. 3
figure 3

Enzymatic profile of Streptomyces species, strain 15 (filled square), strain 28 (filled circle) and strain 30 (open triangle) using barley straw with particle size 12 (1680 µm) as a substrate in SSF for a period of 240 h. A Xylanolytic activity, B CMCase activity

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The CMCase activity of the 38 strains of actinomycetes and the day of maximum production is shown in Table 1. The strain 15 was the one that showed the highest cellulolytic activity with 26.02 AU/gdm at 192 h of incubation. The behavior in the production of CMCase enzymes in SSC systems was very heterogeneous and showed no tendency during the respective kinetics, this same behavior was observed when determining the xylanolytic activity where strain 30 was the one that produced the high more xylanolytic activity with 100.65 AU/gdm at 216 h of incubation (Table 1).

Table 1 Hydrolytic enzymes production (cellulases and xylanases) of streptomycetes isolated from agricultural soils, indicating the time when the maximum enzymatic activity was detected
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The behavior of the strains is interesting, where a greater xylanolytic activity is observed in comparison with the cellulolytic activity. The greater Xyl activity is probably due to the structure of the plant material itself, since structurally, hemicellulose (like xylan) is more exposed, and therefore, it is the polymer that is first degraded.

Longer fermentation times may favor CMCase production; this may be due to the need for a previous Xyl action to expose the cellulose fibers, which will induce the production of cellulases [60]. In addition, it has been reported that the presence of hemicelluloses, xylooligosaccharides and cellobiose in the fermentation medium has an inhibitory effect on the cellulolytic hydrolysis of biomass [61, 62], reason that could justify the low yields obtained for CMCases.

The studies carried out of the production of CMCase and Xyl enzymes in plate and SSC allowed selecting the three strains that presented the highest enzymatic activities. The chosen strains presented both enzymatic activities but in different proportions, in general, the xylanolytic activity was greater than the cellulolytic activity. The production of CMCase and Xyl of the strains using the barley straw as the only carbon source is shown in Fig. 3. Enzyme production increased gradually during the fermentation period, for Xyl activity the maximum values were obtained by strain 30 (100.69 AU/gdm), followed by strain 28 (81.53 AU/gdm) and strain15 (70.47 AU/gdm), at 216 h of incubation (Fig. 3a). The maximum production of cellulolytic activity was observed with strain 15 (26.02 AU/gdm) at 196 h of incubation (Fig. 3b).

In addition, the particle size of 2380 µm was evaluated in order to determine which particle promotes an increase in enzyme production. The results obtained in the production of the hydrolytic enzymes for strains 15, 28 and 30 are shown in Fig. 4, it is observed that for the enzymatic activities of CMCase and Xyl in the first hours of growth there is an increase in catalytic activity, subsequently there is a decay in the enzymatic activity, probably due to factors associated with the decrease of space for growth or depletion of nutrients. For strain 30 the maximum activities of CMCase (15.06 AU/gdm) and Xyl (78.09 AU/gdm) were detected at 240 and 216 h of incubation, respectively. The maximum catalytic activities for strains 15 and 28 were 61.76 and 67.13 AU/gdm of Xyl (120 and 168 h) and 25.73 and 11.47 AU/gdm of CMCase (216 and 192 h), respectively.

Fig. 4
figure 4

Enzymatic profile of Streptomyces species 15 (filled square), 28 (filled circle) and 30 (open triangle) using barley straw with particle size 8 (2380 µm) as a substrate in SSF for a period of 240 h. A Xylanolytic activity, B CMCase activity

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Of the two particle sizes studied, the 1680 µm particle favored the production of Xyl and CMCase due to the increase in the surface area of the substrate. Small particle sizes of lignocellulosic material increase the exposure of cellulose and hemicellulose compartments which benefits microbial binding [63]. Meanwhile, the 2380 µm particle provided larger spaces between the substrate, which generated better conditions for heat and mass transfer, but decreased the surface area for actinomycetes [64].

Most of what is reported by other authors regarding the enzymatic activities of CMCase and Xyl by actinomycetes are in the submerged fermentation (SmF) preferably, because it is possible to control the degree of aeration, agitation, pH, temperature, foam and other environmental factors for better process control [65]. The genus Streptomyces is the most studied in SmF [66,67,68,69,70]. Regarding the SSC, agroindustrial substrates are used as supports for the production of enzymes, Srivastava [71] studied a strain of Thermomonospora 29 in the presence of coffee residues supplemented with mineral salts were arabinosidases, Xyl and β-D-xylosidases production was studied. Beg et al. [72], reported the culture of Streptomyces sp. QG-11–3 in wheat bran and eucalyptus kraft pulp for the production of a thermostable Xyl enzyme. Similarly, Bajaj and Singh [73] used a strain of Streptomyces sp. 7b grown in wheat bran pretreated with steam to detect Xyl and Singh et al. [74], mixed rice bran and wheat in the presence of Streptomyces sp. MSC702 to evaluate amylases and Xyl production.

Postharvest agricultural waste is an inedible fraction of food crops or the plant [75], its use as substrate for the production of enzymes is a way of substantially reducing the total cost of the agricultural processes [76]. In this study it has been shown that barley straw is an interesting material to be used as a substrate for the production of enzymes. In addition to its low economic cost, is has wide availability and a low impact on food production. Straw is the most common organic waste, usually used as feed for cattle, but due to its low digestibility it is often abandoned, or it is incinerated resulting in serious environmental problems [77, 78].

Morphological and Molecular Identification of Streptomyces Species Producing CMCase and Xyl

The macroscopic characteristics of isolates 15, 28 and 30 selected for their potential to produce Xyl and CMCase were observed for 10 days in ISP2 medium (Table 2) Colony 15 showed light gray aerial mycelium, with production of diffusible exopigments to the medium of yellow hue at 6 days. The colony 28 was initially white and opaque since it did not produce any soluble pigment, after 5 days the presence of abundant aerial hyphae was evident that turned the plate pink. In contrast, the colony 30 exhibited light to dark gray aerial mycelium with red exopigment secretion at 7 days. The 3 strains showed a dry, powdery texture and adhered to the substrate, also proved to be Gram positive, typical characteristic of these microorganisms.

Table 2 Morphological identification of the actinomycete strains on ISP2 agar for 10 days at 28 °C. A) Strain 15; B) Strain 28; C) Strain 30
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Molecular identification was performed by sequencing the ribosomal gen16s. Nucleotide similarity search analysis in BLAST revealed that the isolates are closely related to the genus Streptomyces. The phylogenetic analysis included sequences of external strains that indicated that the closest species for strain 15 was Streptomyces flavogriseus with 100% similarity (GenBank accession number MN581662), strain 28 was related to Streptomyces virginiae with 99.89% (GenBank accession number MN581663) of similarity and strain 30 with Streptomyces griseoaurantiacus with 99.89% (GenBank accession number MN581664) of similarity as shown in the phylogenetic tree of Fig. 5. It has been previously described to S. flavogriseus for its capacity to produce high levels of extracellular enzymes capable of hydrolyzing cellulose and xylan, the main enzymes described in this microorganism are: xylanases [79], exoglucanases [80], cellulases and glucose isomerases [81, 82]. There are few studies of microorganisms that produce cellulolytic activity, however S. flavogriseus is particularly interesting because it has hemicellulose de-branching enzymatic activities for the most efficient degradation of lignocellulosic biomass [83]. More recently, Pennacchio et al. [84], formulated a mixture with xylanase and cellulase activity of two strains of S. flavogriseus AE64X and AE63X for use in saccharification of pretreated lignocellulose, obtaining yields of up to 86%. In the case of S. griseoaurantiacus, Chu et al. [85] they isolated strains that grew on amorphous cellulose (carboxymethyl cellulose) and produced endoglucanases at pH 5.0 and 50 °C, in addition to hydrolyzing purified bamboo cellulose with the secretion of β-glucosidases. Like, Kumar [86] they build two mutants of S. griseoaurantiacus with improved activities of endoglucanase and β-glucosidase highly thermostable at 80 °C and tested them in rice straw as the only carbon source. The present study confirms to S. flavogriseus and S. griseoaurantiacus as producers of CMCase and Xyl through the degradation of lignocellulosic residues.

Fig. 5
figure 5

Phylogenetic tree of Streptomyces species selected for their enzymatic potential built with the GenBank code number of each of the strains. Evolutionary relationships based on ribosomal gen 16S sequence alignments are shown. The numbers in the nodes indicate the similarity percentages. The bar indicates 0.001 substitutions per nucleotides position. Strain 15 (Streptomyces flavogriseus, GenBank code number MN581662); strain 28 (Streptomyces virginiae, GenBank code number MN581663); strain 30 (Streptomyces griseoaurantiacus, GenBank code number MN581664); 1[89]; 2[90]; 3[91]; 4[92]; 5[93]; 6[94]; 7[95]

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Meanwhile for S. virginiae, Iizuka and Kawaminami [87] and Macagnan et al. [88] indicated that the species is not capable of producing cellulases, β-1, 3-glucanases and xylanases when using simple substrates, because this type of compounds were not capable of inducing the synthesis of enzymes. The availability of nutrients is more restricted in solid substrates than in liquid cultures, due to the fact that in a liquid state there is more availability of nutrients since they are part of the solution, a situation that does not happen in solid substrates since generally (as in the case of plant matter) they are part of the structure. For microorganisms to have access to substrates, they must first be able to hydrolyze them, therefore it is necessary to develop more efficient enzyme systems for degradation of the solid residuals, so it is suggested that S. virginiae was able to produce CMCase and Xyl in presence of a complex substrate such as barley straw, this being the first report of its enzymatic potential.

Conclusions

In this study, Streptomyces species isolated from semi-arid areas of the MV demonstrated high cellulolytic (endoglucanase) and xylanolytic productivity by using barley straw as the sole substrate in solid-state fermentation. Under the conditions tested in this work, the particle size of 1680 µm favored greater catalytic activity for S. flavogriseus, S. virginiae and S. griseoaurantiacus strains. This study presented the first report of the production of hydrolytic enzymes by S. virginiae.