Screening of Feather Degrading Bacteria and Optimization of Fermentation Conditions from Poultry

Qin Wen-chao, Chen Zhi-hui, Chen Ying-lin, Ma Cheng-zhan, Sun Hao, Lian Li-na, Zhao Ze-yu, Zhao Cheng, Sun Jiayou, Bai Yang, and Xu Liang-mei

College of Animal Sciences and Technology, Northeast Agricultural University, Harbin 150030, China

Abstract: The purpose of this experiment was to isolate and screen the microorganisms from the soil where chicken feathers were piled for a long time and identify them biologically. Single-factor test and response surface methodology (RSM) analyses were used to explore the optimum conditions for the growth and fermentation of a strain. Screening of bacterial strains from the soil where feathers were piled for a long time was performed, and 12 keratinolytic bacterial strains were isolated. One of these isolates, CH-7, was found to be the most effective feather-degrading strain, which was identified as Lactococcus lactis KU568489.1. The growth situation of feather keratin degrading bacterium analysis results showed that the best degradation effect of CH-7 was found in oxygen, inoculation 5%, initial pH=6.5, fermentation temperature 30℃, speed 220 r · min-1 and fermentation time 36 h, and CH-7 had the highest degradation rate of feather keratin. The optimization analysis of RSM showed that there was more significances of the three factors, 32℃, pH 6.3 and amount of inoculum at 5.7% on the degradation of feather keratin. Based on the above results, the feather degrading bacterium, Lactococcus lactis CH-7, was obtained by screening, 30℃ and pH 5.5-6.5 were the best growth conditions. The oxygen, the amount of inoculum 5.7%, the fermentation temperature, 32℃, pH 6.3 and the rotational speed 220 r · min-1 were the best fermentation conditions. Under these conditions, 38.48% degradation rate was obtained after 36 h fermentation, which demonstrated the strain CH-7 was potential to the fermented feather meal for feed.

Key words: feather degradation, degrading bacterium, separation, identification, fermentation

Introduction

The feather is composed of 95%-98% protein. Its main component isβ-keratin, a highly cross-linked fibrous insoluble protein with high stability and difficult degradation. Poultry feather constitutes the most abundant keratinous material in nature. The feather accounts for 5%-7% of the total weight of adult chickens (Latshawet al., 1994). Its accumulation results from the poultry-processing industry and represents a potential environmental pollutant. In particular, the feather from commercial poultry processing is produced more than 8.5 million tons per year around the world (Fellahiet al., 2014). Feather waste represents a good source of nitrogen, protein and amino acid, which may be a cheap and alternative protein feed stuff. This waste product carries potent ecological implications, especially with burgeoning global poultry production. Towards valorization of this waste product into animal feed or fertilizer, many researchers have adopted different methods of processing.

Several different approaches have been used for disposing feather waste, including land filling, burning, natural gas production and treatments for animal feed. Most feather waste is converted to feather meal using rough physical and chemical treatments. However, these methods require substantial energy input, while destroy amino acids and may pollute the air, soil and water (Parket al., 2003). Therefore, a more efficient treatment technique is needed to improve the energy efficiency, economy and nutritional aspects. Hydrolysis of feathers by microorganisms possessing keratinolytic activity represents an attractive alternatives method for improving the nutritional value of feather meal, compared to currently used physiochemical methods.

Both the digestibility and amino acid balance of feather meal may be improved by microbial action combined with thermal or thermo-chemical pretreatment. After hydrolysis, the feather is converted to feed stuff, fertilizers, glues, films and as the sources of rare amino acids, such as serine, cysteine and proline (Tamreihaoet al., 2019; Cai and Zheng, 2009). Microorganisms use feathers as substrates (carbon and nitrogen sources), such asPhaffia rhodozyma(Bhosale and Gadre, 2001) andRhodotorula glutinis(Bhosaleet al., 2001), produce carotenoids, canthaxanthin and asta-xanthin, which may be useful in the deposition of the color of broiler chicken meat and egg yolk pig-mentation. Recently, polypeptides obtained by microbial fermentation of feathers have a variety of biological activities. Some studies have found that the hydrolysed protein from animal and plant sources, such as feather meal, has antioxidant activity (Salamiet al., 2016). It may improve the nutritional values of feather meal to replace the application of soybean and fish meal in livestock diets.

Most microorganisms capable of degrading chicken feathers are gram-positive bacteria ofBacillussp (Haqet al., 2020) andStreptomycessp (Koutchma, 2009) and some gram-negative bacteria ofChryseobacteriumsp (Brandelli and Riffel, 2005),Vibriosp (Sangali and Brandelli, 2000),Pseudomonassp (Torket al., 2010), etc. The degradation of feather byLactococcus lactisis scanty. In this paper,Lactococcus lactisis screened and isolated, which degraded waste chicken feather effectively, and its culture conditions were optimised. It would provide a theoretical basis for the application of fermented feather meal in poultry production.

Materials and Methods

Isolation and identification of feather degrading bacteria

Culture media. (1) Enrichment medium contained 0.5 g NH4Cl, 0.5 g NaCl, 0.3 g K2HPO4, 0.4 g KH2PO4, 0.1 g MgCl, 10.0 g yeast extract, 10.0 g feather meal and 1 000 mL deionized water. pH of each medium was adjusted to 7.0. (2) Each liter of screening medium contained 1.5 g K2HPO4, 0.025 g MgSO4, 0.025 g CaCl2, 0.015 g FeSO4, 10.0 g glucose and 10.0 g feather meal. (3) LB culture medium consisted of 5.0 g NaCl, 10.0 g peptone, 5.0 g yeast extract and 15 g agar per liter. (4) Seed medium consisted of 20.0 g peptone, 10.0 g yeast extract and 20.0 g glucose per liter. (5) Each liter of rescreening medium contained 0.5 g NaCl, 0.7 g K2HPO4, 0.35 g KH2PO4, 0.2 g MgSO4, 0.02 g CaCl2, 10.0 g glucose and 5.0 g feather meal.

Isolation and screening of microorganisms. The 5 g finely powdered soil sample was dissolved in 50 mL sterile distilled water, 1 mL diluted soil suspension was collected into 100 mL in LB culture medium and was incubated at 30℃ and 220 r · min-1for 48 h at a constant temperature to make enrichment medium.

The enrichment nutrient solution was serially diluted and 0.1 mL diluted suspension was spreadly plated on screening medium and was kept incubated at 30℃ for a few days. Morphologically distinct bacterial colonies were selected and cultured on LB plate at constant temperature 30℃ for 3 days. After repeated drawing, a single colony was isolated and purified.

One loopful of bacterial culture was inoculated into the seed medium and incubated at 30℃ and 220 r · min-1for 24 h. The 1 mL of each seed solution was inoculated into 50 mL rescreening medium for 24 h, and the degradation of the feather was closely observed. According to the degradation of the feather by each strain, rescreening was carried out. The fermentation broth was centrifuged in a centrifuge at 4℃ and 8 000 r · min-1for 15 min. The culture supernatant OD600and soluble protein concentrations were determined on microbial and biological identification for microorganisms. The purified cultures were preserved as glycerol stocks (30%, v/v) at -20℃.

Identification and molecular phylogenetic studies. The preserved strains were inoculated in the seed medium and incubated at 30℃ and 220 r · min-1for activation at a constant temperature for 16-17 h. The activated fluid was spreadly plated on screening medium at 30℃ for 2-3 days, until a single colony grew. An appropriate amount of bacterium solution was centrifuged at 12 000 r · min-1for 5 min; bacterial precipitation was used for genome extraction. DNA was extracted by using bacterial genomic DNA extraction kit (Sigma, St. Louis, MO, USA), according to the manufacturer's instructions.

The partial gene sequence of the strain was identified using the GenBank databases and aligned with gene sequences of other related species using Blast (Thompsonet al., 1997). Phylogenetic analyses were performed using the software package MEGA v4.0.

Optimization of strain growth and fermentation conditions

Optimization of growth conditions. AfterLactococcusactivation was cultured in feather meal solid medium at 30℃ for 2-3 days, until a single colony grew, an appropriate amount of bacteria was picked and cultivated at 30℃ for 24 h, and then inoculated in MRS medium at 5% for culture.

To measure the effects of temperatures, the reaction mixture pH maintained at 5.7 which was incubated at 26℃, 28℃, 30℃, 32℃ and 34℃ with agitation at 200 r · min-1for 48 h in a shaker incubator. For the effects of different pHs, the temperature was 30℃, pH was 5.0, 5.5, 6.0, 6.5 and 7.0, respectively, and other conditions were the same as that of temperature optimization experiment.

Optimization of fermentation conditions. Singlefactor test was used to study the factors affecting the fermentation ofLactococcusfeather: oxygen, period, rotation speed, temperature, amount of inoculum and pH on the degradation rate of the feather, and the optimal levels were explored one by one through variance analysis of the test results.

The nylon bags of 400 mesh (6 cm×5 cm) were marked with a pencil and dried to constant weight at 105℃. The weight of each nylon bag was weighed and recorded as M1. The 5 g feather meal was weighed and placed in a nylon bag, then dried at 105℃ to a constant weight; the weight was recorded as M2. CH-7 was cultured at MRS liquid medium for 24 h under the conditions of anaerobic, 30℃, pH of 6.0 and rotation speed 180 r · min-1. The sterilized nylon bag implanted with feathers was inserted into the fermentation medium. Under aerobic and anaerobic conditions, inoculum volume 4% (volume ratio), 30℃, pH 6.0, rotational speed 200 r · min-1and fermentation time 30 h were used as the basic conditions, different gradients were set, other factors were kept unchanged and repeated five times. The time (18, 24, 30, 36 and 42 h), rotating speed (180, 200, 220, 240 and 260 r · min-1), temperature (26℃, 28℃, 30℃, 32℃ and 34℃), amount of inoculum (3%, 4%, 5%, 6% and 7%) and pH (5.0, 5.5, 6.0, 6.5 and 7.0) and the influencing factors of the feather keratin degradation rate were researched.

After fermentation, the nylon bag was removed, the culture medium was washed with tap water, the culture medium was rinsed with distilled water for three times, and then was dried at 105℃, until constant weight. The total weight of the fermented nylon bag and its inner feathers were recorded as M3. The calculation formula was as the following:

Statistical method and validation of degradation rate of the fermented feather. Plackett-Burman (PB) test was conducted using six factors selected by a single- factor as indicators. According to the single factor selection, the optimal level was set as 0, which was divided into two levels: -1 and 1 (Sharma and Manhas, 2013). Design expert 8.0 software was used to find out the significance level of each factor to the response value in the single-factor experiment, and to screen out the key factors that had a significant influence on the response value for further optimization. The model equation was represented as the following:

Where,Ywas the predicted response,β0was the intercept,βiwas the linear coefficient andχiwas the independent variable level. Through PB test results, Box-Behnken (Alaouiet al., 2015) response surface design was used to optimize the maximum feather degradation efficiency. According to the fermentation conditions optimized by the response surface, three replicates were set for verification test to explore the actual degradation rate.

Statistical analysis

The Design-Expert 8.0 method was used for statistical analysis of the optimized data of bacterium species. All of them were measured in parallel for three times, and the results were expressed as mean values. Screening of microorganisms was subjected to ANOVA using the GLM procedure of SPSS 22.0. AP-value of less than 0.05 was considered statistically significant.

Results

Isolation and Identification of feather degrading bacteria

Isolation and screening of feather degrading bacteria

Using feather meal solid medium as single nitrogen source, the strain was separated and purified under the condition of oxygen after anoxia, a total of 48 single strains were isolated from the enrichment medium. During liquid fermentation, 12 groups with the ability to degrade feather keratin were preliminarily screened out, and were named CH-1 to CH-12. According to the absorbance values (OD600) and soluble protein concentrations of the supernatant of the 12 primary strains in the secondary screening medium, the primary strains were rescreened. The absorbance values of the supernatant of CH-7 and CH-2 were significantly higher than those of other strains (P＜0.05), the soluble protein contents of the supernatant of CH-7 were significantly higher than those of other strains (P＜0.05) (Table 1).

Table 1 Absorbance and soluble protein content of supernatant of re-screening strain

According to the rescreening results, CH-2 and CH-7 were selected, and they were inoculated on the solid medium coated with feather suspension, respectively. The generation of "degradation cycle" was observed 48 h later. CH-2 bacterial colonies presented three colors of yellow, green and white from the inside to the outside. CH-7 formed an obvious milky degradation circle. Thus, CH-2 and CH-7 were selected for biological identification (Fig. 1).

Identification and molecular phylogenetic studies

As observed in an electrophoretic image of PCR amplification product for 18S rDNA of CH-2 strain and 16 rDNA of CH-7 strain, the product was a single band, PCR products were sequenced, and the gene sequence of 1 319 bp was obtained by CH-2 and 1 379 bp by CH-7 (Fig. 2), which were consistent with the results of electrophoresis. The 18S rDNA (Fig. 3a) of CH-2 had more than 95% homology with mostMucor circinelloides, and the similarity withMucor circinelloidesf. circinelloidesJ was 88%. The 16S rDNA (Fig. 3b) of CH-7 was more than 95% related to mostLactococcus lactis, and its homology withLactococcus lactisKU568489.1 was 100%. Using MEGA 5.1 cluster analysis to reconstruct the phylogenetic tree, CH-2 and CH-7 were further identified asMucor circinelloidesandLactococcus lactis, respectively (Fig. 3).

Fig. 1 Rescreening degradation CH-2 and CH-7 strains

Fig. 2 Electrophoresis image of PCR amplification products

Optimization of strain growth and fermentation conditions

Optimization of growth conditions

The growth curve of CH-7 went up first and went down as the increasing temperature from 26℃ to 34℃ later. The optimum growth temperature of CH-7 was 30℃. When pH of CH-7 was 5.5-6.5, the growth effect was first increased and then decreased. The optimal growth pH was 5.5-6.0, and the growth ofLactococcusCH-7 was the best, when pH was 6.0 (Fig. 4).

Fig. 3 Phylogenetic tree of strain CH-2 and CH-7

Fig. 4 Effects of temperatures (up) and pH (down) on growth of Lactococcus lactis CH-7

The degradation rate of CH-7 to the feather in the aerobic state was up to 23.08%, which was higher than that of 13.61% in the anaerobic state (Fig. 5).

The effects of different fermentation conditions. The feather degradation rate was increased first and then decreased (Fig. 6 a-e). At the rotating speed of 220 r · min-1, temperature of 30℃, 5% amount of inoculation, pH of 6.5 and fermentation time of 36 h,the optimal degradation rate ofLactococcus lactisCH-7 on feathers was 25.78%, 28.15%, 24.36%, 27.24% and 35.25%, respectively (Fig. 6).

Fig. 5 Effects of oxygen on feather degradation rate of Lactococcus lactis CH-7

Fig. 6 Effects of fermentation conditions on feather degradation rate of Lactococcus lactis CH-7

Optimization of feather ddegradation by response surface methodology (RSM)

Using Design Expert 8.0 software to evaluate five variables, Plackett-Burman's experiment showed that the amount of inoculum, initial fermentation pH and fermentation temperature significantly affected feather meal fermentation within the test ranges (P＜ 0.05). In contrast, the fermentation time and rotation speed did not significantly influence the feather meal fermentation (Table 2). Degradation (R1) by the strain CH-7 was expressed in terms of the following regression equation:

Where,X1,X2 andX3 represented the amount of inoculum, pH and fermentation temperature. The highR2value (97.30%) indicated good agreement between the experimental results and theoretical values predicted by the model. The effects of three parameters amount of inoculum (A), pH (B) and temperature (C) on feather degradation were studied using RSM. A set of 17 experiments with different combinations of the three selected factors was performed (Table 2).

A regression model was developed for the process of microbial degradation of feathers, which was characterized by high suitability. HighR2value (0.99) of the model indicated that the experimental results were coincident with the theoretical value predicted by the model. The adjustedR2of 0.97 presented great significance of this model. According to the model, these three variables had significantly influenced the degradation rate of the feather. The amount of inoculum and the initial pH, the initial pH and the fermentation temperature had significant interaction on the feather degradation rate (P＜0.05). There was no significant interaction between the amount of inoculum and fermentation temperature (P＞0.05) (Table 3).

Table 2 Design proposal and experiment result of RSM

Table 3 Variance analysis of two times model and analysis of variance of regression equation

One factor was fixed at the zero level, and the interaction response surface curve of other two factors was processed using Design-Expert (8.0) software. Under the optimal conditions, the amount of inoculum was 5.69%, the initial pH of fermentation was 6.29, the fermentation temperature was 32℃, and the highest predicted feather degradation rate was 38.48% (Fig. 7a-c).

Fig. 7 Response contour plots and 3D curves showing interactions among variables on feather degradation rate by CH-7

After sorting out the optimized data, the fermentation verification test was carried out under the con and fermentation time 36 h. The experiment was repeated for three times. Overall analyses showed that feather degradation rate was increased by 38.48%, which was close to the theoretically predicted value. Therefore, this model was reliable in analysis and prediction.

Discussion

Feather degrading bacteria

In this research, the principles and methods of microbiology and molecular biology were comprehensively used, preliminary screening medium, soluble protein and supernatant absorbance value were selected, and the feather meal was used as a single nitrogen source to culture and observe by multiple methods. Two strains of bacteria, CH-2 and CH-7, were isolated and screened from the soil where the feather had been piled for a long time. Several keratinolytic microorganisms had been reported, including some species of fungi, such asMicrosporumsp (Essienet al., 2009),Trichophytonsp (Anbuet al., 2008) and the bacteriaBacillussp andStreptomyces(Cai and Zheng, 2009).Lactococcus lactisbelonged toLactobacilli streptococcusfamily in bacteria, andMucor circinelloidesbelonged to trichoderma order trichoderma family,both were not common feather-degrading microorganisms.

In the processing and storage of the feather meal, to ensure the storage time, the mold content needed to be strictly controlled. After entering the animal body,Mucor circinelloidescould absorb nutrients, destroy the metabolism of the body, produce enzymes with strong proteolytic activity, and secrete a large number of toxic substances, endangering human and animal health (López-Fernándezet al., 2018). Therefore, to ensure the feed quality, the degradation rate of the feather byMucor circulanswas not discussed in this experiment.Lactococcuswas a kind of prokaryotic microorganism, which widely existed in dairy products and plant products (Tidonaet al., 2018), had no pathogenicity to human beings and animals, and might prolong the shelf life of food and maintain the nutrition, flavor and color. It was widely used in food, cosmetics, medical care and other fields, and was generally regarded as safe food-grade microorganism (GRAS) (Parket al., 2003). According to the analysis of the principle of microbial fermentation with different feather degradation capabilities, the feather surface fiber might be used as the nutrient substance, during the growth process ofLactococcus, resulting in the dispersion of fiber segments and the fracture, of chemical bonds (Wanget al., 2015). There were no reports about the ability of the feather degradation ofLactococcus lactis. In this study,Lactococcus lactisCH-7 with the prebiotic function was selected as the experimental strain for follow-up study.

Optimization of strain growth and fermentation conditions

Kabanovaet al. (2009) determined the growth ofLactococcus lactisby high-performance liquid chromatography, multichannel calorimetry and spectrophotometry, respectively. The results of this study indicated that there was no significant difference among the three methods. Considering the time and economic cost, spectrophotometry was finally chosen to determine the growth curve of bacteria. In this research, gradient exploration was carried out for the optimal growth conditions ofLactococcus lactisCH-7 obtained by separation and screening. The optimization results of different temperatures for the growth conditions ofLactococcus lactisCH-7 showed that the optimal growth temperature was 30℃, which was consistent with the research results ofLactobacillus plantarum(Wadumesthri and Dianpumepong, 2016).

The results of this study indicated that the optimal pH of CH-7 was between 5.5-6.5, which was higher than the results of other studies about 5.0. It inferred thatLactococcus lactishad certain feather degradation ability, inhibited the formation of lactic acid by fermenting sugar, and pH required for growth was relatively higher (Cheighet al., 2002). Cesselinet al. (2018) found that the growth ofLactococcus lactiswas increased with the increasing acidity of the medium. The growth curve ofLactococcus lactisCH-7 inferred that under the optimum growth condition, the growth was linear in the liquid medium, during 5-20 h of culture. After 24 h, the growth gradually tended to slow down and the platform stage appeared.

For the first time, the method of the nylon bag procedure was used to optimize and analyze the feather conditions of CH-7 fermentation, which were influenced by oxygen, rotation speed, temperature, amount of inoculum, pH and period. The effects of oxygen on the degradation efficiency of CH-7 feather meal byLactococcus lactisinferred that CH-7 degraded the feather meal in an aerobic state. The degradation rate of the feather was 23.08%, which was higher than 13.61% in an anaerobic state.Lactococcus lactiswas a facultative anaerobic microorganism that produced acids, such as lactic acid in the absence of oxygen. Meyeret al. (2012) found that lactic acid degraded feather keratin to some extent; however, the effects were not significant. Under aerobic conditions,Lactococcus lactisincreased the number of the feather keratin degradation rate.

Rotating speed might improve the dissolved oxygen of liquid medium (Hornunget al., 2006) and the fermentation rate ofLactococcus lactisto a certain extent, and the maximum bacterium cell concentration was also increased, thus improving the degradation rate of the feather keratin. In this research, according to the effects of different rotating speeds on feather degradation efficiencies, when the rotating speed was between 180-260 r · min-1, the feather degradation rate was increased first, and then decreased with the increase of the rotating speed. When the rotating speed was 220 r · min-1, the degradation rate was the highest, which reached 25.78%. However, when the rotating speed was higher than 220 r · min-1, the degradation rate of the feather was decreased, because the excessive rotation speed reduced the probability of contact betweenLactococcus lactisand the feather, and the attachment ofLactococcus lactison the feather surface, thus resulting in the decline of the degradation rate of the feather.

In the process of microbial fermentation of the feather meal, temperature mainly affected the degradation rate of the feather meal through two factors. The first was that microbial life activities were composed of biochemistry, which was greatly affected by temperature. The temperature was an important factor affecting microbial growth and reproduction (Duttaet al., 2004). The second was that the temperature affected enzyme activity by affecting the speeds of enzymatic reaction or the structures of active enzyme proteins. Hofvendahlet al. (1999) found that fermentation over the optimal temperature resulted in the production of homogenous lactic acid products, and reduced the growth rate. In this research, the effects of different temperatures on the degradation efficiencies of the feather showed that the degradation rate of the feather was increased first, and then decreased at different temperatures. At 30℃, the degradation rate of the feather by CH-7 was the highest by 28.15%, which was consistent with the optimum growth temperature of the strain, indicating that there was a certain positive correlation between the growing quantity of the strain and the degradation rate.

The environmental pH greatly influenced the life activities of microorganisms. The main function of pH was to change cell membrane, affect the absorption of nutrients and enzyme activities in the process of metabolism, also change the effectiveness of nutrients and toxicity of harmful substances in the growth environment. At the same time, Ramírez-Nuñezet al. (2011) pointed out that pH affected the stability of the product by affecting the charge state of the cell membrane. In this research, the results of different pHs on the degradation efficiency of the feather showed that the maximum degradation rate of the feather was 27.24% at pH of 6.5, which was within pH ranges of the optimum growth condition. However, there was a certain deviation from the optimum growth pH, becauseLactococcus lactisaccelerated the formation of lactic acid under the acid condition, which affected the degradation efficiency of the feather.

For the selection of fermentation time, the transition position between rapid growth and gentle growth during the fermentation process was usually selected, which might not only save costs, but also improve efficiency. Compared to most other isolated strains (Xuet al., 2009; Zhanget al., 2009) in the case of CH-7, complete feather degradation was observed in a much shorter time, which was a noteworthy criterion. The results revealed that the degradation rate of the feather meal byLactococcus lactisCH-7 was increased before fermentation for 36 h, then slowly increased after 36 h and into the plateau stage.

Optimization of feather degradation byLactococcus lactisCH-7 was performed using Plackett-Burman (Mukherjee and Rai, 2011) combined with RSM. RSM was a widely accepted statistical approach for modelling and analyses of problems in which a response was influenced by several variables such as temperature, pH, amount of inoculum and media components (Duttaet al., 2004). In this research, three parameters, pH, amount of inoculum and temperature were selected to study the degradation of the feather meal byLactococcus lactis. Based on the RSM study, the final feather degradation condition was optimized at 5.7% amount of inoculum, pH 6.3 and 32℃. After optimization the soluble degradation of feather meal byLactococcus lactisCH-7 was 38.48%. This study screened out fermentation conditions suitable for mass production ofLactococcus lactisCH-7, which greatly reduced the production cost and raised the possibility of commercial application ofLactococcusdegradation of the feather.

Conclusions

In summary, the present study successfully isolated and screened out microorganism CH-7, which degraded the feather keratin from the feather stacking soil. The homology of 16S rDNA of CH-7 withLactococcus lactisKU568489.1 was 100%. The optimal growth conditions ofLactococcus lactisCH-7 were: temperature 30℃, pH 6.0 and logarithmic growth within 24 h. The optimum conditions for the degradation of the feather meal were as the followings: aerobic, inoculum 5.7%, initial pH 6.3, fermentation temperature 32℃, rotational speed 220 r · min-1and the degradation rate of the feather meal of 38.48% at 36 h of fermentation.

Acknowledgments

Qin Wen-chao and Chen Zhi-hui contributed equally to this work and should be considered co-first authors.