Keratinolytic bacteria isolated from feather waste

Bactérias queratinolíticas isoladas de resíduos de penas

BASIC MICROBIOLOGY

Keratinolytic bacteria isolated from feather waste

Bactérias queratinolíticas isoladas de resíduos de penas

Alessandro Riffel; Adriano Brandelli^*

Laboratório de Bioquímica e Microbiologia Aplicada, Departamento de Ciência de Alimentos, ICTA, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brasil

ABSTRACT

The aim of this study was to characterize keratinolytic bacteria isolated from feather waste. Four isolates were selected after growth on solid medium with feather meal as sole carbon and nitrogen source and screened for proteolytic activity on milk agar plates. Three isolates were Gram-negative (belonged to the genera Burkholderia, Chryseobacterium and Pseudomonas) and one was Gram-positive (Microbacterium sp.). These bacteria grew on diverse keratin wastes such as feather meal, raw feathers, chicken nails, hair and wool. Keratinase activity was detected during growth, but the complete degradation of these substrates was not always achieved. The proteolytic character of crude enzymes was assessed using azokeratin and azocasein as substrates. The keratinases were active on both substrates and were similar in keratin hydrolysis when compared with commercially available microbial peptidases. These novel keratinolytic isolates have potential biotechnological use in processes involving keratin hydrolysis.

Key words: feather, keratin, peptidase, proteolysis

RESUMO

O objetivo deste estudo foi caracterizar bactérias queratinolíticas isoladas resíduos de penas. Quatro isolados foram selecionados após crescimento em meio sólido contendo farinha de pena como única fonte de carbono e nitrogênio e avaliados quanto a atividade proteolítica em placas de ágar leite. Foram identificadas três linhagens Gram-negativas (pertencentes aos gêneros Burkholderia, Chryseobacterium e Pseudomonas) e uma Gram-positiva (Microbacterium sp.). Estas bactérias cresceram em vários resíduos queratinosos como farinha de pena, penas de frango, unhas de frango, pelos e lã. A atividade queratinolítica foi observada durante crescimento, mas a degradação completa dos substratos não foi observada em todos os casos. O caráter proteolítico das enzimas foi determinado usando azoqueratina e azocaseína como substratos. As queratinases foram ativas em ambos substratos e apresentaram hidrólise de queratina comparável a peptidases microbianas disponíveis comercialmente. Estes novos isolados queratinolíticos apresentam potencial uso biotecnológico em processos relacionados com hidrólise de queratina.

Palavras-chave: pena, queratina, peptidase, proteólise

INTRODUCTION

Feathers are produced in large amounts as a waste by-product of poultry processing plant. A current value-added use for feathers is the conversion to feather meal, a digestible dietary protein for animal feed, using physical and chemical treatments. These methods can destroy certain amino acids and decrease protein quality and digestibility (18,26).

The nutritional inferiority and insolubility of native feather protein derive from the composition and molecular configuration of constituent amino acids that ensure the structural rigidity of feathers (22). Resistance to proteolytic enzymes has been attributed to the complex structure of b-keratin filaments. In addition, disulfide cross-links produce a compact three-dimensional network (3), as a result of intermolecular disulfide bonds between rod domains and terminal domains of the constituent molecules (22).

The nutritional upgrading of feather meal through microbial or enzymatic treatment has been described. Feather meal fermented with Streptomyces fradiae and supplemented with methionine resulted in a growth rate of broilers comparable with those fed isolated soybean protein (5). The use of feather-lysate from Bacillus licheniformis with amino acid supplementation produced a similar growth rate in chickens when compared to chickens fed with a diet that included soybean meal (28). The crude keratinase enzyme produced by B. licheniformis significantly increased the total amino acid digestibility of raw feathers and commercial feather meal (12). This enzyme increased the digestibility of commercial feather meal and could replace as much as 7% of the dietary protein for growing chicks (20).

Keratinolytic microorganisms and their enzymes may be used to enhance the digestibility of feather keratin. They may have important applications in processing keratin-containing wastes from poultry and leather industries through the development of non-polluting methods (21). A number of keratinolytic microorganisms have been reported, including some species of Bacillus (1,27), actinomycetes (2,29) and fungi (11,25). Generally, an increase in keratinolytic activity is associated with thermophilic organisms, which require high energy inputs to achieve maximum growth and the decomposition of keratin wastes (6,19). Recently, feather-degrading activity was also observed in studies of Gram-negative bacteria (23,24). These bacteria can degrade raw feathers at mesophilic temperatures, and are therefore useful to develop efficient processes involving keratin substrates.

In this report, we describe the selection and characterization of mesophilic microorganisms showing keratinolytic activity isolated from a poultry processing plant at Porto Alegre, Brazil. Some properties of their keratinases were also determined.

MATERIALS AND METHODS

Microorganisms

Bacterial strains designated as kr7 to kr14 were isolated from feather waste as described elsewhere (23,24). A previously characterized Chryseobacterium sp. strain kr6 that presented keratinolytic activity (23) was used for comparison. Bacterial identification was based on colony morphology, microscopic examination of Gram-stained cells, and biochemical tests (7,16), comparing the data with standard species. Additionally, an API 20E kit was used and the data was analyzed by automated interpretation with the APILAB Plus software (Bio-Mérieux, Marcy-l'Étoile, France).

Effect of temperature on growth and proteolytic activity

Milk agar plates (5 g L^-1 peptone, 3 g L^-1 yeast extract, 100 mL L^-1 sterile UHT non-fat milk, and 12 g L^-1 agar) were prepared for primary screening of proteolytic activity. Bacteria were inoculated onto plates and incubated at 22, 30, 37, 46, and 55ºC for 24 h. Strains that produced clearing zones in this medium were selected.

Enzyme assay

Keratinase activity was assayed with azokeratin as substrate (24). The reaction mixture contained 100 mL of enzyme preparation and 500 mL of 10 g L^-1 azokeratin in 50 mM tris buffer, pH 8. The mixture was incubated at 45ºC for 15 min and the reaction was the stopped by the addition of trichloroacetic acid to a final concentration of 60 g L^-1. After centrifugation at 10,000 g for 5 min the absorbance of the supernatant fluid was determined at 440 nm. One unit of enzyme activity was the amount of enzyme that caused a change of absorbance of 0.01 at 440 nm for 15 min at 45ºC. A similar protocol was used to determine enzyme activity on azocasein (Sigma, St. Louis, USA). Azokeratin was synthesized based on the methodology described for azoalbumin (23).

Degradation of keratin wastes

The capacity of degradation of keratin substrates was tested on medium containing 0.5 g L^-1 NaCl, 0.3 g L^-1 K₂HPO₄, 0.4 g L^-1 KH₂PO₄ and 10 g L^-1 of either feather meal, raw feathers, bovine hair, human hair, wool, or chicken nails powder. Degradation of substrates was visually inspected and aliquots were removed for keratinase activity assay.

Enzyme production

The organisms were cultivated in feather meal broth (10 g L^-1 feather meal, 0.5 g L^-1 NaCl, 0.3 g L^-1 K₂HPO₄, 0.4 g L^-1 KH₂PO₄) for up to 8 days at 30ºC. Enzymes were obtained by centrifugation at 10,000 g for 5 min, and culture supernatants were used as crude enzyme extracts. Keratinase activity was assayed at different cultivation times.

Comparison with commercial enzymes

Commercial enzymes (pronase E, papain and trypsin form Sigma-Aldrich, St. Louis; alcalase from Novo Nordisk, Denmark) were dissolved at 1 mg mL^-1 in 50 mM tris pH 7.5 and then assayed for azoproteins as described previously.

RESULTS

Characterization of keratinolytic strains

Eight isolates were able to grow on medium containing feather meal as sole carbon and nitrogen source. The strains kr8, kr9, kr10, and kr14 produced clearing zones when tested for proteolytic activity on milk agar (Fig. 1). The largest clearing zones were observed for isolate kr10, with a zone diameter similar to the keratinolytic strain Chryseobacterium sp. kr6. Milk agar plates were incubated at several temperatures, and the results are summarized in Table 1. After 24 h, kr8 was able to grow from 22 to 46ºC and peptidase production occurred between 22 and 37ºC. The isolate kr10 was able to grow form 22 to 55ºC, but produced proteolytic activity between 22 and 46ºC. The strain kr14 grew between 22 and 37ºC and produced proteolytic activity between 30 and 37ºC. The other isolates exhibited inferior results in growth and proteolytic activity.

The identification of the keratinolytic bacteria was based on cell morphology, colony morphology, and several biochemical tests. Isolates kr8, kr9 and kr14 were determined to be Gram-negative coccobacilli by microscopic observation, whereas isolate kr10 was determined to be a Gram-positive rod. The isolate kr14 formed golden-yellow colonies on feather meal agar plates. Together with the results of physiological tests and API 20E, the characteristics indicated that kr8 is a Bulkholderia sp., kr9 is a Chryseobacterium sp., and kr14 is a Pseudomonas sp. The strain kr10 is a Microbacterium sp.

Hydrolysis of keratin wastes

The strains were tested for their capacity to degrade diverse keratin wastes. Cultivation on all substrates resulted in the production of keratinase, although maximum values were obtained on feather meal and feathers (Fig. 2). Strains kr6 and kr10 degraded chicken feathers completely. The strain kr14 disintegrated feather barbules but not all rachises. Minor feather degradation was achieved by strains kr8 and kr9. Although keratinase activity was detected during growth on the different substrates, no important degradation of wool or hair was consistently observed (data not shown).

Keratinolytic activity

Keratinolytic activity of the isolates was monitored during growth in feather meal broth. The keratinolytic activity of isolates kr6 and kr10 increased in the first days, while other isolates showed a maximum keratinase activity later (Fig. 3). The highest keratinolytic activity was consistently observed from isolate kr6. An increase in pH was always observed during growth on feather meal broth.

The proteolytic activities of keratinolytic strains were compared with commercial enzymes by determining the hydrolysis of azokeratin and azocasein. Keratinase produced by strain kr10 exhibited higher specific activity degrading azocasein and azokeratin when compared to other enzymes (Table 2). The enzyme preparations were similar in hydrolysis of keratin substrate compared with commercially available microbial peptidases such as pronase, derived from Streptomyces griseus, and alcalase, form B. licheniformis.

DISCUSSION

Bacteria were isolated from a poultry processing plant, that owned keratinolytic activity and ability to degrade keratin wastes. These bacteria present different characteristics, such as a broad temperature range of growth. The optimal proteolytic activities were detected between 30 and 37ºC, whereas previously described keratinolytic bacteria mostly have feather-degrading activity at elevated temperatures (1,6,17). However, these strains behave similar to a Vibrio strain kr2, previously isolated from decomposing feathers (24). An optimum keratin-degrading activity at mesophilic temperatures should be a desirable characteristic because these microorganisms may achieve hydrolysis with reduced energy input.

Preliminary identification tests indicate that strains kr8, kr9 and kr14 belong to the Proteobacteria or Cytophaga-Flavobacterium group, and that strain kr10 is an actinomycetes. In agreement with this data, Lucas et al. (15) noted that feather-degrading Gram-negative bacteria isolated from soils belonged to the Proteobacteria or Cytophaga-Flavobacterium group. The most studied keratinolytic bacteria are Bacillus spp., which have been described to possess feather-degrading activity (9,14). Bacillus licheniformis is a well known keratinolytic organism, possessing the gene kerA, which has been cloned and sequenced (13). However, data on Gram-negative bacteria are relatively scarce and feather-degrading activity has been described only recently for Vibrio sp. (24), Chryseobacterium sp. (23) and Xanthomonas sp. (4).

An increase in pH values was observed during feather degradation, a trend similar to other microorganisms with large keratinolytic activities (8,24). This trend may be associated with proteolytic activity, consequent deamination reactions and the release of excess nitrogen as ammonium ions. The increase in pH during cultivation is pointed as an important indication of the keratinolytic potential of microorganisms (8).

Microorganisms growing on medium containing feather meal as a unique carbon and nitrogen source presented variable activity on azokeratin, suggesting that this enzyme may be inductive. Substrate levels in the medium may regulate enzyme secretion. Strain kr6 showed to be more adapted to keratinase production using keratin as substrate, since the maximum keratinase activity of the isolate was observed during early growth, and the strain displayed a higher total activity during incubation. The azokeratin/azocasein hydrolysis ratio was higher for strains kr6 and kr14, suggesting preferred utilization of keratin as substrate.

Through the strategy of isolation of keratinolytic microorganisms utilized in this work, bacteria presenting high keratinolytic activity were selected. Considering that feather protein has been showed to be an excellent source of metabolizable protein (10), and that microbial keratinases enhance the digestibility of feather keratin (12,20), these keratinolytic strains could be used to produce animal feed protein. In addition, the selected isolates were able to grow and display keratinolytic activity in diverse keratin wastes. This would be beneficial for the utilization of these residues. These novel isolates present potential biotechnological use in processes involving keratin hydrolysis.

ACKNOWLEDGEMENTS

This work was supported by CNPq and FAPERGS (Brazil).

Submitted: December 16, 2004; Approved: July 13, 2006

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Publication Dates

• Publication in this collection
  18 Dec 2006
• Date of issue
  Sept 2006

History

• Accepted
  13 July 2006
• Received
  16 Dec 2004