Feather degradation by strains of Bacillus isolated from decomposing feathers

Abstract

Feather waste is generated in large amounts as a by-product of commercial poultry processing. This residue is almost pure keratin, which is not easily degradable by common proteolytic enzymes. Eight strains of Bacillus, isolated from decomposing feathers were tested for the hydrolysis of feather wastes in the laboratory. Among these strains, Bacillus cereus KB043 was the best feather degrading organism when grown on basal medium containing 1% hen feather as sole source of carbon and nitrogen. It caused 78.16 ± 0.4 % degradation with a significant release of soluble protein (1206.15 ± 14.7 µg mL^-1) and cysteine (20.63 ± 0.4 µg mL^-1) in the cultivation fluid. The strain also showed the highest level of keratinase activity (39.10 ± 0.4 U mL^-1). These data indicates that the Bacillus cereus KB043 could be useful in management of poultry wastes.

Feather is composed of over 90% protein, the main component being keratin, a fibrous and insoluble protein highly cross-linked with disulphide and other bonds. In mature chicken feather accounts up to 5–7% of the live weight. Worldwide, several million tons of feathers are generated annually as waste by poultry-processing industries. Considering its high protein content, this waste could have a great potential as a source of protein and amino acids for animal feed and for many other applications.

Despite the recalcitrance, keratin wastes can be efficiently degraded by specific proteases such as keratinase (15). The production of keratinases has been a domain of saprophytic and dermatophytic fungi, actinomycetes and some Bacillus species (2, 7, 11, 21, 23). 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 (1, 16, 24). Keratinases could also play other important role in biotechnological applications like removal of hairs and feathers in leather and poultry industries, aerobic digestion of poultry waste to generate natural gas, in textile industries to improve shrink proofing wool and for cleaning obstructions in sewage system during wastewater treatment (3).

The present report deals with feather degradation and production of keratinase by selected strains of Bacillus isolated from decomposing feathers. Preliminary screening of 126 isolates of bacteria for degradation of feather in vitro showed that only 35 % of the examined strains were able to grow on feather as sole source of carbon and nitrogen (13). On the basis of extent of feather degradation eight promising isolates were selected for further studies. Bacterial identification was conducted based on morphological, physiological and biochemical tests and the results were compared with Bergey’s Manual of Determinative Bacteriology, 8^th edition (5) and The Genus Bacillus: Agriculture Handbook No. 427 (9). These strains were identified as Bacillus cereus (KB043), B. licheniformis (KB059), B. megaterium (KB008 and KB069), B. subtilis (KB099), and Bacillus sp. (KB037, KB081and KB087) and the results were summarized in Table 1. Previous literatures have also documented the isolation of keratinase producing strains from members of genus Bacillus (8,17, 20).

Table 1.

Morphological, physiological and biochemical comparison of Bacillus strains isolated from decomposing feathers.

Tests	Bacillus strains
	KB008	KB037	KB043	KB059	KB069	KB081	KB087	KB099
Morphology	Rods	Rods	Rods	Rods	Rods	Rods	Rods	Rods
Gram staining	+	+	+	+	+	+	+	+
Endospores	+	+	+	+	+	+	+	+
Motility	+	+	-	+	+	-	-	+
Growth in pH	5.0	-	W	+	+	-	W	-	-
	6.0	+	+	+	+	-	+	+	-
	7.0	+	+	+	+	+	+	+	+
	8.0	+	+	+	+	+	+	+	+
	9.0	+	+	+	+	+	+	+	+
Growth at temperature	10°C	-	-	-	+	-	-	-	-
	25°C	+	+	W	+	+	+	+	+
	35°C	+	+	+	+	+	+	+	+
	45°C	+	+	+	+	-	+	+	+
	50°C	-	-	-	+	-	+	-	-
Growth in NaCl	2%	+	+	+	+	+	+	+	+
	4%	+	+	+	+	+	+	+	+
	7%	+	+	+	+	+	+	-	+
	10%	-	-	-	-	-	+	-	-
Anaerobic growth	-	-	+	+	-	+	+	-
Growth in carbohydrates	Glucose	+	-	+	+	-	+	-	+
	Arabinose	-	-	+	+	-	-	-	+
	Galactose	+	-	+	-	+	+	-	-
	Cellibiose	-	-	-	+	+	-	+	-
	Melizitose	-	-	-	-	-	-	+	-
	Ribose	+	-	-	+	-	-	+	-
	Maltose	+	-	-	+	-	-	+	+
	Fructose	-	-	-	+	+	-	+	-
	Sucrose	-	-	+	+	-	+	+	-
	Xylose	-	-	-	+	-	-	-	+
	Mannitol	-	-	-	+	-	-	+	-
	Citrate	+	+	+	+	+	-	-	+
Hydrolysis of	Starch	+	-	+	+	+	-	-	+
	Casein	+	+	+	+	+	+	+	+
	Gelatin	+	+	+	+	+	+	+	+
	Tween 20	-	-	-	-	-	-	+	-
	Tween 80	-	-	-	-	-	+	+	-
Biochemical tests	Catalase	+	+	+	+	+	+	+	+
	Oxidase	+	-	+	+	+	-	+	+
	VP test	-	-	-	+	-	-	-	-
	NO3 reduction	+	-	+	+	+	+	-	+
	Urease	-	+	-	-	-	-	-	-

+, Positive; -, Negative; W, Weak Growth.

Feather degradation by the selected Bacillus strains was carried out in 150 ml Erlenmeyer flasks containing 50 ml of basal medium (K₂HPO₄ 0.4 g L^-1; MgSO₄.7H₂O 0.05 g L^-1; NaCl 0.05 g L^-1; FeCl₃ 0.01 g L^-1, pH 7.0) with 0.5 g hen feathers. Bacterial culture grown on nutrient broth at 37°C, 150 rev min^-1 for 24 h was used as inoculum (2% v/v). The flasks were incubated at 37°C at 150 rev min^-1 for six days.

Residual feather in the culture broth was harvested by filtration with Whatman number 1 filter paper, washed with distilled water and dried at 65°C to constant weight. The percentage of feather degradation was calculated from the difference in residual feather dry weight between control (Feather without bacterial inoculation) and treated sample (13). The culture filtrates was analyzed for soluble protein content by Folin Phenol method (12). The free cysteine content in the culture filtrate was determined by the method as described by Saville (19).

Keratinase activity was determined using keratin azure as substrate (Sigma, USA) (4). One mL of enzyme sample was incubated with 40 mg of keratin azure in 8mL of Tris-HCl buffer (0.1M, pH 9.0) at 50°C for 1 h. The reaction was stopped using 5% TCA and samples were centrifuged at 10,000 g for 10 min and the absorbance of the supernatant was determined at 540 nm. In enzyme blanks TCA solution were added before reaction. One unit of keratinase was defined as the amount of the enzyme that resulted in an increase in absorbance at 595 nm of 0.01 after the reaction with keratin azure at pH 9.0 and 50°C for 1h.

Among all the strains, Bacillus cereus KB043 showed maximum degradation i.e., 78.16 ± 0.4 % weight loss (Table 2). In the present study local isolates of B. licheniformis KB059 and B. subtilis KB099 showed 76.39 ± 2.1 and 73.41 ± 0.7 % feather degradation, respectively. El-Refai et al. (6) reported 87.2 % degradation in Bacillus licheniformis and 49.4 % weight loss in cultures of Bacillus subtilis when grown on basal medium supplemented with 1% hen feathers.

Table 2.

Keratinolytic activity of Bacillus strains isolated from decomposing feathers.

S.N	Organism	Weight Loss	Final pH	Free Cysteine	Soluble protein	Keratinase
	(%)	(µg mL-1)	(µg mL-1)	(U mL-1)
1.	Bacillus megaterium KB008	68.83 ± 1.3	8.42 ± 0.02	15.56 ± 0.2	1217.69 ± 10.8	26.15 ± 0.3
2.	Bacillus sp KB037	62.04 ± 1.6	8.12 ± 0.04	14.78 ± 0.6	641.15 ± 24.3	16.95 ± 0.4
3.	Bacillus cereus KB043	78.16 ± 0.4	9.38 ± 0.02	20.63 ± 0.4	1206.15 ± 14.7	39.10 ± 0.4
4.	Bacillus licheniformis KB059	74.39 ± 2.1	9.24 ± 0.11	18.85 ± 0.2	1294.84 ± 18.7	31.50 ± 1.2
5.	Bacillus megaterium KB069	68.86 ± 1.3	9.01 ± 0.12	15.22 ± 0.0	1133.84 ± 16.3	18.80 ± 0.3
6.	Bacillus sp KB081	71.50 ± 0.3	9.22 ± 0.01	18.95 ± 1.1	1088.46 ± 22.1	29.35 ± 0.1
7.	Bacillus sp KB087	61.8 ± 0.5	7.90 ± 0.06	13.92 ± 0.1	448.8 ± 15.9	18.05 ± 0.1
8.	Bacillus subtilis KB099	73.41 ± 0.7	9.11 ± 0.05	16.24 ± 1.2	995.38 ± 11.9	25.40 ± 0.1

Test strains were grown on basal medium supplemented with 1% feathers (Initial pH 7.0) and incubated at 37°C, 150 rpm for six days. Readings are presented as Mean ± SD.

An increase in pH values was observed during feather degradation which is indicative of keratinolytic potential of microorganisms. Organism with higher keratinolytic activity turns media more alkaline in comparison with those exhibiting lower keratinolytic activities (18). This observation was based on the facts that keratin degradation involves oxidative deamination which results in production of ammonia and thereby increases the pH value.

Sulphitolysis is the primary process in keratin degradation; it involves breakage of disulphide linkages and release of cysteine residues as thiol. Bacillus cereus KB043 showed the accumulation of highest amount of cysteine i.e. 20.63 ± 0.4 µgmL^-1 while, Bacillus sp. (KB087) showed minimum release of cysteine residue (13.92 ± 0.1 µgmL^-1) in its cultivation fluid. Accumulation of cysteine also suggests the presence of disulfide reductase activity (17).

The amount of soluble protein released into the culture filtrate varied among the different Bacillus strains. Bacillus licheniformis KB059 showed the highest accumulation of soluble protein i.e. 1294.84 ± 18.7 µgmL^-1. The final concentration of protein in cultivation fluid was similar for B. megaterium KB008 and B. cereus KB043. Kim et al. (10) have reported 0.7 mgmL^-1 soluble protein in culture of Bacillus growing on 1% feather medium.

The cultivation fluid of Bacillus cereus KB043 showed the highest keratinase activity (39.1 ± 0.4 UmL^-1), which is followed by B. licheniformis KB059 (31.5 ± 0.2 UmL^-1). Keratinase values were quiet low in case of Bacillus sp. KB037 and KB087 ranging from 16.95 ± 0.4 to 18.05 ± 0.1 UmL^-1. The production of extracellular keratinase during growth of keratinophilic microorganism is well established (14, 22, 25). Suntornsuk and Suntornsuk (20) reported growth and efficient utilization of feather by Bacillus sp. FK 46 with release of 0.9 UmL^-1 of keratinase.

In the light of our results, Bacillus cereus KB043 is a potential keratinolytic strain which is suitable for the bacterial degradation of keratin wastes and its fermentation broth could be useful in processes suitable for the conversion of feather to feed stock additives.