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Poult Sci 2008. 87:2015-2022. doi:10.3382/ps.2007-00501
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METABOLISM AND NUTRITION

Corn Expressing an Escherichia Coli-Derived Phytase Gene: Comparative Evaluation Study in Broiler Chicks1

E. K. D. Nyannor and O. Adeola2

Department of Animal Science, Purdue University, West Lafayette, IN 47907-2054

2 Corresponding author: ladeola{at}purdue.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The efficacy of corn expressing an Escherichia coli-derived gene (corn-based phytase; CBP) and an E. coli-derived microbial phytase (expressed in Pichia pastoris) sprayed onto a wheat carrier (Quantum) was comparatively evaluated in two 14-d broiler chicken studies. In experiment 1, a total of 288 seven-day-old male broiler chicks were grouped by weight into 8 blocks of 6 cages, with 6 birds per cage, and used to measure growth performance, bone mineralization, and nutrient utilization. In experiment 2, a total of 192 seven-day-old male broiler chicks were used, with 4 birds per cage. Six dietary treatment groups were randomly allotted to the cages within each of 8 blocks. The corn-soybean meal-based diets used in each study consisted of a positive control adequate in P and Ca; a negative control (NC) low in P and Ca (no added inorganic P); the NC supplemented with 3,630, 36,300, or 363,000 phytase units (FTU) of CBP/kg; and the NC + 3,630 FTU of Quantum/kg. Growth performance and bone mineralization criteria were reevaluated in the second broiler study. Data from the 2 experiments were combined because there was no diet x experiment interaction and analyzed as a randomized complete block design. Weight gain decreased (P < 0.05) with a reduction in dietary P and Ca in the NC diet. Weight gain, feed efficiency, and percentage of tibia ash of birds fed 3,630 FTU/kg of either CBP or Quantum phytase were not different. There was a quadratic response (P < 0.01) to CBP supplementation of the NC diets in weight gain of the broiler chicks. Feed intake increased quadratically (P < 0.01) with CBP supplementation of the NC diets, but CBP supplementation of the NC diets did not affect feed efficiency. Tibia ash of birds fed the positive control diet was greater (P < 0.01) than that of birds fed the NC diet. There was a quadratic response (P < 0.01) to CBP supplementation of the NC diets in tibia ash of the broiler chicks. Birds fed supplemental phytase had greater (P < 0.01) ileal and total tract P and Ca digestibility than birds fed the NC diet. Data from the study suggest that E. coli phytase expressed in corn is efficacious in P-deficient broiler chick diets for the improvement of growth performance and indices of P utilization, which would minimize the need for supplemental P in broiler diets.

Key Words: broiler chick • corn phytase • digestibility • Escherichia coli-derived phytase • growth


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Phytates are the main storage form of plant phosphorus and inositol (Erdman, 1979; Liu et al., 1998; Maga, 1982), and because of the nominal phytase activity in the gastrointestinal tract of nonruminant animals, digestion of phytates is limited. The level of P in animal waste may contribute to environmental pollution (Biehl et al., 1998). To reduce the amount of P being released into the environment, diets are routinely supplemented with microbial phytase to improve P utilization (Ravindran et al., 1995a; Leske and Coon, 1999; Onyango et al., 2005). Exogenous addition of phytase to diets is effective in improving phytate-P digestibility (Dilger et al., 2004)

Plants accumulate a larger biomass than microbial expression systems and might therefore be more economically attractive as a system of phytase production. Corn constitutes the bulk of ingredients in corn-soybean meal diets and, unlike soybeans, is not subject to excessive processing temperatures. Corn-based phytase (CBP) is a genetically modified corn containing a phytase product expressed in the endosperm of the corn kernel that is identical to an Escherichia coli-derived phytase expressed in Pichia pastoris (Quantum, Syngenta Animal Nutrition, Research Triangle Park, NC). Nyannor et al. (2007) showed that the addition of CBP to a P-deficient diet improved growth performance and indices of P utilization in weanling pigs. It is therefore prudent to evaluate the efficacy of CBP compared with microbial Quantum phytase, which is sprayed onto ground wheat (WP) as a carrier, in terms of growth performance, nutrient utilization, and bone mineralization. We hypothesized that CBP would be as efficacious as the microbial Quantum phytase and that increasing the level of incorporation of CBP would improve the growth performance and bone mineralization of broiler chicks.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
All animal procedures were approved by the Purdue Animal Care and Use Committee.

Experiment 1

Dietary Treatments. The ground CBP was determined to have a phytase activity of 660 phytase units (FTU) per gram of corn. One FTU is defined as the quantity of enzyme required to hydrolyze 1 µmol of inorganic P/min, at pH 5.5, from an excess of 1.5 mM sodium phytate at 37°C (International Union of Biochemistry, 1979). The 6 dietary treatments were a positive control (PC) diet formulated to contain adequate (NRC, 1994) nonphytate P of 5.0 g/kg and Ca of 10.5 g/ kg; a negative control (NC) basal diet low in Ca and P with no inorganic P supplementation (nonphytate P = 1.3 g/kg, and Ca = 5.3 g/kg); NC supplemented with 5.5 g of CBP/kg (CBP1); NC + 55 g of CBP/kg (CBP2); NC + 550 g/kg of CBP (CBP3); or NC + Quantum phytase (WP). The diets were thus formulated to contain 0, 0, 3,630, 36,300, 363,000, or 3,630 FTU/kg, respectively, and were similar in all amino acids, other minerals, vitamins, and energy, as shown in Table 1Go.


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  		Table 1. Composition of diets on an as-fed basis
 
Birds. One-day-old male broiler chicks were wing-banded and maintained in electrically heated (35°C) 0.35 m2 battery cages (model SB 4T, Alternative Design Manufacturing, Siloam Springs, AR). The chicks were fed a standard broiler diet for 7 d, and the following day, 288 chicks were weighed, sorted in decreasing order of BW, and assigned to 1 of the 6 diets such that the average initial weight of chicks was similar across dietary treatments. There were 8 blocks per diet, with 6 birds per cage. Birds were allowed free access to feed and water throughout the entire 14-d period. A 23:1 L:D schedule was provided. Battery cage temperatures from d 0 to 7, 7 to 14, and 14 to 21 were maintained at 35, 32, and 27°C, respectively. Birds were observed daily for any variation in behavior and age, and weights of dead birds were recorded, as well as feeder weight of the cage at the time of mortality. Bird-days, a product of the number of birds alive in each cage and the number of days without incidence of mortality, was used to adjust for mortalities. Excreta samples were collected between d 16 and 19 posthatch and stored at –21°C until analyses. Frozen excreta samples were dried in a forced-air oven at 55°C for 5 d. On d 21, birds were weighed individually and feed consumption was recorded. Birds were killed by CO2 asphyxiation, and the left tibia bones from birds were collected and pooled by cage. Ileal digesta contents, from Meckel’s diverticulum to approximately 5 mm proximal to the ileo-cecal junction, for all birds within a cage were flushed with distilled water, pooled within each cage, and stored at –21°C until analyses. The ileal digesta were freeze-dried, ground to pass through a 0.5-mm screen, and mixed thoroughly before analysis.

Experiment 2

After analyzing the growth performance data of experiment 1, we observed that feed intake tended to decrease when all normal corn was substituted with CBP (CBP3) in the diet. The slightly larger particle size of the CBP corn was therefore reduced by milling through a 12.7-mm screen. Growth performance and bone mineralization of 7-d-old male broiler chicks were reevaluated from d 7 to 21 posthatch as described for experiment 1. However, unlike in experiment 1, 4 birds per cage were used instead of 6. Diet composition was as in experiment 1, except that soybean meal instead of corn starch was used in experiment 2 to replace the chromic oxide premix.

Chemical Analyses

The dried excreta, jejunal and ileal digesta samples, and diets were ground to pass through a 0.5-mm screen and mixed thoroughly before analysis. For DM determination, excreta, ileal digesta, and diets were oven-dried at 105°C until there was no change in weight. Phytase activity was determined according to the method of Engelen et al. (2001) with modifications, optimized for the E. coli phytase, involving the use of 250 mM acetate buffer, 0.1% Tween, and ammonium heptamolybdate and ammonium vanadate as yellow color reagents. Tibia bones (with ends removed) were thawed, defatted, dried, weighed, and ashed at 600°C for 16 h to determine the percentage of tibia ash. The N content of diets, excreta, and ileal digesta samples was determined by the combustion method (AOAC, 2002; method 990.03) with a combustion analyzer (Leco Model FP 2000, Leco Corp., St. Joseph, MI).

The diet, excreta, and ileal digesta samples for the determination of Cr, Ca, and total P contents were prepared by a nitric-perchloric acid wet ash (AOAC, 2002; method 968.08D[b]). The concentration of P was determined by using a colorimetric assay. Briefly, acid molybdate and Fiske-Subbarow reducer solution were added to the wet ash acid digest to form a phospho-molybdenum complex. The blue color intensity, measured with a spectrophotometer (SpectraCount model AS 1000, Packard Intrument Co., Downers Grove, IL) at 620 nm (AOAC, 2002; method 965.17), was proportional to P concentration. Calcium content of the wet acid ash was determined by flame atomic absorption spectrophotometry (AAnalyst 300 Atomic Absorption Spectrometer, Perkin Elmer Instruments, Norwalk, CT), and Cr (Spectronic 21D, Milton Roy Co., Rochester, NY) was determined at a wavelength of 440 nm.

Statistical Analysis

The data collected from the trials were analyzed by using the GLM procedures of SAS (SAS Institute, 2002) appropriate for a randomized complete block design. The growth performance and bone mineralization data from the 2 experiments were combined because there was no diet x experiment interaction. Cage served as the experimental unit, and an a value of less than 0.05 was considered significant. Orthogonal polynomial contrasts were used to determine significant linear and quadratic responses to CBP supplementation of the NC diet, and contrasts were used to separate the means of WP and CBP, PC and NC, and WP and NC.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
The analyzed phytase activity and nutrients in the diets are shown in Table 2Go. Percentage recoveries of phytase activity in the CBP diets were 91, 86, and 99 for CBP1, CBP2, and CBP3, respectively, with percentage recovery for phytase in the WP diet being 74. The analyzed P and Ca reflect the expected amounts in the diets. Phytic acid content of the CBP diets showed a dose-related reduction as the level of corn phytase increased.


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  		Table 2. Analyzed nutritive value of diets on an as-fed basis
 
Growth Performance

Results for the growth performance and bone mineralization of broiler chicks fed CBP or WP in experiment 1 are presented in Table 3Go. Broiler chicks fed the PC diet outperformed (P < 0.05) those fed the NC diets in BW gain. Birds fed the 3,630 FTU/kg of diet, CBP1, gained as much weight as those birds fed the WP diet at a similar phytase activity. In weight gain, birds responded (P < 0.01) in a quadratic manner to the supplementation of NC diets with CBP. Birds fed the NC diet had poorer (P < 0.01) weight gain than chicks fed WP at 3,630 FTU/kg of diet. When compared with the NC diet, phytase supplementation with WP led to an increase (P < 0.01) in feed intake, but birds fed WP or CBP at 3,630 FTU/kg had similar feed intake. Birds fed the NC diets supplemented with CBP had superior (quadratic, P < 0.01) feed intake compared with the NC-fed broilers. There was no difference in feed efficiency between birds fed WP or CBP1. There was no response in feed efficiency to CBP supplementation of the NC diets. There was an improvement in bone mineralization with phytase supplementation. Percentage of tibia ash was improved (P < 0.01) in birds fed WP just as well as in birds fed CBP at 3,630 FTU/kg of diet, compared with the NC-fed broiler chicks.


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  		Table 3. Growth performance of broiler chicks from d 7 to 21 posthatch
 
Apparent Digestibility and Retention of Nutrients

The apparent ileal digestibility of nutrients for broiler chicks is shown in Table 4Go. Dry matter digestibility was unaffected by phytase supplementation. The digestibility of N was not different for birds fed WP or CBP1, but WP-fed chicks had greater (P < 0.05) apparent ileal N digestibility than birds fed the NC diet. Broiler chicks did not respond in terms of N digestibility to CBP supplementation of the NC diets. Corn-based phytase supplementation of the NC diets resulted in linear and quadratic increases (P < 0.01) in apparent ileal P digestibility. Chicks fed the WP-supplemented NC diet had nearly 100% improvement (P < 0.01) in il-eal P digestibility compared with those fed the NC diet. Birds fed the NC diet had greater (P < 0.01) apparent ileal P digestibility than birds fed the PC diet. Digestibility of P in birds fed the WP diet was greater (P < 0.01) than in birds fed CBP1. Supplementing the NC diets with 3,630 FTU/kg of WP resulted in a 51% increase (P < 0.01) in ileal Ca digestibility, but birds fed diets adequate in P and Ca had poorer (P < 0.01) apparent ileal Ca digestibility than birds fed the NC diet. Apparent ileal Ca digestibility for birds fed WP was greater (P < 0.01) than in birds fed CBP1. We observed a linear (P < 0.01) response to graded levels of CBP in the NC diets in apparent ileal Ca digestibility.


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  		Table 4. Apparent ileal digestibility (%) of nutrients for broiler chicks on d 21 posthatch
 
There was a linear (P < 0.01) decrease in DM and an increase in N retention with graded levels of CBP supplementation of the NC diets (Table 5Go). Broiler chicks fed the WP-supplemented NC diet retained P just as well as birds fed the CBP1 diet. There was an improvement (P < 0.01) in P retention for birds fed the NC diet compared with those birds fed the PC diet. Birds responded linearly (P < 0.01) in P retention to CBP supplementation of the NC diets (Table 5Go). The retention of Ca was greater in broiler chicks fed the WP-supplemented NC diets than in birds fed the CBP1 and NC diets. The supplementation of NC diets with graded levels of CBP elicited linear (P < 0.01), but not quadratic, responses in the apparent retention of Ca.


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  		Table 5. Apparent retention of nutrients for broiler chicks
 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Growing environmental concern regarding P pollution and the subsequent incidence of algal blooms, among others, have necessitated a reduction of inorganic P supplementation of nonruminant diets. The annual worldwide seed phytate P production has been estimated to exceed all the P fertilizer applied (Lott et al., 2000). Phytate P is therefore a major potential source for harnessing plant-based P. The use of exogenous phytase to hydrolyze phytate P has been widely acknowledged as an effective means of reducing inorganic P supplementation. Onyango et al. (2005) demonstrated in broiler chicks that supplementing very low P diets with 500 or 1,000 FTU/kg of diet of an evolved E. coli phytase improved BW gain. The work of several researchers corroborates these findings. Dilger et al. (2004), in a 14-d broiler chick study, reported linear increases in weight gain, feed intake, feed efficiency, and tibia ash with microbial phytase supplementation of low-P diets. Improvement in growth performance in broiler chicks fed supplementary phytase has been reported in other research (Ravindran et al., 1995a; Cabahug et al., 1999; Augspurger et al., 2003). The P-releasing ability of microbial phytase has also been demonstrated. Denbow et al. (1995) estimated that feeding 1,000 FTU/kg of diet of a microbial phytase released up to 58% of P from phytate P. Phosphorus and P-containing compounds are intricately involved in growth and metabolic pathways, most notably as the well-known energy currency adenosine triphosphate. The myriad P products derived from phytase hydrolysis of phytate P are involved in DNA repair and recombination, in RNA export, and as ligands in signal transduction pathways (Raboy, 2003). It is therefore not surprising that improvements in growth performance of broiler chicks fed microbial phytase are sometimes attributed to factors other than phytate P release (Ravindran et al., 1995a). Overall, utilization of other minerals appears to be enhanced by phytase supplementation. Adeola et al. (1995) reported increased Zn, Cu, Ca, and P in 9.4-kg pigs fed supplemental Zn, phytase, or both.

The weight gain values of broiler chicks in this study were consistent with the values obtained by Dilger et al. (2004) for broiler chicks of a similar age on a 14-d trial. Most research work on microbial phytase in broiler chicks has used phytase activity levels far below what was used in these studies. However, it has been shown (Shirley and Edwards, 2003; Augspurger and Baker, 2004) that doses of phytase activity (>5,000 FTU/kg of diet) yield improved growth performance and a nearly 100% hydrolysis of phytate P in broiler chicks. Both reports called for a review of the phytase levels used in broiler diets. The linear increase in BW gain with increasing CBP phytase activity in the NC diets, up to the level of 36,300 FTU/kg of diet, is consistent with those reports. The source of phytase (microbial or corn-based) did not influence growth performance. Zhang et al. (2000), evaluating Natuphos and genetically engineered plant phytase (canola) in broiler chicks, concluded that both phytase sources were equally efficacious. Previously, Pen et al. (1993) elegantly demonstrated, in transgenic tobacco seed using the phytase-encoding gene from Aspergillus niger, that phytase expressed in plants was comparable to those genes expressed in microbial expression systems. They reported that not only were the 2 forms of phytase equivalent in promoting growth and bone mineralization, but that they were similar in storage characteristics and enzyme stability as well.

The tibia ash weight and percentage of tibia ash appeared to be more sensitive criteria than BW gain, in contrast to the report of Ravindran et al. (1995b), who found BW and toe ash to be more sensitive measurements than tibia ash. However, it is consistent with the observations of Nelson and Walker (1964), who favored the use of tibia ash over BW as a more accurate measure of P bioavailability in broiler chicks. The tibia ash values obtained in this study are quite consistent with the values reported by Dilger et al. (2004) and Augspurger and Baker (2004).

The effect of phytase supplementation on apparent il-eal P digestibility was quite pronounced. This is consistent with the report of Shirley and Edwards (2003), who, when feeding greater levels of microbial phytase (more than 12,000 FTU/kg of diet), achieved almost complete hydrolysis of phytate P in a corn-soybean meal diet fed to broiler chicks. The results of Augspurger and Baker (2004) corroborate this earlier finding that the current level of phytase supplementation needs revising. The results of the apparent ileal digestibility in this study demonstrate that with CBP supplementation even at 36,300 FTU/kg of diet, improvements in apparent ileal P digestibility could be obtained. There is no known report on nutrient digestibility at such a level of phytase supplementation in broiler chicks. The apparent ileal P digestibility appeared to have plateaued at 36,300 FTU/kg of diet, because a further 9-fold increase in phytase activity did not yield a significant improvement in digestibility. Unlike the growth performance data, WP was more efficacious in improving apparent ileal P digestibility and retention than was CBP at the phytase activity level of 3,630 FTU/kg of diet. Differences in expression systems could account for some posttranslational modification of the phytase gene in terms of glycosylation, which might affect the activity of the enzyme (Pen et al., 1993; Rodriguez et al., 1999). Improvement in Ca digestibility with phytase supplementation of broiler diets has been reported (Ravindran et al., 1995a); however, a lack of response in apparent Ca digestibility in broiler chicks has also been reported (Dilger et al., 2004). As observed in the apparent ileal P digestibility, WP improved apparent ileal Ca digestibility of broiler chicks better than CBP1, by 20 percentage units. The effect of phytase supplementation on protein utilization continues to be unclear (Adeola and Sands, 2003). Although there are some reports of improvements in amino acid and protein digestibility with phytase supplementation (Ravindran et al., 2006), there are differing results from other researchers (Peter and Baker, 2001).

The production cost of microbial phytase was identified previously as one of the obstacles to its application in poultry diets (Ravindran et al., 1995a). The use of phytase-containing transgenic seeds in nonruminant diets could be an economically efficient way to supplement the diet with phytase. The bottleneck associated with the use of nonfeed ingredients such as tobacco seeds (Pen et al., 1993), which necessitates extraction of the phytase from the seeds before being used in feed formulations, has been overcome by using soybeans (Li et al., 1997) and canola (Zhang et al., 2000). However, corn has a unique advantage as the expression vehicle for phytase in that unlike soybeans, it does not require the high temperature associated with processing soybean meal. In addition, corn is a major feed ingredient item in poultry diets in the United States. Nyannor et al. (2007) demonstrated, using CBP in growing pigs, that corn expressing an E. coli-derived gene was indeed as effective in improving growth and P and Ca utilization as Quantum phytase.

The results showed that CBP is as efficacious as the microbial phytase in supporting growth performance and nutrient utilization in low-P and low-Ca diets. Corn-based phytase diets fed to broiler chicks resulted in improvement in bone mineralization comparable to that of phytase expressed in a microbial expression system (Quantum phytase). It can be concluded from this study that the Escherichia coli-derived phytase expressed in corn that was used in the current experiments could be incorporated in a P-deficient diet of the 7-d-old broiler chick for 14 d to improve the growth performance and bone mineralization of broiler chicks and to reduce the excretion of P.


   ACKNOWLEDGMENTS
 
Partial funding, the corn-based phytase, the Quantum phytase, and the analyses provided for this project by Syngenta Animal Nutrition (Research Triangle Park, NC) are gratefully acknowledged. The assistance of Pat Jaynes in chemical analysis of the samples and of members of O. Adeola’s laboratory group is also appreciated.


   FOOTNOTES
 
1 Journal paper No. 2007-18263 of the Purdue University Agricultural Research Programs. Back

Received for publication December 10, 2007. Accepted for publication June 3, 2008.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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E. K. D. Nyannor, M. R. Bedford, and O. Adeola
Corn expressing an Escherichia coli-derived phytase gene: Residual phytase activity and microstructure of digesta in broiler chicks
Poult. Sci., July 1, 2009; 88(7): 1413 - 1420.
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