Journal of Probiotics & Health

Journal of Probiotics & Health
Open Access

ISSN: 2329-8901

Review Article - (2013) Volume 1, Issue 1

Possibility for Probiotic Sources of Methionine for Organic Poultry Nutritional Supplementation: An Early Review

Suwat Saengkerdsub1,2, Corliss A O’Bryan1, Philip G Crandall1 and Steven C Ricke1,2*
1Center for Food Safety and Department of Food Science, University of Arkansas, Fayetteville, AR, 72704, USA
2Department of Poultry Science, University of Arkansas, Fayetteville, AR 72701, USA
*Corresponding Author: Steven C Ricke, Center for Food Safety and Department of Food Science, University of Arkansas, Fayetteville, AR, 72704, USA, Tel: (479) 575-4679, Fax: +(479)575-6936 Email:

Abstract

Methionine is a nutritionally essential amino acid required in the diet of humans and livestock, including poultry. Chickens are unable to produce methionine and therefore must obtain it through their diets. Generally, methionine is one of the first limiting amino acids in poultry nutrition and typically in most diets this amino acid has to be added to the poultry feed. Currently, methionine is produced by chemical processes or hydrolyzing proteins. However, chemical synthesis is expensive and produces a mixture of D- and L-methionine. In addition, these sources of amino acids are problematic as nutritional supplements for organic poultry production. It may be possible to develop microbial sources of methionine that would meet the criteria for organic use but since genetic modification is not allowed this will require isolation of naturally occurring methionine over-producers. Application of such cultures may work as external sources of pure methionine but it may be more cost effective to develop a probiotic approach either by directly administering such cultures or enriching for members of the gastrointestinal population already present that have this ability. This review discusses these strategies and the criteria required to meet the requirements for methionine supplementation in these production systems.

Keywords: Methionine; Bacteria; Detection; Bioavailability

Introduction

An overall development in the poultry industry in the past few years has been the increasing need to supplement animal diets with specific nutrients to compensate for nutritional deficiencies in diets when less than optimal feed ingredient sources are all that are available. One of the nutritional sources most impacted by changes in economics as well as indirect consequences such as environmental quality are the availability of high quality protein supplements and the potential problems associated when lesser quality proteins must be substituted [1-6]. One of the more dramatic examples of a change in economics is the current United States (U.S.) biofuel industry where cereal grains such as corn are being used to generate ethanol at the expense of animal feed [5,7,8]. This has created a variety of problematic issues for the livestock and poultry industry not only from an economical standpoint but also potential food safety concerns when alternative protein sources from biofuel byproducts such as distillers’ grains are used [5,8-11]. In order to reduce nitrogen emissions from poultry fed these lower quality protein sources it is important to meet the animal’s needs for essential amino acids such as lysine and methionine which are the first limiting amino acids in most poultry diets [3,5]. Consequently, animal diets can be supplemented with purified forms of the respective amino acid to compensate for deficiencies in primary protein supplements in order to meet the nutritional requirements of the bird and help restore balance in feed amino acid profiles [2,12,13].

Although amino acid supplementation is fairly straight forward there are circumstances where chemically synthesized amino acids are not appropriate or allowed. The best known example is the organic animal production system where management restrictions have been formalized from a regulatory standpoint and what is allowable for being marketed as organically produced meat is carefully defined [14]. This is reflected in the strict requirements of the organically certified feed components used in feed formulation for organically fed food animals [15]. For some amino acids such as L-lysine and L-threonine there are well developed industrial fermentation sources based on microorganisms such as Corynebacterium glutamicum which over-produce and excrete the amino acid [16] that potentially could be generated as an organic product. This becomes problematic for some of the essential amino acids, particularly methionine where only chemically synthesized forms are commercially available and organic sources of this amino acid are virtually nonexistent [16-18]. Several hurdles remain before a practical solution to developing an organism that potentially could produce commercial levels of methionine, and if any genetic modifications are required would not be allowed for organic use due to the organic regulations [15,18].

Given these difficulties an alternative may be to isolate microorganisms that possess a higher content of methionine which can be directly fed as a supplement or administered as a probiotic culture that can be sustained in the gastrointestinal tract for the life of the bird. Either approach may represent potentially new opportunities for application of probiotic cultures to animal production to meet nutritional needs of animals, particularly where either less than optimal feed ingredients are available or emerging regulatory issues are altering feed supplement choices. In this review, general aspects of probiotics will be discussed as well as the poultry requirements for methionine, potential approaches for generating organic methionine and the need to develop rapid quantitation methods to assess methionine bioavailability.

Probiotics-General Concepts

Probiotics and starter cultures that can be added to animal diets or food products provide several potential benefits to the animal or human consuming either the resulting fermented food product or ingestion of the probiotic culture and subsequent modification in the gastrointestinal microflora [19-21]. Fermented foods have been in human diets for centuries and are generally characterized by a food product being altered in composition, flavor and potential texture due to the addition of a starter culture containing lactic acid bacteria [22]. While the use of starter cultures and the corresponding benefits are relatively straight forward the benefits of establishing probiotic microflora in the gastrointestinal tract of humans and animals are less clear. Generally, probiotic cultures in humans have been promoted for health benefits and to some extent this has been the case for food production animals, but in most animals the focus has been more for food safety purposes [19-21,23,24]. For a number of years, probiotics, also referred to as competitive exclusion or direct fed microbials, have been developed for application in food production animals as a means to limit and potentially prevent establishment of foodborne pathogens such as Salmonella [25-31]. This approach began with feeding undefined mixed cultures and has been refined to certain species of Lactobacillus, Propionobacteria or other lactic acid bacteria [30]. It is believed that such cultures limit pathogens in the gastrointestinal tract by direct competition for nutrients, generation of inhibitory products such as fermentation organic acids, and immunomodulation as well as a variety of other mechanisms which have not been clearly identified or understood [25,29,30,32-35]. Much less work and discussion has focused on their nutritional contributions and most of this has been directed towards impact on gastrointestinal metabolism and competition with pathogens for limiting nutrients [30,32,33,36-38]. The following section is focused on their potential as dietary protein and amino acid sources.

Gastrointestinal Microorganisms as Sources of Amino Acids

Only limited work has been done on probiotic cultures and their ability to serve as potential sources of nutrients. Most of what is known is based on gastrointestinal microorganisms and the research that has been reported has focused primarily on their contributions to the host’s nitrogen balance. In humans it has been shown that microorganisms in the small intestine contribute a fraction of the circulating plasma lysine, urinary lysine and body protein lysine of the host [39]. In poultry, Parsons et al. [40] separated the microbial fraction in the excreta of roosters and estimated that the microbial contribution to amino acid content was 25%. The most extensive characterization of microbial nitrogen metabolism has been done in ruminants. This is because of the ruminant host dependence on the microbial activities of the foregut (also referred to as the rumen) for most of the nutritional value it derives from dietary components [41]. Consequently, as the ruminant animal consumes foods, rumen microorganisms have initial access to the dietary components and hydrolyze dietary polymers including proteins to elements that are more readily fermentable such as sugars, amino acids and associated products [41,42]. These in turn are fermented by the rumen microorganisms to produce short chain volatile fatty acids, carbon dioxide, methane and ammonia [43,44]. The ruminant host uses the organic acids directly while ammonia is either excreted or converted into microbial protein which serves as the primary protein source for the animal [45-47].

The potential for probiotics as sources of dietary protein can probably be best deduced based on what has been learned from studies conducted on rumen microbial protein formation and assessment of protein quality as a dietary source of protein. Ammonia assimilation into rumen microbial protein has been extensively studied both as a function of rumen ecology as well as among individual rumen microorganisms [48-50]. Most rumen microorganisms can scavenge ruminal ammonia very effectively because they generally have a fairly high affinity for ammonia and can in turn convert it into microbial protein through a variety of biosynthetic pathways involving primarily the glutamine synthetase-glutamate synthase enzyme system and glutamate dehydrogenase along with a series of associated enzymes that are involved in the conversion of ammonia nitrogen to microbial protein [51-60]. Several studies have characterized the amino acid composition of ruminal microorganisms [45,46,54,61-63]. Hespell [54] reviewed these early studies and concluded that ruminal protein content varied from 40 to 60% of the cellular dry weight and a wide range of amino acids could be detected in the microbial protein fractions. When digestibility was examined using an in vitro enzymatic method, Bergen et al. [64] concluded that digestibility of rumen microbial protein from sheep ruminal contents was not influenced by diet.

Historically, there has been interest in potentially modifying rumen microorganisms to alter their amino acid composition or even overproduce amino acids that would be of benefit to the host animal [65,66]. However, Russell and Wilson [67] concluded that the highly selective and competitive nature of the rumen may preclude the ability to establish these genetically modified strains. Isolation and/or enrichment for gastrointestinal bacteria may represent a more practical approach particularly if probiotic potential is being considered. The use of prebiotic dietary components such as fructooligosaccharides that select for beneficial bacteria in the gastrointestinal tract have been employed as a means to shift the indigenous bacteria to a microflora more favorable to the host [21,27,31,68,69]. Prebiotics have also been proposed to be used in combination with probiotics to favor selection and establishment of the probiotic cultures in the gastrointestinal tract [27]. Most of the responses associated with these approaches have been identified as resistance to foodborne pathogens but there is the possibility for nutritional improvement as well including the selection of gastrointestinal bacteria that excrete amino acids required by the host. The potential exists for this as some rumen bacteria are known to generate detectable levels of certain extracellular amino acids [70-74]. Several of these studies incorporated the use of structural analogues of amino acids to screen for rumen bacteria resistant to these analogues and isolate those capable of overproducing and excreting the corresponding amino acid. Given the presence of these types of organisms it may very well be possible to administer such compounds in an attempt to selectively enrich for a gastrointestinal microbial population that overproduces amino acids of particular nutritional interest. The remainder of this review will be focused on methionine which is one of the more important limiting amino acids in organic poultry production, along with discussion on estimation of bio-available methionine and finally some potential strategies to select for microorganisms that would be presupposed to overproduce this amino acid.

Dietary Methionine in the Poultry Industry

Methionine is a nutritionally essential amino acid required in the diet of humans and livestock, including poultry. Chickens are unable to produce methionine and therefore must obtain it through their diets. Generally, methionine is one of the first limiting amino acids in poultry nutrition and typically in most diets this amino acid has to be added to the poultry feed [75]. In USA, approximately 90% of poultry feed is composed of corn and soybean which both are insufficient to meet methionine requirements of the bird [76]. The organic poultry industry faces an even tougher challenge regarding methionine supplementation in organic poultry feed. Currently, formulated organic poultry feed contains insufficient amounts of methionine when fed to birds which results in reducing growth rates of broilers [77] as well as lowered egg weight in laying hens [78].

Methionine is an essential amino acid and must be supplemented in most diets for normal growth and function of the body. Methionine supplementation in poultry production is known to enhance feed efficiency, increase protein synthesis, and improve immune systems [79-81]. Methionine supplementation also has been shown to prevent broiler chicks from developing neurological symptoms when raw grass pea seeds (Lathyrus sativus) were used as protein and energy sources in the diet [82]. In addition, methionine in diets demonstrated an improved oxidative stability, an increase in color stability, and a decrease in drip loss in chicken meat [83]. Sufficient methionine levels in the diet have been shown to be necessary for sustaining normal immunocompetence and achieving maximum egg production in laying hens in subtropical conditions [84]. Bunchasak and Silapasorn [85] reported that laying hens under tropical conditions fed a low protein diet (14% crude protein) supplemented with 0.44% methionine improved egg production and egg weight. In the same study bird mortality was reduced and egg shell thickness was improved when these hens were supported with methionine in feed. Conversely, insufficient methionine in organic feed showed a higher incidence of breast blisters in broilers [77] or cannibalism in Hyline hens [78]. The reduction of methionine content also decreased in the percentage of large and extra large eggs in Brown laying hens [86].

Currently, methionine is produced by chemical processes or hydrolyzing proteins [18]. However, chemical synthesis is expensive and produces a mixture of D- and L-methionine. In addition, these processes require hazardous chemicals such as acrolein, methyl mercaotan, ammonia, and cyanide [17]. However, methionine from protein hydrolysis must be separated from the complex mixture [18]. Furthermore, synthetic methionine is currently allowed as an additive to organic poultry feed by the U.S. Department of Agriculture’s National Organic Program (NOP) however NOP is only extending its use until October 1st 2012 (USDA, Agricultural Marketing Service, http://www. paorganic.org/wp-content/uploads/2011/10/USDA-Methionine.pdf).

Methionine Production by Microorganisms

A number of microorganisms capable of producing amino acids have been isolated and the production of amino acids has become an important aspect of industrial microbiology. Amino acids such as L-lysine, L-glutamic acid, L-threonine, and L-isoleucine have been produced successfully by fermentation [87]. Numerous studies have attempted to isolate and mutate microorganisms for overproduction of methionine but commercial methionine production from microorganisms is not available due to the highly branched pathway with complicated metabolic control in methionine biosynthesis [18].

Even though microorganisms use different biosynthetic routes, the pathways of methionine biosynthesis in various microorganisms have many common features.

First, aspartate is converted to aspartyl phosphate by aspartate kinase (EC 2.7.2.4) and then oxidized by aspartaldehyde semialdehyde dehydrogenase (EC 1.2.1.11) to form aspartate semialdehyse. The latter is oxidized by homoserine dehydrogenase (EC 1.1.1.3). Aspartate semialdehyde is subsequently converted to lysine. In one route, homoserine is converted to dihydropicolinate by dihydropicolinate synthase and subsequently threonine and isoleucine production. In another pathway, homoserine undergoes condensation with acetyl CoA to produce O-acetyl homoserine by homoserine acetyl transferase (EC 2.3.1.31). Some yeasts, fungi, and bacteria can directly synthesize homocysteine from O-acetylhomoserine via the direct sulfhydrylation pathway by utilizing sulfide (S2-) as the sulfur donor. Cystathionine is synthesized from O-acetyl homoserine and cysteine by cystathionine γ-synthase (EC 4.2.99.9). After hydrolysis of cystathionine to homocysteine, pyruvate and ammonia are formed by cystathionine b-lyase (EC 4.4.1.8), methionine is formed by the methylation of homocysteine by methionine synthase. Two forms of methionine synthases are involved in the final methylation reactions. Vitamin B12 (cobalamin)-dependent methionine synthase (EC 2.1.1.13, metH) utilizes N5- methyl-tetrahydrofolate or its polyglutamyl derivative as the methyl group donor, while the cobalamin-independent form (EC 2.1.1.14, metE), utilizes N5-methyl-tetrahydropteroyl-triglutamate.

All microorganisms possess mechanisms to regulate enzymes such that excess amino acids production is avoided. For example, in Corynebacterium glutamicum, the activity of enzyme homoserine O-transacetylase was not inhibited by L-methionine, S-adenosylmethionine or S-adenoyl homocysteine [88]. However, the synthesis of the enzyme was strongly repressed by L-methionine. The methionine biosynthesis is also regulated at the transcriptional level. In E. coli, MetJ repressor interacting with S-adenosylmethionine binds at met box, an eight-base consensus sequence, and subsequently leads to repression of the met genes, except metH [89]. In general, met genes have at least two to five contiguous met boxes located at operator sequences. However, the MetR activator in E. coli stimulates the expression of metE and metH, encoding methionine synthases.

Numerous studies have attempted to mutate microorganisms for methionine overproduction by using N-methyl-N'-nitro-Nnitrosoguanidine UV irradiation. Several studies have reported that methionine-analog resistance in mutants correlated to higher methionine production due to an alteration in the regulation of L-methionine biosynthesis [90-98]. Rowbury [99] reported that resistance to norleucine, a methionine analog, in microorganisms is associated with a failure of methionine to repress any of the methionine biosynthetic enzymes by feedback effect. Based on this concept, two methionine analogs (ethionine and norleucine) are typically used to screen for methionine overproduction in either mutants or wild type microorganisms from various natural sources.

The major cause of inhibition appears to be that methionine analogs mimic the means by which methionine regulates its own production [75]. Methionine analogs can effectively function as true feedback inhibitors without participating in other functions in the cells. These analogs may bind to the product site of the enzyme or may bind effectively to the repressor and consequently shutdown the pathway for the synthesis of methionine. Analogs inhibit growth by starving the cell for methionine. Therefore, methionine analogs act as pseudofeedback inhibitors or repressors, thereby inhibiting or repressing the synthesis of methionine. Only strains having resistance to analogs may overproduce methionine. These strains are able to resist the analogs either because of an alteration in the structure of the enzyme or an alteration in the enzyme formation system. Natural methionine analog resistant strains are insensitive to methionine accumulation and thereby will overproduce methionine. Using methionine analogs to screen the methionine overproducing microorganism could be an efficient and robust method for the identification of commercial strains since these strains lack methionine feedback inhibition.

Quantitation of Methionine Availability

Before precise formulation of methionine supplementation of organic diets can be done it is critical to assess total bioavailable methionine in the protein sources already present in the composite feed. This is essential since organic forms of these protein sources are fairly scare and consequently expensive for routine diets formulation. Therefore, methods are needed that allow for rapid bioavailability assessment of essential amino acids including methionine in protein sources prior to adding in pure forms of these amino acids or developing a means to produce these in vivo via some sort of probiotic modification as has been discussed previously in this review. This requires a rapid high-throughput type of bioassay since organic protein sources could be quite variable even within batches of the same protein source. The following addresses some potential in vitro E. coli-based biosensor approaches that may be applicable.

For methionine quantification in animal feed, chemical methods including High Performance Liquid Chromatography (HPLC) are commonly used [100]. By using the chemical methods, feeds are treated with acid digestion and the proteins in samples are completely digested. However, liberation of methionine is different from protein digestion under physiological conditions. Feed-derived methionine, which is available to animals to assimilate, can be more accurately estimated by animal or microbial assays which correspond more directly to the physiological needs of animals [101]. Although considered the biological standard, animal assays are laborious, expensive, and time consuming [102-105].

In contrast to animal assays, microbial assays appear to be easier and more affordable for routine analysis. This method is based on the response of microorganisms to feed nutrients by increasing the population number of organisms [102,105]. In contrast to animal assays, microbiological assays require smaller quantities of nutrients and respond in less time. Rapid development and recent improvements in molecular techniques allow for constructing successful and accurate amino acid biosensors via more precise genetic targeting of specific genes in microbial cells [100]. Among all microorganisms, E. coli is one of the most highly investigated bacteria for the purposes of biosensor fabrication. It is easy to cultivate, with simple nutritive requirements and rapid growth [106]. Based on E. coli auxotroph, threonine, tryptophan, lysine, and glutamine quantification have all been successful [103,107-127]. Since E. coli is an intestinal bacterium of most animals and humans, the assimilation of amino acids would be similar to animals [128]. After feed ingredients treated with enzymes, Erickson et al. [108] demonstrated a correlation of 0.94 between lysine bioavailability determined by using an E. coli lysine auxotroph and previously published chick bioassay data. An E. coli biosensor developed by Chalova et al. [103] proved to be as accurate as the corresponding chick bioassay for lysine bioavailable quantitation in diverse feed ingredients.

E. coli methioine auxotrophs have been constructed, modified, and utilized for methionine quantitation [107,129-134]. However, these strains used in these bioassays originated from strains mutated with using chemical mutagens and isolated based on the methionine requirement [135]. As a result, the mutation is not target specific and various non-methionine related genes can be affected. Revertants or compensatory mutations may occur to abolish the desired functionality [136]. In the case of methionine, the auxotrophic requirements for this amino acid are not specific and can also be satisfied by a variety of compounds including methioninyl peptides, α-hydroxy methionine, N-acetylmethionine, and the α-keto analog α-keto-λ-methiol butyrate [137]. When a chemically generated E. coli methionine auxotroph (ATCC 23798) was used, Froelich et al. [130] observed no differences based on substrate affinities of an E. coli methionine auxotroph to methionine and methionine hydroxy analog, respectively. Estimated maximum growth rate of the E. coli auxotroph when grown on both substrates was also found to be similar. To avoid problems as mentioned above, Bertels et al. [126] constructed a single gene deletion mutant from wild-type E. coli K12 for methionine quantitation using green fluorescent protein emission for detection.

Conclusions

Many studies have attempted to isolate methionine producing microorganisms from environments or by genetic modification by using N-methyl-N'-nitro-N-nitrosoguanidine. Several studies have reported that methionine-analog resistance in bacterial strains correlated to higher methionine production due to an alteration in the regulation of L-methionine biosynthesis [90,92,96-98]. Although the mutation was successful in producing methionine overproducing microorganisms, any genetically-modified organisms are considered unacceptable for use in organic food production (Electric Code of Federal Regulations: U.S. National Organic Program). Therefore, wild type strains with methionine-producing ability are necessary for the organic poultry industry.

There are a couple of possibilities for application of wild-type methionine producing microorganisms as potential probiotic cultures. One approach would be use them as external sources of methionine that could then be directly applied to the feed ration during formulation and mixing similar to other sources of pure amino acids are done now. However, sufficient quantities would need to be excreted to make this process cost effective otherwise some sort of extraction would need to be done which would add to the cost of production. The organisms could also be administered directly as a probiotic but this could be confounded by whether they could colonize and sustain establishment through the bird production cycle. A better alternative may be to use organic forms of methionine analogues as feed supplements to select for methionine over-producers already present in the gastrointestinal tract. Such organisms have been isolated and characterized from the rumen and it is not inconceivable that similar microorganism could also be present in the avian gastrointestinal tract. To achieve this will require isolation and identification of these microorganisms from the avian gastrointestinal tract and determining how to selectively enrich for them in the gastrointestinal tract by specific dietary amendments that are organically acceptable.

Acknowledgements

This review was supported by the Methionine Task Force, Coleman Natural Foods, Petaluma, CA, USA.

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Citation: Saengkerdsub S, O’Bryan CA, Crandall PG, Ricke SC (2013) Possibility for Probiotic Sources of Methionine for Organic Poultry Nutritional Supplementation: An Early Review. J Prob Health 1: 103.

Copyright: ©2013 Saengkerdsub S, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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