Abstract
Methionine is an essential amino acid for animals and is typically considered one of the first limiting amino acids in animal feed formulations. Methionine deficiency or excess in animal diets can lead to sub-optimal animal performance and increased environmental pollution, which necessitates its accurate quantification and proper dosage in animal rations. Animal bioassays are the current industry standard to quantify methionine bioavailability. However, animal-based assays are not only time consuming, but expensive and are becoming more scrutinized by governmental regulations. In addition, a variety of artifacts can hinder the variability and time efficacy of these assays. Microbiological assays, which are based on a microbial response to external supplementation of a particular nutrient such as methionine, appear to be attractive potential alternatives to the already established standards. They are rapid and inexpensive in vitro assays which are characterized with relatively accurate and consistent estimation of digestible methionine in feeds and feed ingredients. The current review discusses the potential to develop Escherichia coli-based microbial biosensors for methionine bioavailability quantification. Methionine biosynthesis and regulation pathways are overviewed in relation to genetic manipulation required for the generation of a respective methionine auxotroph that could be practical for a routine bioassay. A prospective utilization of Escherichia coli methionine biosensor would allow for inexpensive and rapid methionine quantification and ultimately enable timely assessment of nutritional profiles of feedstuffs.
Highlights
Methionine is an essential amino acid for animals and is involved in numerous metabolic processes [1,2,3,4]
Feed compounds such as cysteine, vitamin B12, arginine, choline, and sulfate that are related to methionine metabolism can affect the apparent methionine requirement of animals and complicate the estimation of the optimal dosage of this amino acid in animal diets [12]
The first unique step in bacterial methionine biosynthesis involves the activation of homoserine, which in E. coli is accomplished through a succinylation reaction catalyzed by homoserine transsuccinylase (HTS)
Summary
Methionine is an essential amino acid for animals and is involved in numerous metabolic processes [1,2,3,4]. According to Klasing and Austic [10] and Baker [11], excess of individual amino acids due to feed mixing errors can be potentially harmful to the animal, with methionine considered to be the amino acid possessing the highest toxicity. Feed compounds such as cysteine, vitamin B12, arginine, choline, and sulfate that are related to methionine metabolism can affect the apparent methionine requirement of animals and complicate the estimation of the optimal dosage of this amino acid in animal diets [12]. This review discusses methionine biosynthesis and regulation in Escherichia coli and the potential of genetically modifying this microorganism into practical whole cell biosensors for methionine bioavailability quantification
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