ASSESSMENT OF AN ESCHERICHIA COLI METHIONINE AUXOTROPH GROWTH ASSAY FOR QUANTIFYING CRYSTALLINE METHIONINE SUPPLEMENTED IN POULTRY FEEDS

Estimating availability of methionine is relevant to feed formulation since diets can be supplemented with crystalline methionine to meet the minimum requirements of rapidly growing birds. Bacterial assays have been developed to measure the bioavailable levels of several essential amino acids in feeds, including methionine. The E. coli methionine auxotroph strain used in this study exhibited a linear extent of growth response to increasing concentrations of methionine added to the minimal test media, in the range of 0 to 4 μg/mL. In addition the growth rates of the E. coli auxotroph were significantly (P < 0.01) different when the methionine concentrations were varied (0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 μg/mL) in minimal media. To assay feeds, feed grade methionine was added to poultry feed mixtures and samples were diluted with M9 media. Using this assay for estimating crystalline methionine added to feed, the extent of growth of the methionine auxotroph was correlated with the levels of crystalline methionine supplemented in the feed (R2= 0.9873). For all supplementation levels methionine recovery percentages ranged from 71 to 80% indicating that the bacterial assay response to crystalline methionine was relatively constant in the presence of the feed matrix. The overall results indicate that the rapid detection of crystalline methionine added to feeds is possible using this E. coli methionine auxotroph growth-based assay.