Regulation of aspartokinase in Lactobacillus plantarum

As a rational approach to the genetic development of a stable lysine overproducing strain of Lactobacillus plantarum for the fermentation of 'ogi', a Nigerian fermented cereal porridge, regulation of lysine biosynthesis in this species was investigated. Spontaneous lysine overproducing mutants of Lact. plantarum were obtained and their aspartokinase activities compared with those of wild-type strains under different conditions. Results showed that aspartokinase activity of Lact. plantarum cell extracts was not inhibited by either lysine, threonine, methionine or combinations of lysine and threonine. Instead, methionine enhanced aspartokinase activity in vitro. Results indicated that lysine biosynthesis in Lact. plantarum could be regulated by lysine via the control of aspartokinase production in a way different to that described for other bacteria.     

Utilization of high lysine-producing strains of Lactobacillus plantarum as starter culture for nutritional improvement of ogi

A high lysine-producing mutant of Lactobacillus plantarum (OG 261-5) derived from a wild type strain (OG 261) previously isolated from fermenting ogi was evaluated for nutritional improvement of ogi in a modified fermentation process. Results indicate that ogi obtained by fermentation with the pure culture of the mutant compared to traditional ogi increased in concentration of available lysine from 228.5 ± 12.0 mg/100 g to 525.1 ± 25.8 mg/100 g, tryptophan from 58.6 ± 8.0 mg/100 g to 114.3 ± 11.0 mg/100 g and tyrosine from 408.5 ± 13.7 mg/100 g to 4636.5 ± 11.3 mg/100 g. However, the contents of valine, leucine, isoleucine and phenylalanine were substantially reduced which may affect the protein quality of the modified ogi. The modified process shortened the period of traditional ogi production from five days to one by reducing the two-stage fermentation (i.e. soaking and souring stages) to a one-stage process and the total protein recovery was better compared to traditional ogi processing. There was no significant difference (P > 0.05) in organoleptic quality attributes of colour, flavour, and overall acceptability of ogi produced by the modified process and the traditional ogi.