The invention relates to amino acid-producing bacteria from the family Enterobacteriaceae, in particular from the species Escherichia coli, which contain a stop codon chosen from the group amber, ochre and opal in the nucleotide sequence for the coding region of the rpoS gene and a suppressor for a stop codon chosen from the group amber suppressor, ochre suppressor and opal suppressor. This invention also relates to a process for preparing amino acids, in particular L-threonine, using these bacteria.