SECRETION OF AMINO-ACIDS BY BACTERIA - PHYSIOLOGY AND MECHANISM

Although representing a common property of microorganisms and being widely used for biotechnological purposes, solute secretion has been relatively poorly studied in terms of biochemistry. In this review, various examples of metabolite secretion processes by bacteria are discussed with the emphasis on the mechanisms of amino acid secretion by coryneform bacteria. Among the metabolic concepts which may be applied to explain the physiological meaning of metabolite secretion, mainly two concepts are dealt with, i.e. the so-called 'overflow metabolism' on the one hand and the situation where non-metabolizable intermediates are accumulated and finally secreted on the other. In the central part of this review, the different concepts are discussed which have been put forward to mechanistically explain amino acid secretion under particular metabolic conditions and in particular strains of bacteria, i.e. secretion mediated (i) by diffusion, (ii) by the participation of amino acid uptake systems, and (iii) by the use of specific secretion systems. These concepts are then applied to amino acid secretion in Corynebacterium glutamicum, and exemplified by detailed studies on the mechanism and regulation of the secretion of lysine, isoleucine and glutamate by C. glutamicum.