Lactic acid and 3-hydroxypropanoic acid are industrially relevant microbial products. This paper reviews the current knowledge on export of these compounds from microbial cells and presents a theoretical analysis of the bioenergetics of different export mechanisms. It is concluded that export can be a key constraint in industrial production, especially under the conditions of high product concentration and low extracellular pH that are optimal for recovery of the undissociated acids. Under these conditions, the metabolic energy requirement for product export may equal or exceed the metabolic energy yield from product formation. Consequently, prolonged product formation at low pH and at high product concentrations requires the involvement of alternative, ATP-yielding pathways to sustain growth and maintenance processes, thereby reducing the product yield on substrate. Research on export mechanisms and energetics should therefore be an integral part of the development of microbial production processes for these and other weak acids.
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