SUMMARY: Lactic acid is readily utilized as a carbon and energy source by The oxidation of lactate is coupled to electron transport via a membrane-bound lactate dehydrogenase (iLDH) which is independent of pyridine nucleotide. The broad substrate specificity of iLDH endows with the novel ability to convert phenyllactate to L-phenylalanine via phenylpyruvate. ATCC 27628 typifies a class of clinical isolate whose growth is inhibited by phenylpyruvate (or L-phenylalanine). Exploiting resistance to growth inhibition by phenyllactate as a strategy of positive selection, mutant derivatives of strain ATCC 27628 lacking iLDH activity were readily obtained. These mutants are incapable of oxidizing phenyllactate, and lack the parent-strain ability to reduce -type cytochromes in the presence of lactate, phenyllactate or 4-hydroxyphenyllactate. They retain, however, a cytoplasmic NAD-linked lactate dehydrogenase (nLDH). Since the mutants retained the ability to grow on lactate as a sole source of carbon, nLDH presumably can function in an opposite-to-normal physiological direction in the absence of iLDH. This would explain the failure to isolate iLDH-deficient mutants by selection for inability to grow on lactate.


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