Free and bound intracellular lactic acid in Lactobacillus casei and other organisms.

Studies of extracellular and intracellular n-Lu-hydroxy acids produced by Lactobadlus casei 7469 were prompted by the earlier discovery that certain nutritional mutants derived from this organism, notably L. casei 280-16 and 280-16A (l-3), give a growth response to any one of a wide variety of n-ac-hydroxy acids. n-Lactic acid is the simplest growth-promoting acid for these organisms, although longer chain length n-cr-hydroxy acids are generally more active, and one of the most active substances is a 16 carbon compound, 2-D) 15-D, 16-trihydroxyhexadecanoic acid (4). Altogether more than twenty different cr-hydroxy acids, including aromatic as well as straight and branched chain aliphatic types are known to be nutritionally active ((14) and unpublished data). In many cases the D and L acids have been tested separately, and growth-promoting activity is invariably limited to the D form. The simplest explanation of these observations seems to be that compounds with nutritional activity for L. casei 280-16A either replace or function as metabolic precursors for essential n-a-hydroxy acids normally synthesized by L. casei 7469 in the absence of an external n-cY-hydroxy acid source. It was anticipated that if this explanation were correct, it would be possible to detect the L. casei 7469 acids by microbiological assay with L. casei 280-16A, and that identification of the L. casei 7469 acids would help to clarify the role of n-cr-hydroxy acids in L. casei 280-16 nutrition. A study of the extracellular growth-promoting acids produced by L. casei 7469 has been reported separately (5). The intracellular products of this organism have now been investigated, and it has been found that the only growth-promoting substance present in appreciable amounts is o-lactic acid. A minor portion of the intracellular n-lactic acid appears to occur as a free solute together with a large excess of microbiologically inactive L-lactic acid. Most of the intracellular n-lactic acid, however, is firmly bound to insoluble cellular elements, is detected only after hydrolysis with acid or alkali, and occurs together with little or no L-lactic acid. Details of these studies, together with preliminary data on intracellular n-lactic acid in organisms other than L. casei, are given in the present report.