Effect of Ascorbic Acid on Cysteine Requirement of Lactobacilli. ∗

Abstract

Summary

1. The requirement of some lactobacilli for cysteine, as determined in growth tests with usual chemically defined media, disappears when ascorbic acid in appropriate amounts, or other reducing substance, is incorporated in the medium. Of 5 strains of lactobacilli in this category encountered in a collection of 36 cultures, one was L. casei, the other 4 were heterofermentative L. fermenti. For these organisms the apparent cysteine requirement appears to be a need for a reducing substance and not for cysteine per se. Many L. fermenti strains evidently do not require cysteine though this may not always be apparent with some of the commonly-used synthetic media. 2. In contrast to L. fermenti, other lactobacilli—most L. casei, lactose-negative L. casei, L. acidophilus, and the majority of L. plantarum—require cysteine for growth in the presence of ascorbic acid. The rate of growth of these lactobacilli often is accelerated by ascorbic acid in the presence of either suboptimal or optimal amounts of cysteine. 3. Some implications of these findings concerning microbiological assay of cysteine and the distinguishing characteristics of heterofermentative L. fermenti and homofermentative lactobacilli are discussed.