Mutants of incapable of utilizing citrate (C −) were discovered in glucose cultures by a modification of the mutant-concentrating penicillin technique. These C− mutants utilize glucose as efficiently as does the wild-type (C +), despite their inability to utilize a wide range of other metabolites as carbon sources for growth, including compounds that would be oxidized if glucose were aerobically dissimilated via the Krebs cycle. The compounds that the C − mutants cannot utilize are citrate, α-ketoglutarate, glutamate, succinate, fumarate, aspartate and acetate, which are all substrates to which glucose-grown C + cells slowly adapt. A non-genetic loss of ability to grow in media containing these substrates also occurs in glucose-grown populations under certain conditions. The C − mutant can revert to the prototrophic condition directly, or indirectly by first mutating to a stable state (C− F +) in which fumarate and aspartate can be utilized whereas the other compounds cannot.


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