A new episomic element controlling fermentative metabolism and excretion of amino acids by Citrobacter intermedium C3.

Glutamate excretion by colonies of Citrobacter intermedium C3 was detected by using the auxotrophic strain Leuconostoc mesenteroides P-60. A constant ratio of strain C3 colonies did not excrete glutamate. These colonies were subcultured, and colonial analysis of their descendants established that the change from non-excretor to excretor (Sg(-) --> Sg(+)) is a spontaneous and random process with occurs at a high rate, and that an equilibrium state results from the back-transition Sg(+) --> Sg(-) in large populations. Acridine orange, ethidium bromide, and shaking have a strong influence on Sg(+)-to-Sg(-) interconversion, which suggests that a genetic element like an episome is implicated (S factor). Various auxotrophic mutants of bacterial strain C3 have been cured of the S factor. Strains lacking the S factor (S(-) strains) do not excrete glutamate and lose their fermentative metabolism completely. Consequently, the S factor is different from other extrachromosomal genetic factors whose elimination does not modify central metabolism. The gain of the S factor by infectious transfer has been shown with different C3 auxotrophic mutant strains. Also, the S factor has been transferred to Paracolobactrum intermedium ATCC 11606. These findings suggest that phenotypic changes observed are a consequence of elimination or infectious gain of the S factor, with its autonomous or integrated multiplication.