Spontaneous mutations in bacteria are generally reversible. Consequently, in a medium where prototrophy confers no selective advantage, various auxotrophs (and other mutant types) should acclumulate until for each type the number is such that the lossby mutation to prototrophy should equal the gain by mutation to the corresponding auxotrophic condition. That is to say, mutational equilibria should be established.Nevertheless it is common experience that prototrophic bacterial populations continuously maintained on complex media remain essentially prototrophic and contain at any time a very small proportion of auxotrophs. Conversely, under the same conditions, populations started from an auxotroph remain essentially auxotrophic and contain at any time a very small proportion of prototrophs. The forward and reverse mutation rates for any pair of alleles can lead to only one equilibrium, yet everyday experience shows us that two stable equilibria exist for each-known mutant in bacteria. We may conclude therefore that for each mutant at least one of the equilibrium ratios cannot be attributed to a balance of mutation rates.The above considerations serve to emphasize the ubiquitous role of selection in controlling the stability of bacterial populations which would otherwise become extremely heterogeneous as a result of mutation pressure. Perhaps it is not too surprising then, that we have found as a result of experiments to be described below that there exists in Escherichia coli a mechanism for stabilization of the major component of the population by the simultaneous suppression of all mutants.Materials and Methods.-The four initial stocks of E. coli (strain 15) used in these experiments were histidine requiring, lactose non-fermenting (h-lac-); histidine independent, lactose non-fermenting (h+ lac-); histidine requiring, lactose fermenting (h-lac+); and histidine independent, lactose fermenting (h+ lac+). Stocks were kept on nutrient agar slants transferred every three months, grown for 36 hours at 370C., then kept refrigerated between transfers. The main experimental procedure was the serial transfer of populations comprised of various initial ratios of the above stocks through Gray and Tatum synthetic medium containing 25y of L-histidine monohydrochloride per cc. (H+). The growth rate and final level of growth of h-and h+ are indistinguishable on H+.' The serial transfer experiments (STs) consisted in transferring 0.5 cc. of cultures in stationary phase to 50 cc. of the above medium in a 125-cc. Erlenmeyer

No abstract

No abstract

SUMMARY: During a stationary phase induced and maintained by the exhaustion of histidine, the total number of histidineless Escherichia coli (h -) remains constant as does the cytological appearance of the cells, If glucose is available to the starved bacteria they die at a rate of c. 10-2 per hr., while mutations t o a histidine-independent ( h + ) condition occur at a rate of c. 10-8 per bacterium per hr. Bacteria adapted to use lactose behave essentially the same way when it, instead of glucose, is available during starvation; but if the starved cells are not fully adapted, death does not occur or is very slow (c. 1 0 -5 per hr.) and the rate of mutation is c. 1O-lo. When no carbon source is available to the starved cells, mutations cannot be detected.The following predictions served as tests of the hypothesis of cell-turnover, wherein some bacteria lyse only to be replaced at the same rate by the growth of othersthe mutations are presumed to have occurred during this cryptic growth:(1) Mixtures of h-lac+ and h-lac-bacteria in lactose medium where the h-lac+ has a selective advantage should show population shifts if cryptic growth were occurring.(2) During the hypothesized lysis and growth the enzyme /?-galactosidase should, at predictable rates, be lost to the medium from'adapted cultures in the absence of lactose and developed in unadapted cultures in its presence.(3) Penicillin should kill those cells that grow to replace others, causing an accelerated death and preventing the mutations from taking place.(4) The lysis and death might be microscopically observable on an agar surface. The hypothesis of turnover did not withstand any of these tests. It was concluded, therefore, that. the bacteria under investigation were not dividing and that mutations -genotypic changes (Ryan, 1955 a)-were taking place among them.Reasons are given to suppose that the mutations result from errors in the replication of genetic material which is in the process of turnover within the non-dividing cells.

No abstract