SUMMARY: During a stationary phase induced and maintained by the exhaustion of histidine, the total number of histidineless (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 10per hr., while mutations to a histidine-inde-pendent (h +) condition occur at a rate of 10per 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 ( 10per hr.) and the rate of mutation is 10. When no carbon source is available to the starved cells, mutations cannot be detected.

The following predictions served as tests of the hypothesis of -turnover, wherein some bacteria Iyse only to be replaced at the same rate by the growth of others—the mutations are presumed to have occurred during this cryptic growth:

(1) Mixtures of hlacand hlacbacteria in lactose medium where the hlachas a selective advantage should show population shifts if cryptic growth were occurring.

(2) During the hypothesized lysis and growth the enzyme β-galactosidas 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, 1955530)—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.


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