The lethal effect and the inhibition of colicin production by leucine on a phenylalanine requiring E. coli strain as a function of age and dilution of culture.

The lethal effect caused by leucine on a phenylalanine requiring strain of Escherichia coli K-12 (7), was found to be strongly affected by the age of the culture. Early log cells were the least sensitive, while older cells became more sensitive to leucine. Another important factor affecting the sensitivity of this strain to leucine in liquid medium was the range of dilution of the culture. The age sensitive culture reacted to leucine only when its dilution was at a certain specific range. The sensitivity to leucine of a culture treated at the "right" age and at the proper dilution was proportional to the concentration of leucine. Colicine production was studied with cultures treated in various ways using a colicinogenic derivative of the leucine sensitive strain. It was found that leucine in a rather high concentration (2 mg/ml) prevented colicine production when given in the absence of phenylalanine, while phenylalanine at a very low dose (0.2 microgram/ml) could reverse this inhibition. The effect of leucine on colicine production, like lethality, operated only under certain conditions of culture dilution, but unlike lethality, was not very sensitive to the age of the culture. Tryptophane was found to resemble leucine in its effects on viability and on colicine production in liquid medium. Its effect was likewise reversed by a small amount of phenylalanine, and like leucine, tryptophane acted best only under very specific conditions of culture dilution.