CHANGES IN U.V. SURVIVAL CURVES OF ESCHERICHIA COLl B/r CONCOMITANT WITH CHANGES IN GROWTH CONDITIONS

Abstract

Abstract— For E. coli B/r u.v.‐irradiated while in logarithmic growth, the nature of the dose‐response curve was strongly dependent on both pre‐ and post‐irradiation conditions of growth. Survival curves for cells grown in nutrient medium, or minimal medium with glucose, and plated immediately after irradiation, demonstrated an initial insensitive or ‘shoulder’ region provided the plating medium was such that no derepression was required of operons controlling inducible enzyme systems. If, however, such derepression was called for, survival curves were of exponential form. Delay in plating resulted in the return of the survival curve to the shouldered form even when ‘shift‐down’ media were used. Of those cells grown before u.v.‐irradiation in minimal media and plated thereafter with the same sugar as carbon source, only those grown with glucose (or lactose) demonstrated the shouldered survival curve.

Relation Between Survival and Deoxyribonucleic Acid Replication in Ultraviolet-Irradiated Resistant and Sensitive Strains of Escherichia coli B/r

“…Effect of UV irradiation on survival in log-phase and prestarved wild-type E. coli B/r. When E. coli B/r are grown to the logarithmic phase in arabinose minimal medium, exposed to UV light, and then plated on glucose minimal medium, the resultant survival curve is exponential in nature (26). Figure 1 demonstrates the transition from exponential to shouldered survival curve, characteristic of cells which have been starved for an essential amino acid prior to UV exposure (16).…”

“…For this purpose, bacteria were grown on arabinose-minimal medium before irradiation. Under these conditions, the UV dose survival curve for logarithmic-phase E. coli B/r is exponential in nature (26), RUDE AND DOUDNEY whereas for AA-prestarved cells it still has a large shoulder (see Fig. 1).…”

“…Strains of E. coli which are deficient in these repair mechanisms demonstrate increased UV sensitivity characterized by a greatly increased slope of the dose survival curve (20,21,23). Changes in growth conditions for wild-type strains, however, usually affect the magnitude of the "shoulder" of the survival curve without any effect on the slope (16,24,26). One attempt to get around this apparent discrepancy has been to postulate that the repair system has become saturated after a high enough dose, t-hereby causing the appearance of the shoulder (for arguments pro and con, see references 1, 3,17).…”

“…AA prestarvation has been shown to increase considerably the UV resistance of DNA replication of glucose-grown E. coli B/r, implying a relation between colony inactivation and inactivation of DNA replication by UV light (13). In view of the importance of growth conditions in determining survival curve shape (26), we decided to compare the UV inactivation of DNA replication under growth conditions which gave an exponential UV survival curve and one with a large shoulder. For this purpose, bacteria were grown on arabinose-minimal medium before irradiation.…”

Relationship Between Survival, Photoproduct Production and Repair Capacity in a Variant of Bacillus Cereus

“…Both pre-and post-irradiation cultural conditions modify the shapes of survival curves (Hanawalt, 1966; Morton and Haynes, 1969;Rude and Alper, 1972 ; Town et al, 1970). The 'shoulder' which appears at lower dose levels has been attributed to various conditions (Alper and Gillies, 1960;Haynes, 1966; Rude and Alper, 1972) which allow id I. ELIZABETH YOUNG cells to cope with damage more efficiently. Survival curves derived from the same organism when growing vegetatively or when sporulating may differ considerably without reflecting any novel mechanism for coping with radiation damage.…”