Quantitative aspects of the interactive killing effects between X rays and other mutagens in microorganisms

Recently we presented a mathematical description of the synergistic interaction which occurs when Escherichia coli B/r is exposed to both X rays and 254 nm ultraviolet light (D. D. Ager and R. H. Haynes, Radiat. Res. 110, 129-141 (1987]. Here we extend this approach to other bacteria and describe a graphical technique which can be used to determine the nature and relative importance of second and third degree terms in the function h(x, y), which describes the dose dependence of such effects. In most cases, interaction functions appear to be dominated, in the biologically interesting dose range, by a second degree term in the product, xy, of the doses of the two agents. We find that the magnitudes of these interactions vary among the organisms examined and can be surprisingly large. Finally, we show that the simple xy dependence observed for most interactions does not carry any unambiguous implications with respect to previous speculations on the mechanisms of these effects.