Further studies on the ionizing radiation inactivation of bacterial spores.

where N/No is the fractional survival of the spore colony-forming ability, V is the "target volume" parameter in appropriate units, I is the radiation dose, and n is an integer (Woese, 1958a, b; Donnelan and Morowitz, 1957). The second equation is derived on the assumption that the radiosensitive portion of the spore consists of n pairs of targets, and survival occurs so long as one member of each pair remains intact (Woese, 1958a). A plot of In N/No vs. I gives for the case of equation 1 a straight line of slope V, while for the case of equation 2, the plot approaches asymptotically a straight line of slope -nV and intercept ln 2n. Equation 1 describes the radiation inactivation of spores of Bacillus subtilis, Bacillus mesentericus, and Bacillus brevis, while equation 2 describes the inactivation of spores of Bacillus megaterium, Bacillus cereus, and Bacillus mycoides (XWoese, 1958b). An understanding of this difference in shape of survival curves seems important to an understanding of spore structure. Although no unequivocal explanation of the data has been given to date, it has been suggested that the difference is due to the fact that in the case of the multiple target spores only, a duplication of spore genetic material exists (Woese, 1958b). The work reported here attempts to analyze this difference further by correlating it to other spore properties. A clue as to a possible correlation is given in the results of Powell (1953), who found that the dipicolinic acid content of B. megaterium spores is higher than that of other spore types (Perry and Foster, 1956). MATERIALS AND METHODS