GAMMA RADIATION FOR PRODUCT STERILIZATION 713 with evidence pointing to induced changes in DNA as being responsible for inhibition of cell division. Apart from difficulties in location of the site of primary damage, there is still controversy as to whether the majority of radiation effects are due directly to ionization or to the indirect action of the radiolysis products of water. Whatever the detailed mechanisms involved, much is already known both qualitatively and quantitatively in relation to the radiation inactivation of microbial populations, lethality being measured by the loss by cells of colony forming ability in nutrient media. Comparative radiation resistance In general, viruses are more resistant than bacterial spores, resistance increasing with decreasing particle size, and in turn spores are more resistant than vegetative organisms, yeasts and moulds. For comparative purposes it is convenient to refer to dose/survival curves such as those presented in Fig. 1 for several vegetative bacteria and spores. Surviving fraction (number of surviving organisms expressed as a fraction of the original number) is plotted on a logarithmic scale against dose on a linear scale. The reduction in numbers of viable organisms with increasing dose follows an exponential law, i.e. with increasing dose there is a decreasing probability that there will be a survivor in a given sample. This relationship holds for other sterilization agents as pointed out in a comparison of radia- tion and heat (2). An outstanding exception to the above general statement on comparative resistance between micro-organisms can be seen in Fig. 1. The curve for Micrococcus radiodurans produced from data of Krabbenhoft, Andersen and Elliker (3), shows that this organism is more resistant than the spores illustrated and requires a radiation dose for a 5 log cycle reduction in a population which is 30 times that required for Pseudomonas sp. Fortunately this resistant organism is non-pathogenic and unlikely to occur as a con- taminant in the commercial products of current interest. However, Christen- sen et al (1) have drawn attention to the existence of other gram positive organisms of high resistance and in particular to strains of Streptococcus faecium. There are also examples of yeasts of high inherent resistance (4). Erdman, Thatcher and MacQueen (5), examining the comparative resistance of specific bacteria of public health significance, concluded that in broth suspension Streptococcus faecalis was more resistant than the staphylococci, salmonellae, coliforms and Micrococcus tuberculosis. It is important to note that because of the particularly low resistance to radiation of the coliforms,

714 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS i i I I I O Pseudomonc•s sp. • S. typhimurium (• Strep. f(•ecium A21 (• B. pumi[us (• Ci. botu[inum type A (• M. r(•diodur(3ns o 10 -2 z cc 10- 10 0-25 0-5 0.75 1.0 1.25 1.5 1.75 DOSE (M r(•d) Figure 1. The difference in radiation sensitivity between bacteria illustrated by dose survival curves prepared after irradiation of the organisms in buffer suspension in the presence of air. their absence after treatment should not be used as a measure of effective elimination of other pathogens as is sometimes used to check heat pasteur- i zation. Influence of environ•nent It is particularly relevant to the subject of product sterilization to stress the considerable influence of environment during irradiation on the resist-