Survival of microorganisms, as measured by their retention of unlimited growth potential, may be influenced by post-irradiation treatments. Holding in suspension before plating (Roberts and Aldous, '49) ; incubation at suboptimal (Latarjet, '43; Stapleton et al., '56) or supraoptimal (Stein and Meutzner, '50; Anderson, '51; Buzzell, '56) temperatures; starvation (Alper and Gillies, '58a); and the use of metabolic inhibitors ( Alper and Gillies, '58b; Doudney and Haas, '58; Kimball, '58; and Gillies and Alper, '59) have been reported as lessening the inactivation response to irradiation. Alper and Gillies ('58b, '60) have suggested that a common feature of these procedures is their ability to slow down metabolism after irradiation, so preventing death due to "unbalanced growth." Thus, the greatest lethality from irradiation could be expected under conditions optimal for growth. Yet, Hollaender and Stapleton ('56), Stapleton et al. ('55), and Balduzzi ('57) have shown greater colony formation on a complex natural medium than on a synthetic basal medium.
The importance of growth in influencing recovery from ionizing radiation could best be evaluated by use of spores in which germination could be largely prevented for several days at near optimal temperatures for growth while maintaining an active metabolic rate. Work in this laboratory suggested that sporangiospores of the Phycomycetous fungus, Rhizopus stolonifer (Erh. ex Fr.) Vuillemin, approach this condition if held in a sufficiently concentrated suspension. Germination is effectively inhibited under apparently satisf actory conditions of temperature, moisture, and media. Unfortunately, the respiratory rate is also materially reduced in the concentrated suspension.