Effects of continuous and interrupted radiation on microorganisms.

Various bacterial spores exhibited a wide range of radiation resistance to doses of 0.25 to 2.5 Mrad from a cobalt-60 radiation facility. Bacillus pumilus and Clos-tridium tetani were shown to have the highest degree of resistance when compared with other bacterial sporeformers. B. subtilis E163 was the least resistant of the bacterial spores studied. Dried spores contained on cellulose discs were more readily destroyed by gamma-rays than were wet spores under similar conditions. Mycobacterium tuberculosis was destroyed by radiation doses much lower than that required by the least resistant bacterial spores. Interrupted dosimetry tests performed with materials of various types showed that sutures and other similar materials were effectively sterilized when the total radiation dose was given in two separate exposures with periods of interruption of 1 to 19 days. When "agar dosimeters" were employed in similar interrupted dosimetry series, B. pumilus spores were recovered in a few tests after administration of a total combined dosage of 2.5 Mrad with interruption periods of 2 to 19 days. When the experiment was repeated with interruption for 14 days, no survivors were found after a total dose of 2.0 to 2.8 Mrad.