Radiation-resistant micro-organisms isolated from textiles.

Towels from private homes and public offices and underwear contaminated by being used by employees at a public health laboratory were examined for occurrence of radiation-resistant bacteria and fungi. Three different methods were used for isolation of the most resistant organisms, one with multiplication of the microbial population prior to an irradiation used for selection, and two without this multiplication and with the organisms placed on membrane filters or in situ on the textiles, respectively. A total of 44 different strains were isolated. Differences in the three methods used for selection of the most radiation-resistant microorganisms were not reflected in the results. 16 pigment-producing Gram-positive cocci, tentatively classified as Micrococcus radiodurans, were the most radiation-resistant and were isolated in about half of the examinations. Other Gram-positive cocci, non-spore forming rods, some Nocardia and Candida parapsilosis strains and two Bacillus strains constituted the rest of the collection. Wtih few exceptions dose-response curves for the strains were upward convex. D-6 values were determined to be between 1.5 megarad for the most radiation sensitive, a Candida, and 5.7 megarad for the most resistant,, tentatively classified as M. radiodurans. The D-6 values for the Bacillus strains were in both cases 1.8 megarad, consistent with a D-value of 0.3 megarad. The same resistance is reported to be the maximum resistance for B pumilus, strain E601, commonly used as reference strain in the literature on radiation sterilization of medical devices and supplies.