RADIATION‐RESISTANCE OF MICRO‐ORGANISMS FROM AIR IN CLEAN PREMISES

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Abstract
A total of more than 20,000 colonies of bacteria and fungi collected by means of slit‐sampler in controlled environments for aseptic microbiological work were included in a screening test of the occurrence of micro‐organisms with high radiation‐resistance. A D‐6 value, the dose necessary to reduce the initial population of micro‐organisms with a factor of one million, of 1.8 megarads or more, was used as criterion. A total of 117 strains were isolated. No differences in the proportion of radiation‐resistant strains could be demonstrated between different laboratories or at various seasons of the year. In the lower half of the resistance spectrum for the 117 isolates with unusually high radiation‐resistance, half of the strains were spore‐forming. In the uppermost part of the spectrum, spores were not observed. Rods dominated the lower part of the resistance spectrum, cocci the upper part. A large proportion of the strains were pigment‐producing. 50 strains with D‐6 values of 3.2 megarads or higher, and 15 strains with D‐6 values of 4.5 megarads or higher, were found. In the upper part of the resistance spectrum the dose‐response curves for the organisms were all upward convex, when the organisms were irradiated in a dry state and glued together with small amounts of organic compounds. On the basis of the proportion of radiation‐resistant bacteria demonstrated and the assumptions that a medical device with an average total count of 20 contaminating organisms per product unit prior to irradiation should comply with an established norm (1:106), or a suggested norm (1:103), respectively, for radiation sterilized devices, the minimum dose necessary for routine sterilization was estimated as about 3.8 megarads and about 2.0 megarads, respectively. In parallel screening tests for heat‐resistant organisms, only one resistant, thermophilic strain was found among organisms equivalent to about 7,000 colony‐forming units as total counts on slit‐sampler plates, and no resistant mesophilic organisms were detected among more than 30,000 colonies from slit‐sampler plates.