Effect of incremental doses of radiation on viability of the microbial population on synthetic operating room gowns

A total of 700 25-cm2 samples of surgical gown material were exposed to doses of cobalt-60 radiation of 0.0 to 0.6 Mrad in 0.1-Mrad increments. Pour plates were made, and the microbial colonies that arose were enumerated, isolated, and identified as to species. The death rate of the microbial population was calculated, and the mean D10 value of 0.269 Mrad was obtained. Analysis showed that the initial population on unirradiated material had been underestimated; when the counts obtained by homogenization of unirradiated material were substituted, a corrected mean D10 value of 0.249 Mrad was obtained. The isolates obtained were identified, and 70.7% were found to be Bacillus spp. with 12 different species identified, 16.2% were Micrococcus spp. with 6 different species identified, and 8.2% were fungi with 10 different species identified. Calculations were made for appropriate doses of radiation to sterilize gowns with this contaminating microbial population. These calculations gave an estimated dose of radiation of 1.98 to 1.81 Mrad to reduce the observed population to 0.001, a standard where 1 gown in 1,000 might contain a living organism. Comparison of the radiation resistance of this population with that of others reported in the literature showed good agreement.