Sewage samples collected from animal wastes and from effluents at an animal disease laboratory were inoculated with known numbers of pathogenic organisms and subjected to various doses of gamma radiation from a 60Co source. Surviving test organisms were quantitatively determined by selective and enrichment techniques. The experiment was modeled as a quantal assay in which probit analysis was applied to obtain D10 values. The D10 value represents the irradiating dose required to reduce the population by 90%. The D10 value ranged from 13.4 krad for Campylobacter fetus to 156.6 krad for Streptococcus faecalis in animal sewage. However, the D10 value for the laboratory effluent was generally lower. Based on the estimated D10 values, the rating of the test organisms in decreasing order of radiosensitivity appeared as follows: Brucella abortus, Campylobacter fetus subsp. fetus, Campylobacter jejuni, Campylobacter coli, Campylobacter laridis, Mycobacterium fortuitum, Aspergillus fumigatus, Salmonella muenster, Candida albicans, Clostridium difficile and Streptococcus faecalis. If the D5 and D1 values were utilized, this listing would be only slightly altered.
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