Inactivation of the Radiation-resistant Spoilage Bacterium Micrococcus Radiodurans. I. Radiation Inactivation Rates in Three Meat Substrates and in Buffer.

A simplified technique permitting the pipetting of raw puréed meats for quantitative bacteriological study is described for use in determining survival of these non-sporing bacteria, which are exceptionally resistant to radiation. Survival curves, using gamma radiation as the sterilizing agent, were determined in raw beef with four strains of Micrococcus radiodurans. Survival curves of the R(1) strain in other meat substrates showed that survival was significantly greater in raw beef and raw chicken than in raw fish or in cooked beef. Resistance was lowest in the buffer. Cells grown in broth (an artificial growth medium) and resuspended in beef did not differ in resistance from cells that had been grown and irradiated in beef. Survival rate was statistically independent of the initial cell concentration, even though there appeared to be a correlation between lower death rate and lower initial cell concentrations. The initial viable count of this culture of the domesticated R(1) strain in beef was reduced by a factor of about 10(-5) by 3.0 megarad, and 4.0 megarad reduced the initial count by a factor of more than 10(-9). Data suggest that M. radiodurans R(1) is more resistant to radiation than spore-forming spoilage bacteria for which inactivation rates have been published.