During laboratory studies on noiisporeforming food spoilage microorganisms which are difficult to destroy by high level gamma radiation ( l ) , it was observed that such species also were resistant to ultraviolet light (UVL) irradiation. Their resistance when irradiated in phosphate buffer was greater than other nonsporeformers or the spores of Bacillus globigii when treated under identical conditions. The response of these microorganisms to ultraviolet light is similar in some respects to their response to gamma radiation. Mechanisms of resistance to UVL and to gamma rays may have some factors in common. It has been shown that both forms of radiation cause the formation of peroxides in the medium (5,7). If similar resistance mechanisms were indicated it was anticipated that by using UVL in place of gamma radiation perhaps some answers to problems associated with radiation sterilization could be obtained without the difficulties associated with the use of gamma rays. The ultimate objective in these experiments was to obtain information on the response of such microorganisms to irradiation under various physical and chemical environments. This paper describes the apparatus and techniques used, together with some completed studies on a UVL and gamma resistant micrococcus. MATERIALS AND METHODS Cultures subjected to irradiation included: a radiation resistant micrococcus, Micrococcits pyogenes var. aureus, Sarcina lutea ATCC 9341, Bacillus subtilis morphotype globigii ATCC 9372 and Escltrrichia coli FDA strain 198.Cultures to be irradiated were grown at 32' C. on a rotary shaker in 50 ml. quantities of tryptone-glucose-yeast extract (TGY) broth contained in 8 02. prescription bottles. A 2% inoculum from a 24 hour old cdture was used. Twenty-four hour cultures (except where stated) were centrifuged and resuspended in 0.05 M phosphate buffer. This resuspended cell stock on adjustment to 90% transmittance at 550 mp with a Bausch and Lomb Spectronic-20 spectrophotometer, gave a viable plate count of 2-5 X 1V per ml. A 25 ml. aliquot of this suspension was placed in a 1 X 8 in. tube of UVL-transmitting glass (Vycor 7910) and irradiated in a specially constructed tank.The irradiation tank (Figure 1) consists of a 3-chambered plastic box with a
of the radiation-resistant spoilage bacterium Micrococcus radiodurans. II. Radiation inactivation rates as influenced by menstruum temperature, preirradiation heat treatment, and certain reducing agents. Appl. Microbiol. 11:413-417. 1963.-The R1 strain of Micrococcus radiodurwans, previously determined to be more resistant than three other strains exposed to gamma radiation, was studied further to determiine the influence of certain environmental factors on resistance to radiation inactivation. The frozen state offered insignificant protection to the organisms irradiated in raw pureed beef. Resistance was reduced by higher menstruum temperatures (40 and 50 C) during irradiation. Preirradiation heat treatment was found to lower resistance to subsequent irradiation. When the cells were irradiated in buffer at pH 5, 7, or 9, no differences in resistance were noted. Cell suspensions in buffer were protected to some extent by cysteine but not by thioglycolate. Ascorbate enhanced radiation inactivation. Earlier papers have described the radiation-resistant spoilage organism MIicrococcus radiodurans (Anderson et al., 1956; Anderson, Rash, and Elliker, 1961; Duggan et al., 1959; Raj et al., 1960). The R1 strain of this organism, which has been shown to be more resistant than the other cultures examined (Duggan, Anderson, and Elliker, ' This paper reports research undertaken in cooperation with the Quartermaster Food and Container Institute for the Armed Forces, Q. M. Research and Engineering Command, U.S. Army, and has been assigned no. 2239 in a series of papers approved for publication. The views or conclusions contained in this report are those of the authors. They are not to be construed as necessarily reflecting the views or endorsements of the Department of Defense. 2 Technical Paper no. 1598, Oregon Agricultural Experiment Station; contribution of the Department of Microbiology. I Presented at the 62nd Annual Meeting of the American Society for Microbiology, Kansas City, Mo., 8 May 1962.
SUMMARY The thermal resistance of Micrococcus radiodurans R1, a spoilage bacterium highly resistant to ionizing radiation, was characterized. A modified “thermal‐death‐time tube” was used, with cells uniformly suspended in a raw meat puree reconstituted from freeze‐dried and powdered beef that had been screened to remove pipette‐plugging fibers and irradiated to eliminate viable aerobic organisms before use. Thermal death rate, unlike the radiation death rate, seemed to approximate an exponential form, as indicated by survival curves. A D1A0= 0.75 and z= 10.65 describes the heat resistance in beef. Simple calculations suggested that if all parts of the samples of beef reached 150° F, the lower level of the “medium‐done” range, a 1‐min hold at that temperature should reduce viable nymbers by a factor of 10−10 and a 2‐min hold should reduce numbers by a factor of 10−20.
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