Comparative Resistance of Nonsporogenic Bacteria to Low-Temperature Gamma Irradiation

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A total of 36 microorganisms, comprising 19 species of 11 genera, were screened for radiation resistance with 60Co gamma rays at a radiation temperatore of -80 ± 2 C in phosphate buffer (pH 7.0) under vacuum. Micrococcus radiodurans was the most resistant organism. An initial population of 2.8 × 105 cells per dose of this species survived 2.4 but not 2.7 Mrad. Of the remaining 18 species with initial populations of about 106 cells per dose, Streptococcus faecium survived 0.9 to 1.5 Mrad, depending on the strain tested. S. faecalis QM survived 0.9 but not 1.2 Mrad. S. faecalis 1539 and Alcaligenes faecallis survived 0.6 but not 0.9 Mrad. Three species of Salmonella, one strain each of Escherichia coli, Streptococcus lactis, and Aerobacter aerogenes survived 0.3 but not 0.6 Mrad. The remaining 22 bacteria did not survive 0.3 Mrad, the lowest dose tested. Detailed survival curve determinations for four strains of S. faecium, the most resistant of the test bacteria of public health significance, indicated the following order of resistance at -80 C: α21 > θ12 = F6 > FEC. Each strain produced two exponential survival curves with different slopes, the breaks occurring at 0.3 to 0.5 Mrad. The D values (doses which reduce the microbial population by 90%) of the more resistant cell fractions were two- to three-fold higher than the more sensitive cell fraction. The resistance of strain α21 was determined at different radiation temperature (+5, -30, -80, -140, -196 C). The D value-radiation temperature relationship followed a quadratic equation. Computations of Ea and Q10 values (activation energy and temperature coefficient, respectively) showed a very small thermodynamic effect on radiation death. An Arrhenius evaluation of the temperature effect on cell kill indicated that there was no simple physicochemical mechanism which might explain the change in D value as a function of temperature.

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