The efficiency of ionizing radiation in detoxifying the lethal determinant(s) of the lipopolysaccharide (LPS) of Salmonella typhimurium, S. enteritidis, and Escherichia coli in aqueous solution and associated with heat-killed S. typhimurium cells in suspension decreased with doses above 1 Mrad. The 50% end point of inactivation was more than 7.0 Mrad for heat-killed salmonellae and 4.8, 4.5, and 1.0 Mrad for the LPS of S. typhimurium, S. enteritidis, and E. coli, respectively. After exposure to 20 Mrad, S. typhimurium LPS retained a small portion of its lethal properties although the LD50 was much greater than 9.5 mg per 20-g mouse. However, at-184 C, no inactivation of the lethal determinant(s) occurred after exposure to as much as 20 Mrad. This demonstrated the significance of the indirect effect and the mobility and formation of free radicals. At 22 C, the optical density at 400 mA increased and the pH decreased with increasing radiation dose, but no qualitative changes were observed in the infrared spectrum. No change was observed in the pyrogenicity of S. typhimurium LPS; a slight decrease in antigenicity was revealed when 6 days, but not when 1 day, elapsed between vaccination and challenge in the mouse protection test. The results were interpreted as evidence of the existence of two or more lethal and antigenic determinants. The differential effect of radiation on these properties and on the pyrogenic component(s) probably are indicative of separate functional sites for lethal, antigenic, and pyrogenic activities.
Studies on detection of bacteria by radiometric techniques have been concerned primarily with aerobic species in clinical specimens. The data presented here are related to detection of aerobic and anaerobic species that are of significance in foods, by measurement of "4CO2 evolved from the metabolism of 14Cglucose. Salmonella typhimurium and Staphylococcus aureus were inoculated into tryptic soy broth containing 0.0139 ,gCi of 14C glucose/ml of medium. Detection times ranged from 10 to 3 hr for inocula of 100 to 104 cells/ml of broth. Heat-shocked spores of Clostridium sporogenes or C. botulinum were incubated in tryptic soy broth supplemented with Thiotone and NaHCO,. The medium was rendered anaerobic with N2. Spores were detected when 0.0833 M1Ci of labeled glucose was available/ml of medium but not when 0.0139 1Ci of glucose was present/ml. The spores required 3 to 4 hr longer for detection than did comparable numbers of aerobic vegetative cells. The results demonstrate the importance of availability of sufficient label in the media and the potential of the application of this technique for sterility testing of foods.
Susceptibility to enteric infection with Salmonella was studied in mice housed at different temperatures. The oral doses of S. typhimurium SR-11 and RIA, which caused 50% mortality in animals at 10 C, were about J%0 and M00 of the respective values at 23 C, and that of an avirulent strain of S. enteritidis was also lower in the cold-exposed mice. The frequency of mortality due to Salmonella infection was essentially the same in mice exposed to 34 or 23 C. The divergent responses in the cold and heat probably stem from basic differences in the physiological changes mediated by the two extremes. The pattern and extent of change in weight and rectal temperature were the same among infected mice housed at 10 and 23 C and controls at 10 C, but differed from controls at 23 C. The incidence of Salmonella in samples of liver-spleen, lung, colon, blood, or feces was similar among infected mice housed at 10 and 23 C during a 14-day period of observation. The increased frequency of mortality in cold-exposed infected animals is not due to alterations in invasiveness of the bacteria nor to greater impairment of the thermoregulatory capacity of the mice. It may be attributable, in part at least, to a greater effectiveness of Salmonella toxins as metabolic poisons at low temperatures.
The radiation resistance (D, in megarads) of six strains of Salmonella irradiated at 4C in brain heart infusion suspension ranged from 0.042 for S. enteritidis to 0.084 for S. thompson. The resistance values were 0.048 for S. typhimurium, strain SR-11, 0.060 for S. typhimurium, strain RIA, and S. newport, and 0.078 for S. heidelberg. The mutation frequency to tetracycline resistance of S. typhimurium, strain SR-11, increased between 0 and 0.05 Mrad and declined thereafter with increasing radiation dose. After 0.5-Mrad exposure, the mutation frequency was essentially the same as in control populations. The mutation frequency to streptomycin resistance of S. typhimurium, strain SR-11, decreased at doses greater than 0.05 Mrad and increased only slightly for the more radiation-resistant serotypes, S. typhimurium, strain RIA, S. thompson, and S. heidelberg. The average mutation frequency of the four Salmonella cultures tested was essentially unchanged (less than 1 log difference) between 0.05 and 0.5 Mrad, while the difference in reduction in viable numbers was 5 logs or greater. The evidence presented indicates that with the proper choice of processing parameters, the application of radiation pasteurization could drastically reduce the possibility of transmission of Salmonella by poultry and thereby decrease the public health hazards associated with this microorganism.
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