Effects of radiation pasteurization on Samonella. 3. Radiation lethality and the frequency of mutation to antibiotic resistance

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.

Effects of radiation pasteurization on Salmonella. II. Influence of repeated radiation-growth cycles on virulence and resistance to radiation and antibiotics

Populations derived from Salmonella typhimurium surviving 1, 5, or 10 exposures to 0.5 Mrad did not show any increase in resistance to antibiotic discs containing 5 μg of tetracycline, 5 μg of chloromycetin, or 50 units of polymyxin. The frequency of resistance to 3 μg of tetracycline, 10 units of ampicillin, 120 μg of chloromycetin, or 30 μg of streptomycin per milliliter of agar, did not increase among progeny derived from either of two strains of S. typhimurium surviving 5–10 exposures to 0.5 Mrad. So, too, the virulence and radiation resistance of three strains of S. typhimurium exposed to radiation-growth cycling remained essentially unchanged or decreased.A mixture which originally contained 10 different serotypes of Salmonella was cycled at 0.23 Mrad. After 10 exposures the frequency of resistance of the progeny to ampicillin, chloromycetin, and streptomycin increased 87-, 55- and 13-fold respectively. The LD50 was essentially the same, but the radiation resistance (D value) increased from 0.053 Mrad in controls to 0.074 Mrad.The evidence presented seems to indicate that mutants are attained more readily from Salmonella cultures recycled at 0.25 Mrad or less than from those recycled at 0.5 Mrad. The pathogenicity of either one was usually essentially unchanged or reduced.

Effects of radiation pasteurization on Salmonella. I. Parameters affecting survival an recovery from chicken

Growth of Salmonella typhimurium, strain RIA, in minimal broth supplemented with methionine, produced a population of cells that was more sensitive to 60Co irradiation than were those grown in brain heart infusion. The resistance values (D) were 0.056 and 0.066 Mrad, respectively. D was not significantly different when the bacteria were irradiated at 4 °C on fresh or frozen-thawed germ-free chicken, or on conventional chicken sterilized by autoclaving, or by exposure to 2 Mrad of ionizing radiation. It was generally higher when the cells were irradiated in the original growth medium than on chicken. D of five different strains of S. typhimurium irradiated at 4 °C on autoclaved chicken varied from 0.052 to 0.068 Mrad. The greatest increase in resistance was produced by decreasing the temperature during irradiation. S. typhimurium, RIA, attained a D value of 0.293 Mrad at −80 °C. The effect of the recovery media on survival is equivocal. In some instances nutrient and minimal agar seemed to provide slightly better recovery of strains SR-11 and RIA. Enrichment of complex media with yeast extract did not increase recovery. The results indicate that of those parameters studied, the most significant effects on resistance were due to the temperature during irradiation.

Detoxification of Salmonella typhimurium lipopolysaccharide by ionizing radiation

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 ld(50) 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 mmu 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.

The chemistry of pyrrolic compounds. VI. Chlorins and related compounds

A novel bromination of porphyrins is described in which a compound of the chlorin class is produced. Evidence is presented in support of structure (VIa) for this compound. Mesochlorin and deuterochlorin have been prepared as model compounds during this investigation, and it has been shown by N.M.R. spectroscopy that both these chlorins have been reduced in either rings A or B and not in ring D as occurs in the chlorophylls a and b.