Gamma-Ray Sterilization and Residual Toxicity Studies of Ground Beef Inoculated with Spores of Clostridium botulinum

Inoculated packs of cooked and raw ground beef were sterilized with gamma radiation from cobalt-60. With inocula of 5,000,000 Clostridium botulinum 213B spores per g of cooked ground beef, 3.8 megarad were required for sterilization; in raw ground beef, 3.72 megarad sterilized the meat when inocula of 1,700,000 C. botulinum 213B spores were used per g. Using C. botulinum 62A spores, cooked ground beef inoculated with 5,200,000 spores per g was sterilized with 3.85 megarad; raw ground beef, inoculated with 2,670,000 spores per g, was sterilized with 3.6 megarad. Cans of meat that were considered sterile by lack of culture growth after incubation for at least 6 months and, in some instances, as long as 5 years, were tested for the presence of botulinus toxin. No toxin was found in any meat taken from inoculated packs prepared from C. botulinum 213B spores; however, all cans of meat that had been inoculated with more than 2,670,000 C. botulinum 62A spores per g of meat, contained type A toxin. It was shown that these latter inocula of heat-shocked spores, by themselves, contained sufficient toxin to kill mice. However, more toxin appeared to be present than could be ascribed to the unirradiated spores alone. This finding is discussed.     

Effects of irradiation on growth and toxigenicity of Clostridium botulinum types A and B inoculated onto chicken skins.

This study was conducted to examine the effects of 0.3-Mrad irradiation on growth and toxigenicity of Clostridium botulinum types A and B on chicken skins. Irradiation followed by aerobic or anaerobic incubation at 30 degrees C extended the shelf life of skin samples and delayed growth and toxin production by C. botulinum. During 2 weeks of incubation at 10 degrees C, the irradiated and nonirradiated C. botulinum spores failed to grow or produce toxin.     

Effects of irradiation on growth and toxigenicity of Clostridium botulinum types A and B inoculated onto chicken skins

This study was conducted to examine the effects of 0.3-Mrad irradiation on growth and toxigenicity of Clostridium botulinum types A and B on chicken skins. Irradiation followed by aerobic or anaerobic incubation at 30 degrees C extended the shelf life of skin samples and delayed growth and toxin production by C. botulinum. During 2 weeks of incubation at 10 degrees C, the irradiated and nonirradiated C. botulinum spores failed to grow or produce toxin.     

Radiation Injury of Clostridium botulinum Spores in Cured Meat

Cans of chopped ham, inoculated with spores of Clostridium botulinum strains 33A and 41B at levels of 2,500 and 250 per gram, were subjected to an enzyme-inactivating heat treatment and irradiation with 0.5, 1.5, 2.5, or 3.5 Mrad of Co(60). A portion of the pack was not irradiated, and received a commercial thermal process (F(0) = 0.2). Viable spores were enumerated after treatment and after 6 months of incubation at 30 to 37.7 C. Toxic spoilage occurred at 0 and 0.5, but not at 1.5, 2.5, or 3.5 Mrad. More spoilage and toxin formation occurred in the product irradiated at 0.5 Mrad than in identical product receiving no radiation treatment. Confirmed botulinal spores were isolated from all of the radiation variables of 2,500 per gram-inoculated product and from all but the 3.5 Mrad low-inoculum cans. However, neither growth nor toxin was observed in unspoiled product. The "injury" phenomenon previously described in thermally processed cured meats (survival of botulinal spores without capacity for multiplication or toxigenesis) apparently occurs also in irradiated cured meats.     

Production of ochratoxin A by Aspergillus ochraceus NRRL-3174 before and after exposures to 60Co irradiation.

Spores from the toxigenic organism Aspergillus ochraceus NRRL-3174 were exposed to specific levels of gamma irradiation and then allowed to germinate on selected media. Increases in ochratoxin A production by irradiated, compared to non-irradiated, spores were observed after inoculation of spores onto a cracked red wheat or into a synthetic liquid medium. Variations in daily ochratoxin production were also observed for control and irradiated spore-derived cultures developing on both media, with maximum toxin production varying from 7 to 11 days of incubation. The most notable increases in ochratoxin A production occurred from cultures developing from spores having been irradiated with 10, 25, or 50 krad. Exposures to 400 or 600 krad resulted in complete inhibition of spore germination and, consequently, no ochratoxin production. Of the two substrates used, wheat and synthetic, the quantities of ochratoxin A produced were significantly lower in the synthetic media than on the natural substrate. Higher and more rapid toxin production occurred from spores having been irradiated with 10, 25, 50, and 100 krad than occurred from the non-irradiated control spores when grown on synthetic media. Cultures derived from spores having been exposed to 10, 25, 50, and 100 krad produced significantly higher levels of ochratoxin A after 8 days of incubation on natural substrate than did the controls. Analysis of variance revealed that substrate, length of incubation, as well as irradiation levels all affected the time required to produce maximum levels of ochratoxin A.     

A Comparison of Toxin Production by Clostridium botulinum Type E in Irradiated and Unirradiated Vacuum Packed Fish

S ummary . The rates of toxin production at 10° by inocula of 10⁵, 10⁴, 10³ and 10² spores of Clostridium botulinum type E in vacuum packed herring, cod and haddock were compared with that of equivalent numbers of spores surviving γ-irradiation at 0.3 Mrad. There was little difference between the rates of toxin production in unirradiated and irradiated fish. More toxin was produced in irradiated herring than in unirradiated, but in haddock the reverse was obtained. In cod about equal amounts of toxin were found without trypsinization; after trypsinization toxin levels were generally higher in irradiated samples.     

Influence of postirradiation incubation temperature on recovery of radiation-injured Clostridium botulinum 62A spores.

The number of colonies formed by unirradiated Clostridium botulinum 62A spores was independent of temperature, in the range from 20 to 45 degrees C (in 5 degrees C increments); no colonies developed at 50 degrees C. Spores irradiated at 1.2 or 1.4 Mrads produced more macrocolonies at 40 degrees C than at higher or lower temperatures. Apparently, radiation-injured spores were capable of repair of 40 degrees C than at the other temperatures studied. More than 99% of the radiation (1.2 Mrads) survivors were injured and were unable to form macrocolonies in the presence of 5% NaCl. The germinated radiation-injured spores were also sensitive to dilution, resulting in the loss of viability of 77 to 79% of the radiation survivors. At 30 and 40 degrees C, the irradiated spores did not differ significantly in the extent of germination (greater than 99% at both 30 and 40 degrees C), emergence (64% at 30 degrees C and 67% at 40 degrees C), and the maximum number of emerged cells that started to elongate (69% at 30 degrees C and 79% at 40 degrees C). However, elongation was remarkably more extensive at 40 degrees C than at 30 degrees C. Many elongated cells lysed within 48 h at 30 degrees C, indicating an impaired repair mechanism. If the radiation-injured spores were incubated at 40 degrees C in the recovery (repair) medium for 8 to 10 h, they germinated, emerged, and elongated extensively and were capable of repair. If, after 8 to 10 h at 40 degrees C, these cultures were shifted to 30 degrees C, the recovery at 30 increased by more than eightfold, resulting in similar colony counts at 30 and 40 degrees C. Thus, repair appeared to be associated with outgrowth. Repair did not occur in the presence of chloramphenicol at 40 degrees C, whereas penicillin had no effect, suggesting that the repair involved protein synthesis but did not require multiplication.     

Effect of Sodium Chloride and pH on the Outgrowth of Spores of Type E Clostridium botulinum at Optimal and Suboptimal Temperatures

The sodium chloride inhibition of spore outgrowth of four strains of type E Clostridium bolulinum was determined in a Trypticase-peptone-glucose (TPG) medium. At 16, 21, and 30 C, spores of three strains required 5.0% and one strain 4.5% salt for complete inhibition during 1 year of incubation. At 8 and 10 C, spores of the four strains required 4.5% salt for definite inhibition. Salt concentrations slightly lower than those providing inhibition tended to extend spore outgrowth time at low temperatures. The minimal pH permitting outgrowth of type E spore inocula was affected by the concentration of reducing compound present in the system. When either 0.02% sodium thioglycolate or 0.05% L-cysteine hydrochloride was used, outgrowth at 30 and 8 C occurred at much lower pH levels than when 0.2% thioglycolate was added. At 30 C, spores of one strain showed outgrowth in TPG medium as low as pH 5.21 with an inoculum of 2 million spores per replicate tube. At a 10-fold higher inoculum, the same strain showed outgrowth at pH 5.03 in one of five replicate tubes. At 8 C, spore outgrowth of the four strains occurred at pH 5.9, but not at pH 5.7, in TPG medium containing L-cysteine hydrochloride.     

The influence of gamma radiation and substrate on mycotoxin production by Fusarium culmorum IMI 309344

Mycotoxin production (deoxynivalenol (DON), acetyl deoxynivalenol (A DON) and zearalenone) by Fusarium culmorum inoculated on to maize (heat sterilized, irradiation sterilized and non-sterile) and irradiated to 1 kGy or 3 kGy, or unirradiated, was investigated over a period of time. Lowest mycotoxin production was observed on non-sterile maize which may be due to the presence of a competitive microflora on non-sterile maize. In general, mycotoxin production was higher on heat-sterilized grain as compared to irradiation-sterilized maize. It was suggested that this pattern of mycotoxin production was possibly caused by changes in the grain brought about by autoclaving, which favoured mycotoxin production and possibly induced changes in irradiation-sterilized maize which inhibited mycotoxin production. On sterile maize, there was no significant difference in DON production by unirradiated, 1 kGy and 3 kGy irradiated cultures up to 56 d of incubation; between days 56 and 77 of incubation, DON production increased rapidly with largest increases occurring in irradiated (1 kGy and 3 kGy) cultures. On non-sterile grain, neither DON nor A DON were detected in unirradiated cultures of F. culmorum but were detected in cultures irradiated to 1 kGy and 3 kGy. In practice grain should be stored under conditions of temperature and moisture content which prevent fungal growth. However, in this study, the grain was stored under conditions that were approaching ideal for growth of the test organism. The results highlight that irradiation disinfestation of grain must be combined with good grain handling practices so that excessive mycotoxin production can be prevented during storage.     

Use of Immunofluorescence and Animal Tests to Detect Growth and Toxin Production by Clostridium botulinum Type E in Food

The appearance of Clostridium botulinum type E organisms and of toxin in experimentally inoculated packages of turkey roll was followed to study the time relationship between the presence of vegetative cells and the demonstration of toxin. The presence of vegetative cells was determined by immunofluorescence, and animal tests were used to assay toxin production. Growth initiated from detoxified spores of C. botulinum type E resulted in toxin formation within 24 hr. Presence of fluorescing vegetative cells and of toxin coincided from 1 to 14 days of incubation. Beginning with the next testing date, day 21, differences were observed. Toxin could be detected for a longer time than vegetative cells. Neither toxin nor organisms could be found after 56 days of incubation. The mouse lethal dose tests (MLD per gram of turkey roll) showed fluctuations in the amount of toxin present throughout the period of testing. Maximal amounts of toxin were present during the period when fluorescing organisms were also more numerous. The applications of immunofluorescence in the study and in the diagnosis of botulism is discussed.     

Factors affecting the production of eremofortin C and PR toxin in Penicillium roqueforti.

Eremofortin C (EC) and PR toxin are secondary metabolites of Penicillium roqueforti. Of 17 strains from the American Type Culture Collection that were studied for their ability to produce EC and PR toxin, 13 produced these metabolites. Toxin production by strains grown in solid media (10 cereals and 8 other agricultural products) was also investigated. Production of EC and PR toxin by fungi grown on cereals was greater than production of EC and PR toxin by fungi grown on legumes; fungi grown on corn produced the greatest amount of PR toxin. Addition of corn extracts to the culture medium greatly increased the production of EC and PR toxin in a coordinated manner, with no significant change in mycelial dry weight. The fungi produced the highest levels of EC and PR toxin at 20 to 24 degrees C depending on the strain. Toxin production was higher in stationary cultures than in cultures that were gently shaken at 120 rpm. The optimum pH for production of both EC and PR toxin was around pH 4.0. With regard to spore age, toxin levels did not change significantly when we used spores obtained from fungi that were grown at 24 degrees C for 3 up to 48 days.     

Factors affecting the production of eremofortin C and PR toxin in Penicillium roqueforti.

Eremofortin C (EC) and PR toxin are secondary metabolites of Penicillium roqueforti. Of 17 strains from the American Type Culture Collection that were studied for their ability to produce EC and PR toxin, 13 produced these metabolites. Toxin production by strains grown in solid media (10 cereals and 8 other agricultural products) was also investigated. Production of EC and PR toxin by fungi grown on cereals was greater than production of EC and PR toxin by fungi grown on legumes; fungi grown on corn produced the greatest amount of PR toxin. Addition of corn extracts to the culture medium greatly increased the production of EC and PR toxin in a coordinated manner, with no significant change in mycelial dry weight. The fungi produced the highest levels of EC and PR toxin at 20 to 24 degrees C depending on the strain. Toxin production was higher in stationary cultures than in cultures that were gently shaken at 120 rpm. The optimum pH for production of both EC and PR toxin was around pH 4.0. With regard to spore age, toxin levels did not change significantly when we used spores obtained from fungi that were grown at 24 degrees C for 3 up to 48 days.     

Suppression of Spoilage Rates of Bombay Duck (Harpodon nehereus) Homogenates by Gamma Radiation

The spoilage rates of unirradiated and irradiated (0.1 Mrad) Bombay duck (Harpodon nehereus) homogenates stored at 0°C, 10°C and 30°C in terms of organoleptic score (OS) total bacterial count (TBC), trimethylamine content (TMA), and total volatile basic nitrogen (TVBN) followed a linear response with increase in temperature. Unirradiated Bombay duck stored at 0°C were found to spoil faster than those irradiated at 0.1 Mrad; the spoilage rates derived for TBC, TMA, TVBN and OS were 0.42, 0.67, 1.93 and ?0.86 respectively for unirradiated samples as compared with corresponding values of 0.31, 0.55, 1.19 and ?0.19 for the irradiated samples. The spoilage rates of unirradiated Bombay duck stored at 10°C and 30°C measured as a function of OS, TMA and TVBN were consistently higher than their irradiated (0.1 Mrad) counterparts. In the case of irradiated samples, TBC was not a satisfactory index of spoilage.     

The action of caffeine on X-irradiated HeLa cells. IX. Hypothermic effects

Hypothermic enhancement of the lethal effect of 3.5 Gy of 220-kV X rays in the absence of caffeine as well as in its presence (4 mM) was examined at temperatures between 10 and 34 degrees C in monolayer cultures in the G1 phase of the cell cycle. Correction has been made for the toxicity of low temperatures, and of caffeine at low temperatures, by concomitantly measuring cell killing in unirradiated cells. In the absence of caffeine, incubation of irradiated cells for up to 34 h at temperatures in the range 15 to 30 degrees C (or possibly 34 degrees C) enhances killing compared to that observed at 38 degrees C; the amount of enhancement is about the same throughout this range, but is nil at 10 degrees C. The enhanced killing induced by caffeine at 38 degrees C decreases as the temperature is lowered to 15 degrees C; there is no enhancement at 10 degrees C. Less killing is manifested in the range 15 to 25 degrees C in the presence of caffeine than in its absence. Recovery (loss of sensitivity to caffeine) and fixation of potentially lethal damage were studied in late-S/G2-phase cells at reduced temperatures by delaying treatment with caffeine for increasing times after irradiation. As the temperature is progressively lowered to 20 degrees C, less recovery is manifested after 5 h of incubation; no recovery is detected in the range 10 to 20 degrees C. Despite extensive recovery at 34 degrees C, no fixation is observed at that (or any lower) temperature in G2-phase cells: the cells are able to recover essentially fully when returned to 38 degrees C. In addition, responses of unirradiated control series to incubation at low temperatures appear to differ from those reported by others for longer treatment times of different cell systems.     

Bystander apoptosis in human cells mediated by irradiated blood plasma

Following exposure to high doses of ionizing radiation, due to an accident or during radiotherapy, bystander signalling poses a potential hazard to unirradiated cells and tissues. This process can be mediated by factors circulating in blood plasma. Thus, we assessed the ability of plasma taken from in vitro irradiated human blood to produce a direct cytotoxic effect, by inducing apoptosis in primary human peripheral blood mononuclear cells (PBM), which mainly comprised G(0)-stage lymphocytes. Plasma was collected from healthy donors' blood irradiated in vitro to 0-40Gy acute γ-rays. Reporter PBM were separated from unirradiated blood with Histopaque and held in medium with the test plasma for 24h at 37°C. Additionally, plasma from in vitro irradiated and unirradiated blood was tested against PBM collected from blood given 4Gy. Apoptosis in reporter PBM was measured by the Annexin V test using flow cytometry. Plasma collected from unirradiated and irradiated blood did not produce any apoptotic response above the control level in unirradiated reporter PBM. Surprisingly, plasma from irradiated blood caused a dose-dependent reduction of apoptosis in irradiated reporter PBM. The yields of radiation-induced cell death in irradiated reporter PBM (after subtracting the respective values in unirradiated reporter PBM) were 22.2±1.8% in plasma-free cultures, 21.6±1.1% in cultures treated with plasma from unirradiated blood, 20.2±1.4% in cultures with plasma from blood given 2-4Gy and 16.7±3.2% in cultures with plasma from blood given 6-10Gy. These results suggested that irradiated blood plasma did not cause a radiation-induced bystander cell-killing effect. Instead, a reduction of apoptosis in irradiated reporter cells cultured with irradiated blood plasma has implications concerning oncogenic risk from mutated cells surviving after high dose in vivo irradiation (e.g. radiotherapy) and requires further study.     

Inhibition of gamma-ray dose-rate effects by D2O and inhibitors of poly(ADP-ribose) synthetase in cultured mammalian L5178Y cells

Effects of deuterium oxide (D2O) and 3-aminobenzamide, an inhibitor of poly(ADP-ribose) synthetase, on cell proliferation and survival were studied in cultured mammalian L5178Y cells under growing conditions and after acute and low-dose-rate irradiation at about 0.1 to 0.4 Gy/hr of gamma rays. Growth of irradiated and unirradiated cells was inhibited by 45% D2O but not by 3-aminobenzamide at 10 mM, except for treatments longer than 30 hr. The presence of these agents either alone or in combination during irradiation at low dose rates suppressed almost totally the decrease in cell killing due to the decrease in dose rate. The D2O did not inhibit the radiation-induced increase in poly(ADP-ribose) synthesis as measured by the incorporation of [14C]NAD into the acid insoluble fraction, contrary to 3-aminobenzamide. Among other inhibitors tested, theobromine and theophylline were found to be effective in eliminating the dose-rate effects of gamma rays. Possible mechanisms underlying the inhibition are discussed.     

Resistance of some common fungi to gamma irradiation

Ten species of fungi representing the genera Alternaria, Aspergillus, Caldosporium, Curvularia, Fusarium, and Penicillium were examined for their relative resistance to gamma irradiation from a 137Cs source. Inactivation doses for dematiaceous fungi in agar medium ranged from 0.6 to greater than 1.7 megarads, whereas those for moniliaceous fungi were less than 0.3 megarad. D10 values (the dose required to reduce the inoculum by 1 log) for Curvularia geniculata (greater than 0.29 megarad) exceeded those for control spores of Bacillus pumilus (0.15 megarad).     

Evaluation of a monoclonal antibody-based immunoassay for detecting type B Clostridium botulinum toxin produced in pure culture and an inoculated model cured meat system

A monoclonal antibody-based amplified ELISA method for detecting Clostridium botulinum type B toxin was evaluated for its ability to detect the toxin in the supernatant fluid of pure cultures and after growth from Cl. botulinum spores inoculated into pork slurries. Slurries containing NaCl (1.5-4.5% w/v) and polyphosphate (0.3% w/v) were either unheated or heated 80 degrees C/5 min followed by 70 degrees C/2 h before incubation at 15 degrees, 20 degrees or 27 degrees C. Presence of specific toxin was confirmed by mouse bioassay and results were compared with those of the amplified ELISA method. A total of 48 strains, consisting of 38 Cl. botulinum and 10 Cl. sporogenes (putrefactive anaerobes), and 140 slurry samples were tested. Cultures of eight out of nine strains of type B Cl botulinum and 73 of 101 slurry samples containing type B toxin were positive by ELISA; the remaining 28 slurry samples contained type B toxin at levels below or close to the detection limit (20 LD50/ml) of the type B ELISA. No false-positive reactions occurred with Cl. botulinum types A, C, D, E or F, or with the 10 strains of Cl. sporogenes. Toxin produced by one strain of Cl. botulinum type B (NCTC 3807) was not detected by this single monoclonal antibody-based amplified ELISA. With a mixture of two monoclonal antibodies, however, the toxin from NCTC 3807 could be detected without reducing the sensitivity of the ELISA.     

Comparison of the growth of Listeria monocytogenes in unirradiated and irradiated cook-chill roast beef and gravy at refrigeration temperatures

Specific growth rates of two strains of Listeria monocytogenes in unirradiated and irradiated (2 kGy) roast beef and gravy stored at 5° and 10°C were found to be similar. However, exponential growth of L. monocytogenes after irradiation was preceded by an extended lag period of 6–9 d at 5°C and 3–4 d at 10°C, compared with lag periods of 1–2 d and <0.1 d in unirradiated beef and gravy stored similarly.     

Evaluation of a monoclonal antibody-based immunoassay for detecting type B Clostridium botulinum toxin produced in pure culture and an inoculated model cured meat system

A monoclonal antibody-based amplified ELISA method for detecting Clostridium botulinum type B toxin was evaluated for its ability to detect the toxin in the supernatant fluid of pure cultures and after growth from Cl. botulinum spores inoculated into pork slurries. Slurries containing NaCl (1.5–4.5%w/v) and polyphosphate (0.3%w/v) were either unheated or heated 80°C/5 min followed by 70°C/2 h before incubation at 15°, 20° or 27°C. Presence of specific toxin was confirmed by mouse bioassay and results were compared with those of the amplified ELISA method. A total of 48 strains, consisting of 38 Cl. botulinum and 10 Cl. sporogenes (putrefactive anaerobes), and 140 slurry samples were tested. Cultures of eight out of nine strains of type B Cl. botulinum and 73 of 101 slurry samples containing type B toxin were positive by ELISA; the remaining 28 slurry samples contained type B toxin at levels below or close to the detection limit (20 LD₅₀/ml) of the type B ELISA. No falsepositive reactions occurred with Cl. botulinum types A, C, D, E or F, or with the 10 strains of Cl. sporogenes. Toxin produced by one strain of Cl. botulinum type B (NCTC 3807) was not detected by this single monoclonal antibody-based amplified ELISA. With a mixture of two monoclonal antibodies, however, the toxin from NCTC 3807 could be detected without reducing the sensitivity of the ELISA.