Effect of temperature history on the growth of Listeria monocytogenes Scott A at refrigeration temperatures

The effect of pre-inoculation temperature on the subsequent growth of Listeria monocytogenes Scott A at 5 degrees C was examined in microbiological medium, UHT milk, canned dog food, and raw ground beef (untreated and irradiation-sterilized). In microbiological medium, the duration of the lag phase was decreased when aerobic and anaerobic cultures were initially grown at less than or equal to 28 and less than or equal to 13 degrees C, respectively. Subsequent exponential growth rates and maximum population densities of the 5 degrees C cultures were not affected by temperature history. Differences in lag phase durations were also observed when L. monocytogenes initially cultured at 19 and 37 degrees C were grown at 5 degrees C in UHT milk and some of the canned dog food varieties. Growth of L. monocytogenes was not observed in either untreated or irradiation-sterilized raw ground beef. While temperature history can affect the growth kinetics of L. monocytogenes at 5 degrees C, it did not account for the lack of growth in raw meat, suggesting that there is an inhibitory condition or component in ground beef that is lost upon cooking.

Cisplatin resistance and mechanism in a viral test system: SV40 isolates that resist inhibition by the antitumor drug have lost regulatory DNA

Isolates of SV40 that have enhanced ability to survive inhibition by the antitumor drug cisplatin were selected by serial drug challenge in vivo. These mutant viruses have acquired specific deletions within the repeated regulatory motif (GGGCGG)6 or GC box. This DNA element was shown previously to be a strong target of drug attack by cisplatin and other anticancer drugs in vitro and is an important viral and cellular DNA control sequence. Thus, drug resistance in this viral test system is dependent on the loss of important target DNA sequences. The results also indicate that drug efficacy may be related to the ability of certain anticancer drugs to attack regulatory DNA sequences containing strings of guanosines.

Programmed factor binding to simian virus 40 GC-box replication and transcription control sequences

Nuclear footprinting revealed a temporal program involving factor binding to the repetitive GC-box DNA elements present in the simian virus 40 regulatory region. This program specified ordered and directional binding to these tandem regulatory sequences in vivo during the late phase of infection. The program was interrupted by the DNA replication inhibitor aphidicolin or by inactivation of the viral replication factor simian virus 40 T antigen, suggesting a link between viral DNA replication and new factor binding. Measurements of DNA accumulation in viruses lacking either the distal or proximal halves of the GC-box region suggested that the region has a dual role in replication control. Overall, the data point to important relationships between DNA replication and factor binding to the GC-box DNA, a multifunctional regulatory region.

Characterization of the Aeromonas hydrophila group isolated from retail foods of animal origin

During a recent survey of retail fresh foods of animal origin (fish and seafood, raw milk, poultry, and red meats) for organisms of the Aeromonas hydrophila group, we isolated representative strains from the various foods. In this study, we sought to characterize these isolates for biochemical properties and virulence-associated factors and to compare the food isolates with clinical isolates. We identified all food and clinical isolates as A. hydrophila and found that all isolates were typical in their biochemical reactions. Examination of the isolates for various virulence-associated factors indicated that most food and clinical isolates were serum resistant, beta-hemolytic, cytotoxin positive (against Y1 adrenal cells), hemagglutinin positive, Congo red positive, elastase positive, and staphylolysin positive. Mouse 50% lethal doses were log10 8 to 9 CFU for most isolates. All isolates had biotypes identical to those of enterotoxin-positive strains. The public health significance of these organisms in foods is not known at present, although their widespread occurrence and ability to grow competitively in foods kept at 5 degrees C represents a potential hazard.

Comparison of lithium chloride-phenylethanol-moxalactam and modified Vogel Johnson agars for detection of Listeria spp. in retail-level meats, poultry, and seafood

The effectiveness of Modified Vogel Johnson agar and lithium chloride-phenylethanol-moxalactam agar for detection of Listeria spp. in foods was compared by using the media to analyze retail-level meat, poultry, and seafood both by direct plating and in conjunction with a three-tube most-probable-number enrichment. The most-probable-number protocol detected Listeria species, including Listeria monocytogenes, in a substantial portion of the fresh meat and seafood samples. In most instances the Listeria levels were less than 2 CFU/g, which precluded detection by direct plating. Modified Vogel Johnson agar performed as well as did lithium chloride-phenylethanol-moxalactam agar and was considerably easier to use because of its ability to differentiate Listeria spp. from other microorganisms.

Listeria methods development research at the Eastern Regional Research Center, U.S. Department of Agriculture

Listeria methods research at the U.S. Department of Agriculture, Eastern Regional Research Center, has concentrated on 2 areas during the past year. The first was development of techniques for assessing isolation methods for their ability to detect sublethally stressed cells. It appears that a number of widely used media do not accurately detect Listeria that have been injured by thermal processing or acidification. The second was development of improved plating media. One, modified Vogel-Johnson agar, shows promise; it is highly selective and quantitative, and eliminates the need to select colonies on the basis of a blue color when illuminated with reflected light.

Effect of miconazole on growth and aflatoxin production by Aspergillus parasiticus

At 5 microM, miconazole prevented the growth of Aspergillus parasiticus Speare in a number of media. Sensitivity to miconazole was increased approximately 10-fold in a medium containing glycerol. At sub-inhibitory concentrations, miconazole stimulated aflatoxin synthesis on media which normally support toxin formation. Miconazole inhibited respiration and altered mitochondrial ultrastructure, suggesting that miconazole inhibits growth and stimulates aflatoxin production by depressing mitochondrial activity.

Thermal Resistance of Aeromonas hydrophila

Thermal resistance of five strains of Aeromonas hydrophila (three clinical and two food isolates) was studied at 45 to 51°C in saline solution and raw milk. In addition, effects of growth temperature and growth phase on thermal resistance of the cells were also studied. Survivors after various heat treatments were plated on starch phenol red agar; colonies were counted after 24 h at 28°C. Cells heated at 48°C and 51°C exhibited a diphasic response and the data presented are from the initial and final linear phases. Data were expressed as D- and z-values. Most variables caused small but statistically significant changes in D-value of the initial linear phase. At 48°C, D-values for stationary phase cells heated in saline solution ranged from 3.49 to 6.64 min; for cells heated in raw milk, the D-values ranged from 3.20 to 6.23 min. At 48°C, D-values for log-phase cells heated in saline solution ranged from 2.23 to 3.73 min, and z-values ranged from 5.22 to 7.69°C. These results indicate that A. hydrophila should be killed by many of the heat treatments given foods during processing. The thermal resistance of A. hydrophila appears similar to that of other gram-negative bacteria associated with food.

Determination of glucose level needed to induce aflatoxin production in Aspergillus parasiticus

Previous studies have established that aflatoxin production is induced by high levels of glucose (or other compatible carbohydrates). However, the minimal amount of glucose needed to achieve this effect had not been determined. Aspergillus parasiticus was induced to produce aflatoxin when incubated for 18 h in the presence of greater than or equal to 0.1 M glucose before addition of cycloheximide. Toxin formation was not induced at any glucose concentration in the mycelia incubated for 8 h. These results suggest that aflatoxin synthesis would not be expected in substrate containing less than 0.1 M of a carbohydrate source.

Regulation of aflatoxin biosynthesis: assessment of the role of cellular energy status as a regulator of the induction of aflatoxin production

Possible relationships among cellular energy status and the induction and initiation of aflatoxin synthesis were studied by using replacement culture techniques in conjunction with aflatoxin-supporting and-nonsupporting media. Transcription and translation processes associated with the induction of aflatoxin synthesis occurred 3 to 6 and 6 to 10 h, respectively, after mycelia were transferred to glucose-containing media. From adenylate energy charge determinations and in situ 31P nuclear magnetic resonance analyses, a relationship between overall energy status and the induction or initiation of aflatoxin synthesis could not be identified; however, electron microscopic evaluations indicated that aflatoxin synthesis occurred in association with a glucose-mediated inactivation of mitochondria. The results suggest that aflatoxin synthesis is not regulated by the overall energy status of the fungal cell but may be controlled by the energy status of specific subcellular compartments.

Effect of NaCl, pH, temperature, and atmosphere on growth of Salmonella typhimurium in glucose-mineral salts medium

The interactions of pH (5.0, 6.0, and 7.0), temperature (19, 28, and 37 degrees C), and atmosphere (aerobic versus anaerobic) with NaCl (0, 1, 2, 3, 4, and 5%) on the growth of Salmonella typhimurium ATCC 14028 in defined glucose-mineral salts culture medium were evaluated. Response surface methodology was used to develop equations describing the response of S. typhimurium to environmental changes. The response to an increasing concentration of NaCl at any temperature tested was nonlinear. The maximum growth was predicted to occur at an NaCl concentration of 0.5%, a temperature of 19 degrees C, and an initial pH of 7.0 under aerobic growth conditions. The relative amounts of aerobic growth at 19 degrees C, pH 7.0, and NaCl concentrations of 0, 0.5, 1, 2, 3, 4, and 5% were predicted to be 99.2, 100.0, 98.8, 90.2, 73.5, 48.6, and 15.6%, respectively. Anaerobic growth conditions repressed the amount of growth relative to that under aerobic conditions, and the effects of NaCl and pH were additive at low salt concentrations; however, at higher salt levels anaerobiosis provided protection against the effects of NaCl.

Factor interactions at simian virus 40 GC-box promoter elements in intact nuclei

Primer extension footprinting was used to probe late simian virus 40 regulatory elements in intact infected cell nuclei. Specific protection was observed over the viral "GC-box" transcription elements. The participation of the bound templates in gene activation is addressed by quantitation that shows that their abundance greatly exceeds that of transcription complexes but is comparable to that of open chromatin.

Inhibition of Clostridium botulinum 62A by fumarates and maleates and relationship of activity to some physicochemical constants

A series of n-monoalkyl maleates and n-mono-, di-, and methyl n-alkyl fumarates were prepared, 18 esters of each, with R = CH3 to C18H37. Their activity against Clostridium botulinum was determined in culture medium. The n-monoalkyl maleates and fumarates possessed significant activity, particularly those esterified with higher C13 to C18 alcohols. Somewhat lower activity was exhibited by methyl n-alkyl fumarates, while symmetrical esters, di-n-alkyl fumarates, were almost inactive. An attempt was made to correlate the activity of n-monoalkyl maleates and fumarates with chain length, solubility in water, apparent dissociation constant (pKa'), and infrared and UV absorption frequencies. The active esters may have potential as preservatives in foods.

Enterotoxin A production in Staphylococcus aureus: inhibition by glucose

In this study, we investigated the relationship between carbohydrate metabolism and repression of staphylococcus enterotoxin A (SEA) in Staphylococcus aureus 196E and a pleiotrophic mutant derived from strain 196E. The mutant, designated at strain 196E-MA, lacked a functional phosphoenolpyruvate phosphotransferase system (PTS). The mutant produced acid, under aerobic conditions, from only glucose and glycerol. The parent strain contained an active PTS, and aerobically produced acid from a large number of carbohydrates. Prior growth in glucose led to repression of SEA synthesis in the parent strain; addition to the casamino acids enterotoxin production medium (CAS) led to more severe repression of toxin synthesis. The repression was not related to pH decreases produced by glucose metabolism. When S. aureus 196E was grown in the absence of glucose, there was inhibition of toxin production as glucose level was increased in CAS. The inhibition was related to pH decrease and was unlike the repression observed with glucose-grown strain 196E. The inhibition of SEA synthesis in mutant strain 196E-MA was approximately the same in cells grown with or without glucose and was pH related. Repression of SEA synthesis similar to that seen with glucose-grown S. aureus 196E could not be demonstrated in the mutant. In addition, glucose-grown S. aureus 196E neither synthesized beta-galactosidase nor showed respiratory activity with certain tricarboxylic acid (TCA) cycle compounds. Glucose-grown strain 196E-MA, however, did not show suppressed respiration of TCA cycle compounds; beta-galactosidase was not synthesized because the mutant lacked a functional PTS.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of Sugar and Low-Dose Irradiation on Toxin Production by Clostridium botulinum in Comminuted Bacon

Comminuted bacon, processed to contain target levels of 40 μg NaNO₂/g and 0, 0.25 or 0.75% sucrose or 0.75% glucose, was inoculated with a mixture of spores of 20 strains of Clostridium botulinum (400 spores per g) and was canned under vacuum. Portions were irradiated using ¹³⁷Cs at doses of 0, 0.19, 0.38, 0.75, 1.12 and 1.5 Mrad. Cans were incubated for 1, 2, 4 or 8 wk at 30°C. Some cans of nonirradiated bacon without or with 0.25% sucrose became toxic in 2 wk; with 0.75% sucrose, toxin production was delayed to 8 wk. Bacon irradiated at 0.75 Mrad, made with or without sucrose, became toxic in 2 to 4 wk, whereas most cans of bacon irradiated at 1.5 Mrad remained toxin-free for the 8-wk incubation period. A comparison of bacon made with 0.75% sucrose or glucose showed no difference between the sugars in the rates of toxin production by C. botulinum in irradiated cans of bacon. Irradiation at 0.19 Mrad increased the rate of toxin formation over nonirradiated bacon in sugar-containing (0.75%) bacon, but had no effect in sugar-free bacon. The pH of nonirradiated bacon containing 0.75% glucose or sucrose decreased from pH 6.12 and 6.11, respectively, to pH 5.63 and 5.67 after 8 wk of incubation at 30°C. The titratable acidity showed a concurrent increase. The pH and titratable acidity of bacon irradiated at 0.19 Mrad or higher showed no changes.

Effect of 2-deoxyglucose, alpha-methylglucoside, and glucosamine on aflatoxin production by Aspergillus parasiticus

The effects of 2-deoxyglucose (2-DOG), alpha-methylglucoside (alpha-MG), and glucosamine (GA) on aflatoxin production by Aspergillus parasiticus were studied using conidia-initiated and replacement cultures. In conidia-initiated, 2-DOG, alpha-MG, and GA supported varying amounts of growth when employed as sole carbon sources. In both conidia-initiated and replacement cultures, 2-DOG, but not alpha-MG nor GA, as sole carbon sources support toxin formation. None of the compounds inhibited aflatoxin production when used in combination with glucose. It appears that neither 2-DOG, alpha-MG, nor GA can be considered nonmetabolizable analogs of glucose in A. parasiticus.

Regulation of aflatoxin biosynthesis: effect of glucose on activities of various glycolytic enzymes

Catabolism of carbohydrates has been implicated in the regulation of aflatoxin synthesis. To characterize this effect further, the activities of various enzymes associated with glucose catabolism were determined in Aspergillus parasiticus organisms that were initially cultured in peptone-mineral salts medium and then transferred to glucose-mineral salts and peptone-mineral salts media. After an initial increase in activity, the levels of glucose 6-phosphate dehydrogenase, mannitol dehydrogenase, and malate dehydrogenase were lowered in the presence of glucose. Phosphofructokinase activity was greater in the peptone-grown mycelium, but fructose diphosphatase was largely unaffected by carbon source. Likewise, carbon source had relatively little effect on the activities of pyruvate kinase, malic enzyme, isocitrate-NADP dehydrogenase, and isocitrate-NAD dehydrogenase. The results suggest that glucose may, in part, regulate aflatoxin synthesis via a carbon catabolite repression of NADPH-generating and tricarboxylic acid cycle enzymes.

Caffeine inhibition of aflatoxin synthesis: probable site of action

Aflatoxin production by pregrown cultures of Aspergillus parasiticus was completely inhibited by incorporation of 2 mg of caffeine per ml into the medium. This was accompanied by a decrease in glucose utilization and an inhibition of oxygen uptake and carbon dioxide evolution. Enzyme analyses indicated no significant differences in specific activities on glucose-6-phosphate dehydrogenase, mannitol dehydrogenase, phosphofructokinase, fructose 1,6-diphosphatase, pyruvate kinase, or malate dehydrogenase. Glucose uptake kinetics indicated a linear dose-related inhibition of glucose uptake. It appears likely that caffeine inhibits aflatoxin synthesis by restricting the uptake of carbohydrates which are ultimately used by the mold to synthesize this family of mycotoxins.

Caffeine inhibition of aflatoxin production: mode of action

Evaluation of caffeine and a number of related methylxanthines indicated that the ability of the compound to inhibit growth and aflatoxin production by Aspergillus parasiticus is highly specific and does not involve an inhibition of cyclic AMP phosphodiesterase. Supplementation of the culture medium with purine bases, nucleosides, and nucleotides suggested that the inhibition of fungal growth could be partially overcome by adenine or guanine but that the purines had little effect on the inhibition of aflatoxin production. Likewise, increasing the levels of trace minerals did not overcome the inhibition of toxin production. Electron microscopic evaluation of caffeine-treated and -untreated cultures indicated that the compound produced observable changes in the ultrastructure of the fungus.

Effect of Food Environment on Staphylococcal Enterotoxin Synthesis: A Review

Effects of various nutritional and environmental factors on growth and enterotoxin synthesis by Staphylococcus aureus in model systems and foods are reviewed. Factors discussed include effects of inoculum size, competing microflora, gaseous atmosphere, carbon source, temperature, pH, sodium chloride, water activity, mineral ions and sublethal stress. Areas where additional research is needed are also discussed.

Detection of genotoxicity in fried bacon by the Salmonella/mammalian microsome mutagenicity assay

The potential for mutagen formation in fried bacon and the possible reduction or elimination of this hazard was examined in the Salmonella/mammalian microsome mutagenicity assay using Salmonella typhimurium strain TA98. Alkaline dichloromethane extracts were prepared from green pork bellies, commercial bacon (nitrite-treated and nitrite-free), and pilot-plant bacon (nitrite-free). When fried, all forms of bacon and the green belly samples gave positive mutagenic responses with the plate-incorporation technique. Unfried samples were not mutagenic. Aroclor-activated rat-liver S-9 fractions plus NADPH were essential to demonstrate a mutagenic response. When the frying temperature was held constant (171 degrees C) maximum mutagen formation was observed in samples fried for 6 min; when samples were fried for 6 min a mutagenic response which increased with temperature, in a linear manner, was observed at temperatures above 125 degrees C. Volatile nitrosamines were not detected in the bacon samples. The data indicate the generation of one or more mutagens in fried bacon and green pork belly, the levels of which can be reduced by decreasing heating temperature and/or time.

Effect of initial pH on aflatoxin production

The effect of initial pH on aflatoxin production by Aspergillus parasiticus NRRL 2999 was examined in a semisynthetic medium. Maximal growth, aflatoxin production, and aflatoxin production per unit of growth occurred at initial pH levels of 5.0, 6.0, and 7.0 respectively. Initial pH levels less than pH 6.0 favored production of the B toxins, whereas levels greater than pH 6.0 favored production of the G toxins.