Starch-Ampicillin Agar for the Quantitative Detection of Aeromonas hydrophila

Interest in Aeromonas hydrophila as a food-borne and human pathogen is increasing. Isolation media from the clinical laboratory were evaluated for food use and either did not give quantitative recovery of A. hydrophila or did not permit ready differentiation of A. hydrophila from the background microflora. A new medium was developed which permitted quantitative recovery of A. hydrophila from foods. The medium consisted of phenol red agar base (Difco Laboratories), soluble starch (10 g/liter), and ampicillin (10 mg/liter). All foods surveyed contained A. hydrophila. Foods sampled included red meats, chicken, raw milk, and seafood (fish, shrimp, scallops, crab, and oysters). The count of A. hydrophila at the time of purchase ranged from 1 x 10/g (lower limit of detection) to 5 x 10/g. In most instances, the count of A. hydrophila increased during 1 week of storage at 5 degrees C. The starch-ampicillin agar developed permitted rapid quantitative recovery of A. hydrophila from foods in the presence of very large numbers of competing microflora.

Determining the microbiological criteria for lot rejection from the performance objective or food safety objective

The Microbiological Criteria (MC) is a set of parameters used to determine whether a specific lot of food is acceptable or not. These parameters are the microbial test protocol and its sensitivity, the confidence level that an unacceptable lot will be detected, the number of samples to be taken and the number of positive samples that are allowed before rejecting the lot. Determining the microbiological criteria begins with knowledge of the distribution of contamination from samples within a lot, particularly within a lot that is just at the unacceptable level of the microbial hazard. The just unacceptable lot can be defined by the Food Safety Objective (FSO) or Performance Objectives (PO), the small fraction of samples that can exceed these values and the standard deviation of the samples from the lot. With this information, a microbial test protocol is chosen to have a sensitivity level that would detect between approximately 15% and 45% of the samples. A confidence level for the MC and the number of positive samples that would be acceptable (c value which is usually zero) are also chosen. With this information the number of samples (n) required can be calculated. A critical factor in setting the microbiological criteria is the sensitivity of the microbiological test (m value). The sample size (weight) and sampling procedure can affect the standard deviation of the samples, particularly foods with non-homogeneous distribution and low numbers of microorganisms. Sampling, sample preparation and analytical procedures that reduce the variation between the samples will affect the choice of m value and maximum lot mean that meets the MC.

Microbial risk assessment: dose-response relations and risk characterization

Characterizations of the risks associated with foodborne pathogens are dependent on the availability of information on the population's exposure to the biological agents. However, by itself, exposure data are insufficient to assess the public health impact of pathogenic microorganisms. This requires the availability of effective dose-response models. Successful development of models that describe dose-response relations for enteric pathogens is dependent on a sound understanding of the mechanisms of pathogenicity associated with individual pathogens. This includes knowledge of how the various pathogen, host, and food matrix factors influence pathogenicity. Currently, a group of sigmoidal mathematical equations are used to empirically describe dose-response relations. While these have proven to be highly useful, advances in microbial food safety risk assessment will likely require the development of mechanistic models that more effectively consider the range of factors that influence the frequency and severity of foodborne infections in a population.

Acquisition of microbiological data to enhance food safety

The routine acquisition and archiving of microbiological data is undertaken for two reasons. The first is the development of historical microbiological profiles of foods, ingredients, or processes in order to determine or verify that microorganisms of concern are being controlled to the level desired. The second reason is data concerning the pathogenicity or virulence of foodborne pathogens and their behavior in foods in order to develop strategies and criteria for assuring microbiological safety. Both types of microbiological data are essential to effective food safety programs. A firm understanding of the uses and limitations of both is essential to correct acquisition, interpretation, and use of such data.

Contamination of intact apples after immersion in an aqueous environment containing Escherichia coli O157:H7

The extent and location of Escherichia coli O157:H7 contamination after intact apples were immersed in cold (2 degrees C) 1% peptone water containing approximately 3x10(7) CFU/ml was assessed using four apple varieties, Golden Delicious, McIntosh, Red Delicious, and Braeburn. Room temperature and refrigerated apples were used to determine the effect of temperature differential on E. coli infiltration. The highest levels of E. coli were associated with the outer core region of the apple, followed by the skin. Apples were subsequently treated by immersing them for 1 min in 2,000 mg/liter sodium hypochlorite, followed by a 1-min tapwater rinse. This treatment reduced pathogen levels by 1- to 3-log cycles but did not eliminate the microorganism, particularly from the outer core region. While E. coli was not detected in the inner core of most apples, warm fruit immersed in cold peptone water occasionally internalized the pathogen. The frequency and extent of internalization of the pathogen was less when cold apples were immersed in cold peptone water. Subsequent dye uptake studies with Golden Delicious apples indicated that approximately 6% of warm apples immersed into a cold dye solution accumulated dye via open channels leading from the blossom end into the core region. However, dye uptake did not occur when the dye solution was warmer than the apple.

pH-dependent stationary-phase acid resistance response of enterohemorrhagic Escherichia coli in the presence of various acidulants

The effect of acidulant identity on the pH-dependent stationary-phase acid resistance response of enterohemorrhagic Escherichia coli was studied. Nine strains of E. coli (seven O157:H7, one O111:H-, and one biotype 1 reference strain) were cultured individually for 18 h at 37 degrees C in tryptic soy broth (TSB) plus 1% dextrose and in TSB without dextrose to yield acid resistance induced and noninduced stationary-phase cells, respectively. These cultures were then inoculated into brain heart infusion broth (BHI) supplemented with 0.5% citric, malic, lactic, or acetic acid and adjusted to pH 3.0 with HCl. The BHI tubes were incubated at 37 degrees C for up to 7 h and samples were removed after 0, 2, 5, and 7 h and plated for counting CFU on BHI agar and MacConkey agar (MA). The results were compared to data previously obtained with HCl only. Acid resistance varied substantially among the isolates, being dependent on the strain, the acidulant, and the induction of pH-dependent acid resistance. Hydrochloric acid was consistently the least damaging to cells; lactic acid was the most detrimental. The relative activity of the other acids was strain dependent. Inducing pH-dependent acid resistance increased the already substantial acid tolerance of stationary-phase E. coli. The extent of injury also varied with acid and strain, with as much as a 5-log-cycle differential between BHI agar and MA CFU counts. The accurate determination of the survival of enterohemorrhagic E. coli in acidic foods must take into account the biological variability of the microorganism with respect to its acid resistance and its ability to enhance survival through the induction of physiological stress responses.

Effects of pH and acid resistance on the radiation resistance of enterohemorrhagic Escherichia coli

The effects of pH and the induction of pH-dependent stationary-phase acid resistance on the radiation resistance of Escherichia coli were determined for seven enterohemorrhagic strains and one nonenterohemorrhagic strain. The isolates were grown in acidogenic or nonacidogenic media to pH levels of approximately 4.7 and 7.2, respectively. The cells were then transferred to brain heart infusion (BHI) broth adjusted to pH 4.0, 4.5, 5.0, and 5.5 (with HCl) that was preequilibrated to 2 degrees C, and cultures were then irradiated using a 137Cs source. Surviving cells and the extent of injury were determined by plating on BHI and MacConkey agars both immediately after irradiation and after subsequent storage at 2 degrees C for 7 days. Decreasing the pH of the BHI in which E. coli was irradiated had relatively little effect on the microorganism's radiation resistance. Substantial differences in radiation resistance were noted among strains, and induction of acid resistance consistently increased radiation resistance. Comparison of E. coli levels immediately after irradiation and after 7 days of refrigerated storage suggested that irradiation enhanced pH-mediated inactivation of the pathogen. These results demonstrate that prior growth under conditions that induce a pH-dependent stationary phase cross-protects E. coli against radiation inactivation and must be taken into account when determining the microorganism's irradiation D value.

Fate of Escherichia coli O157:H7 on fresh-cut apple tissue and its potential for transmission by fruit flies

Pathogenic Escherichia coli O157:H7, as well as nonpathogenic strains ATCC 11775 and ATCC 23716, grew exponentially in wounds on Golden Delicious apple fruit. The exponential growth occurred over a longer time period on fruit inoculated with a lower concentration of the bacterium than on fruit inoculated with a higher concentration. The bacterium reached the maximum population supported in the wounds regardless of the initial inoculum concentrations. Populations of E. coli O157:H7 in various concentrations of sterilized apple juice and unsterilized cider declined over time and declined more quickly in diluted juice and cider. The decline was greater in the unsterilized cider than in juice, which may have resulted from the interaction of E. coli O157:H7 with natural populations of yeasts that increased with time. Experiments on the transmission of E. coli by fruit flies, collected from a compost pile of decaying apples and peaches, were conducted with strain F-11775, a fluorescent transformant of nonpathogenic E. coli ATCC 11775. Fruit flies were easily contaminated externally and internally with E. coli F-11775 after contact with the bacterium source. The flies transmitted this bacterium to uncontaminated apple wounds, resulting in a high incidence of contaminated wounds. Populations of the bacterium in apple wounds increased significantly during the first 48 h after transmission. Further studies under commercial conditions are necessary to confirm these findings.

Inactivation of Escherichia coli O157:H7 in apple juice by irradiation

Three strains (932, Ent-C9490, and SEA13B88) of Escherichia coli O157:H7 were used to determine the effectiveness of low-dose gamma irradiation for eliminating E. coli O157:H7 from apple juice or cider and to characterize the effect of inducing pH-dependent, stationary-phase acid resistance on radiation resistance. The strains were grown in tryptic soy broth with or without 1% dextrose for 18 h to produce cells that were or were not induced to pH-dependent stationary-phase acid resistance. The bacteria were then transferred to clarified apple juice and irradiated at 2 degrees C with a cesium-137 irradiator. Non-acid-adapted cells had radiation D values (radiation doses needed to decrease a microbial population by 90%) ranging from 0.12 to 0.21 kGy. D values increased to 0.22 to 0.31 kGy for acid-adapted cells. When acid-adapted SEA13B88 cells were tested in five apple juice brands having different levels of suspended solids (absorbances ranging from 0.04 to 2.01 at 550 nm), radiation resistance increased with increasing levels of suspended solids, with D values ranging from 0.26 to 0.35 kGy. Based on these results, a dose of 1.8 kGy should be sufficient to achieve the 5D inactivation of E. coli recommended by the National Advisory Committee for Microbiological Criteria for Foods.

Risk assessment: a means for linking HACCP plans and public health

HACCP plan adoption has greatly enhanced the food industry's ability to systematically design programs to ensure the microbiological safety of foods. Yet, this widening acceptance of the HACCP system has revealed several areas where its application is limited due to reliance on qualitative consideration of hazards and their control. In particular, HACCP planning is limited both conceptually and practically by its inability to quantify the potential combined influence of multiple control-point deviations and to relate the successful operation of a HACCP system to a measurable public-health impact. Recent advances in quantitative microbiological risk assessment appear to offer a means of overcoming these limitations. The integration of HACCP plans with the development of dynamic risk-assessment models offers a means for considering the entire farm-to-table continuum and for relating food-manufacturing operations to public health goals. Such capabilities may be critical to establishing equivalence among HACCP systems.

Development of a quantitative risk assessment model for Salmonella enteritidis in pasteurized liquid eggs

The performance of hazard analyses and the establishment of critical limits by the food industry are both hampered by the inability to directly relate food processing operations from farm-to-table with their public health impact. Using a 'unit operations' and stochastic simulation approach, data on the frequency of pathogens in raw ingredients, predictive microbiology models for growth and inactivation (thermal and non-thermal), and dose-response models for infectivity were integrated to create a quantitative risk assessment model for a Salmonella enteritidis infection from thermally processed liquid whole eggs made into mayonnaise in the home. The risk assessment indicated pasteurization provides sufficient consumer protection from a high incidence of infected birds and from temperature abuse between the farm and the egg breakers. However scenarios showed how inadequate pasteurization temperatures and/or temperature abuse during storage leads to a hazardous product. This dynamic approach to modeling risk should aid in identification and setting critical control points and assessing the impact of altering food formulations or processes.

Expanded models for the non-thermal inactivation of Listeria monocytogenes

Previously developed four-variable response surface models for describing the effects of temperature, pH/lactic acid, sodium chloride and sodium nitrite on the time to achieve a 4-log, non-thermal inactivation (t4D) of Listeria monocytogenes in aerobic, acidic environments were expanded to five-variable models that distinguish the effects of pH and acidulant concentration. A total of 18 new variable combinations were evaluated and the inactivation kinetics data appended onto a consolidation of two data sets from earlier studies. The consolidated data set, which included 315 inactivation curves representing 209 unique combinations of the five variables, was analysed by response surface analysis. The quadratic model without backward elimination regression was selected for further evaluation. Three additional quadratic models were generated using the concentrations of undissociated lactic and/or nitrous acids as variables in place of percentage lactic acid and sodium nitrite concentration. Comparison of predicted t4D values against literature values for various food systems indicated that the models provide reasonable initial estimates of the inactivation of L. monocytogenes. The models based on the concentration of undissociated lactic and nitrous acids support the hypothesis that antimicrobial activity is associated with this form of the compounds. Evaluation of several examples suggests that these models may be useful for predicting the equivalent of the compounds' "minimal inhibitory concentrations' for accelerating inactivation under various conditions.

Identifying and controlling emerging foodborne pathogens: research needs

Systems for managing the risks associated with foodborne pathogens are based on detailed knowledge of the microorganisms and the foods with which they are associated--known hazards. An emerging pathogen, however, is an unknown hazard; therefore, to control it, key data must be acquired to convert the pathogen from an unknown to a known hazard. The types of information required are similar despite the identity of the new agent. The key to rapid control is rapid mobilization of research capabilities targeted at addressing critical knowledge gaps. In addition, longer-term research is needed to improve our ability to respond quickly to new microbial threats and help us become more proactive at anticipating and preventing emergence. The type of contingency planning used by the military in anticipating new threats serves as a useful framework for planning for new emergence.

Culturing enterohemorrhagic Escherichia coli in the presence and absence of glucose as a simple means of evaluating the acid tolerance of stationary-phase cells

Prior growth of seven enterohemorrhagic and one nonenterohemorrhagic strains of Escherichia coli in tryptic soy broth with (TSB+G) and without (TSB-G) 1% glucose was evaluated for its effect on acid tolerance. The final pHs of 18-h TSB+G and TSB-G cultures were 4.6 to 5.2 and 6.9 to 7.0, respectively. Cells were then transferred to brain heart infusion broth adjusted to pH 2.5 or 3.0 with HCl, incubated at 37 degrees C for up to 7 h, and assayed periodically for viable populations with brain heart infusion and MacConkey agars. All enterohemorrhagic strains were acid resistant (< 0.5 log decline after 7 h) when initially cultured in TSB+G, but substantial differences in acid tolerance were observed among strains cultured in TSB-G (log declines ranged from < 0.3 to > 3.8). The results indicated that prior growth in a medium with and without a fermentable carbohydrate is a convenient way to studying the induction of acid tolerance, that acid inactivation is preceded by a period of acid injury, and that pH-independent and pH-dependent stationary-phase acid tolerance phenotypes may exist among strains of enterohemorrhagic E. coli.

Expanded response surface model for predicting the effects of temperatures, pH, sodium chloride contents and sodium nitrite concentrations on the growth rate of Yersinia enterocolitica

The previously reported data set for the low temperature (5, 12 and 19 degrees C) of Yersinia enterocolitica was expanded to include higher abusive temperature (28, 37 and 42 degrees C). In addition to temperature, the data set included the effects and interactions of pH (4.5-8.5), sodium chloride (0.5-5%) and sodium nitrite (0-200 micrograms ml-1) on the aerobic growth of Y. enterocolitica in brain heart infusion broth. Growth curves were modeled by fitting viable count data to the Gompertz equation. Quadratic models of natural logarithm transformations of the Gompertz B and M values and the derived values for lag phase durations and generation times were obtained using response surface analyses. Predictions based on the models for B and M values were comparable to predictions based on the derived values. These revised models provide an expanded means for rapidly estimating how the bacterium is likely to respond to any combination of the four variables within the specified ranges.

Model for the combined effects of temperature, initial pH, sodium chloride and sodium nitrite concentrations on anaerobic growth of Shigella flexneri

A fractional factorial design was used to measure the effects and interactions of temperature (12-37 degrees C), initial pH (5.5-7.5), NaCl (0.5-4.0%) and NaNO2 (0-1000 ppm) on the anaerobic growth kinetics of Shigella flexneri in Brain-Heart Infusion broth. Anaerobic conditions were established by flushing the culture flasks with N2. A total of 375 cultures representing 124 variable combinations were analyzed, with growth curves being generated using the Gompertz equation. Growth rates decreased with decreasing temperature, decreasing pH and increasing NaCl level. NaNO2 in combination with low temperature, low pH and high NaCl content effectively inhibited S. flexneri. Response surface analysis was used to obtain models for estimating the growth of S. flexneri in terms of temperature, initial pH, and NaCl and NaNO2 concentrations. A third-order equation using the natural logarithm transformations for the Gompertz B and M terms gave reasonable estimates of bacterial growth in response to any combination of the variables studied within the specified ranges.

Comparison of the Tecra VIA kit, Oxoid BCET-RPLA kit and CHO cell culture assay for the detection of Bacillus cereus diarrhoeal enterotoxin

Two commercial serological kits (Oxoid BCET-RPLA and Tecra VIA) and a Chinese hamster ovary (CHO) cell cytotonicity assay for the detection of Bacillus cereus diarrhoeal enterotoxin were compared. Eleven B. cereus strains and one enterotoxigenic B. thuringiensis strain were evaluated. Both kits and the CHO cell assay yielded positive toxin responses for cell-free culture filtrates from eight out of 11 diarrhoeal enterotoxigenic strains. An emetic enterotoxin producing strain was negative with all three assays. Two B. cereus strains were negative using the BCET-RPLA kit, but positive with the Tecra VIA kit and CHO cell assay. The BCET-RPLA indicated significant levels of enterotoxin after samples were boiled, whereas the CHO cell and Tecra assays were negative. Overall, the cell culture assay was the most sensitive. However, the Tecra VIA kit provided similar results and was better suited for the rapid detection of B. cereus diarrhoeal enterotoxin.

Expansion of response surface models for the growth of Escherichia coli O157:H7 to include sodium nitrite as a variable

The previously published (Buchanan et al., 1993a) response surface models for estimating the aerobic and anaerobic growth of Escherichia coli O157:H7 as a function of temperature, initial pH, and sodium chloride content have been expanded to include sodium nitrite as a further variable. A fractional factorial design was employed to quantitate the effect of NaNO2 in conjunction with the four other variables by culturing a three-strain mixture in brain heart infusion broth. The activity of NaNO2 was strongly pH-dependent, with inhibition being significant at pH values < or = 5.5 and enhanced by lowering the incubation temperature. The effects of the variables on Escherichia coli O157:H7 growth kinetics were modeled by response surface analysis using quadratic and cubic polynomial models of the natural logarithm transformation of both the Gompertz B and M parameters (Gompertz parameters) and the lag phase duration (LPD) and generation time (GT) values (kinetics parameters) calculated for individual growth curves. All models provided reasonable estimates for most variable combinations; however, comparisons of predicted versus observed values indicated that overall the most useful models were the cubic models based on LPD and GT values. Although additional validation of the models is required, comparisons of predicted times to a 1000-fold increase in population density against those calculated from previously published growth studies indicate that the models are an effective means for acquiring 'first estimates' of the growth characteristics of E. coli O157:H7.

Response surface model of the effect of pH, sodium chloride and sodium nitrite on growth of Yersinia enterocolitica at low temperatures

A fractional factorial design was used to measure the effects and interactions of temperature (5, 12, 19 degrees C), pH (4.5-8.5), sodium chloride (0.5-5%) and sodium nitrite (0-200 micrograms/ml) on the aerobic growth of Y. enterocolitica in brain heart infusion broth. Growth curves were modeled by fitting plate count data to the Gompertz equation. Quadratic models of natural logarithm transformations of the Gompertz B and M values and the derived values for lag phase durations and generation times were obtained using response surface analysis. Predictions based on the models for B and M values were comparable to predictions based on the derived values. These models provide a means for rapidly estimating how the bacterium is likely to respond to any combination of the four variables within the specified ranges.

Virulence of an Escherichia coli O157:H7 sorbitol-positive mutant

Virulence and pathogenicity of an Escherichia coli O157:H7 sorbitol-positive mutant were investigated with an infant rabbit animal model as well as a battery of in vitro assays. Total cell lysate protein profiles, outer membrane protein profiles, plasmid profiles, and levels of cytotoxic activity against Vero cells were similar in the wild-type and mutant strains. Both adhered to intestinal epithelial cells in culture and reacted with fluorescein isothiocyanate-labeled antiserum against E. coli O157:H7. The mutant appeared to be similar to the wild type in all respects except in its ability to ferment sorbitol. [14C]sorbitol uptake and sorbitol-6-phosphate dehydrogenase activities were notably increased in the mutant strain. Diarrhea developed in rabbits administered the wild-type strain and in those fed the sorbitol-positive mutant. There was greater bacterial attachment and mucosal damage in the cecum and large intestine than in the small intestine. Scanning electron microscopy revealed bacteria adhering as single cells and as aggregates closely associated with mucus. Mucosal lesions consisted of areas of tissue necrosis with sloughing of epithelial cells. By transmission electron microscopy, electron-dense necrotic epithelial cells were visible in areas where bacteria were present, and epithelial cell debris containing bacteria was observed between the villar luminal surfaces. Light microscopy of epithelial cells of intestinal sections of infected rabbits revealed noticeable vacuolation and spherical, pyknotic nuclei. These data indicate that the sorbitol-negative phenotype is not associated with the pathogenicity of E. coli O157:H7.

Studies on Escherichia coli serotype O157:H7 strains containing a 60-MDa plasmid and on 60-MDa plasmid-cured derivatives

Seventeen strains of Escherichia coli serotype O157:H7 producing Shiga-like toxin were examined for the presence of plasmids and for the ability to adhere to HEp-2 and Intestine 407 cells. All of the strains possessed a common 60-MDa plasmid. To determine the role of the 60-MDa plasmid, plasmid-cured strains were compared with the parent strains for their ability to produce pili and to adhere to epithelial cells in culture. The total cell lysate protein and outer-membrane protein (OMP) profiles were also compared. Both the parent strains and their plasmid-cured derivatives produced pili. Immunofluorescence assay results indicated that the plasmid-cured and parent strains adhered equally well to HEp-2 and Intestine 407 cells; overall adherence was greater with intestinal cells than HEp-2 cells. SDS-PAGE of polypeptides synthesised in an E. coli system in vitro showed that plasmid DNA encodes c. 35 proteins. SDS-PAGE of OMP preparations demonstrated that the 60-MDa plasmid appears to be involved in the synthesis of a 33-kDa OMP. Two strains cured of the 60-MDa plasmid, one that possessed no plasmids and one that still contained a 2.2-MDa plasmid, produced exopolysaccharide (EPS) when cultured on solid medium at 25 degrees C. Two other strains, which were cured of the 60-MDa plasmid but contained a 4.5-MDa plasmid, did not produce visible amounts of EPS. Gas chromatography analysis showed that the EPS consisted of fucose, glucose and galactose in an approximate molar ratio of 2.0:0.9:1.1 and also had 7% of a uronic acid sugar as part of its structure.

Differentiation of the Effects of pH and Lactic or Acetic Acid Concentration on the Kinetics of Listeria Monocytogenes Inactivation

The effects of pH and lactic acid or acetic acid concentration on Listeria monocytogenes inactivation were studied in brain heart infusion broth using a three strain mixture. Combinations of lactic acid/sodium lactate and acetic acid/sodium acetate were used to achieve concentrations of 0.1, 0.5, 1.0, and 2,0 M in conjunction with pH values of 4.0, 5.0, 6.0, and 7.0. Cultures adjusted with HCl to pH 3.0 to 7.0 in 0.5 pH unit intervals were used as 0.0 M controls. Each pH/concentration combination was inoculated to a level of 10⁸ CFU/ml and incubated at 28°C for up to 60 d. Bacterial populations were determined periodically by plate counts. Inactivation was exponential after an initial lag period. Survivor curves (log# versus time) were fitted using a linear model that incorporated a lag period. The model was subsequently used to calculate D values and "time to a 4-D (99.99%) inactivation" (t_4-D); t_4-D values were directly related to pH and inversely related to acid concentration. At acid/pH combinations that supported growth, the level of the organism increased slightly (2- to 10-fold) before declining. In the HCl-adjusted controls with pH's ≤5.5, the rate of inactivation was linearly related to pH. In the presence of the monocarboxylic acids, the duration of the lag period and the rate of inactivation were dependent on the pH, as well as the identity and concentration of acid. 4-D inactivation times were related to the level of undissociated lactic and acetic acids. That relationship was described by the equations, t_4-D = exp (-0.1773*LA^0.5 + 7.3482) and t_4-D = exp (-0.1468*AA^0.5 + 7.3905) for lactic and acetic acids, respectively, where LA and AA are mM of undissociated acid. These relationships were used in conjunction with the Henderson-Hasselback equation to develop a model for predicting the rate of inactivation as a function of pH and total organic acid concentration.

Defective glia in the Drosophila brain degeneration mutant drop-dead

To understand better the cellular basis of late-onset neuronal degeneration, we have examined the brain of the drop-dead mutant of Drosophila. This mutant carries an X-chromosomal recessive mutation that causes severe behavioral defects and brain degeneration, manifested a few days after emergence of the adult. Analysis of genetically mosaic flies has indicated that the focus of the drop-dead mutant phenotype is in the brain and that the gene product is non-cell autonomous. We examined the adult drop-dead mutant brain prior to onset of symptoms and found that many glial cells have stunted processes, whereas neuronal morphology is essentially normal. Adult mutant glial cells resemble immature glia found at an earlier stage of normal brain development. These observations suggest that defective glia in the drop-dead brain may disrupt adult nervous system function, contributing to progressive brain degeneration and death. The normal drop-dead gene product may prevent brain degeneration by providing a necessary glial function.

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.