Influence of solute, pH, and incubation temperature on recovery of heat-stressed Wallemia sebi conidia.

The influences of glucose, sorbitol, and NaCl in a basal enumeration medium at water activities (aw) from 0.82 to 0.97 on colony formation by sublethally heat-stressed Wallemia sebi conidia were determined. Over this aw range, glucose and sorbitol had similar effects on recovery, whereas at an aw of 0.82 to 0.92, NaCl had a detrimental effect. Colony diameters were generally largest on media containing sorbitol and smallest on media containing NaCl. Maximum colony size and viable population of heat-stressed conidia were observed on media at an aw of ca. 0.92. When the recovery incubation temperature was 20 degrees C, the number of uninjured conidia detected at an aw of 0.82 was reduced compared with the number detected at 25 degrees C, while at 30 degrees C, the number recovered at an aw of 0.97 was reduced. The effect on heat-stressed conidia was magnified. This suggests that W. sebi conidia may be more tolerant of aw values higher than the optimum 0.92 when the incubation temperature is decreased from the near optimum of 25 degrees C and less tolerant of aw values greater than 0.92 when the incubation temperature is higher than 25 degrees C. The sensitivity of heat-stressed conidia increased as the pH of the recovery medium was decreased from 6.55 to 3.71. W. sebi conidia dispersed in wheat flour at aw values of 0.43 and 0.71 and stored for up to 65 days at both 1 and 25 degrees C neither lost viability nor underwent sublethal desiccation or temperature injury.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of water activity and temperature on survival of and colony formation by heat-stressed Chrysosporium farinicola aleuriospores.

The ability of sublethally heat-stressed aleuriospores of Chrysosporium farinicola to form colonies on yeast extract-glucose agar (YGA) supplemented with sufficient glucose, sorbitol, glycerol, and NaCl to achieve reduced water activity (aw) in the range of 0.88 to 0.95 was determined. The effects of the aw of diluent and incubation temperature during recovery and colony formation were also investigated. Aleuriospores harvested from 14-day-old cultures grown at 25 degrees C were less resistant to heat inactivation compared with aleuriospores from 20-day-cultures. Increased populations of heat-stressed aleuriospores were recovered as the aw of YGA was decreased from 0.95 (glucose and glycerol) and 0.94 (sorbitol) to 0.89 and 0.88, respectively. In NaCl-supplemented YGA, populations recovered at an aw of 0.94 were greatly reduced compared with populations detected at an aw of 0.92; no colonies were formed on NaCl-supplemented YGA at an aw of 0.88. Tolerance to aw values above 0.88 to 0.89 as influenced by solute type was in the order of glucose greater than sorbitol greater than glycerol greater than NaCl. Incubation at 20 degrees generally resulted in an increase in recoverable aleuriospores compared with incubation at 25 degrees C or at 30 degrees C for 14 days followed by 20 degrees C for 10 days. The lethal effect of NaCl on heat-stressed aleuriospores was enhanced at 30 degrees C. The retention of viability of aleuriospores held in sucrose-peptone water diluent (aw, 0.936) for 20 min was essentially the same as that observed when aleuriospores were held in peptone water (aw, 0.997).(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of water activity and temperature on survival of and colony formation by heat-stressed Chrysosporium farinicola aleuriospores

The ability of sublethally heat-stressed aleuriospores of Chrysosporium farinicola to form colonies on yeast extract-glucose agar (YGA) supplemented with sufficient glucose, sorbitol, glycerol, and NaCl to achieve reduced water activity (aw) in the range of 0.88 to 0.95 was determined. The effects of the aw of diluent and incubation temperature during recovery and colony formation were also investigated. Aleuriospores harvested from 14-day-old cultures grown at 25 degrees C were less resistant to heat inactivation compared with aleuriospores from 20-day-cultures. Increased populations of heat-stressed aleuriospores were recovered as the aw of YGA was decreased from 0.95 (glucose and glycerol) and 0.94 (sorbitol) to 0.89 and 0.88, respectively. In NaCl-supplemented YGA, populations recovered at an aw of 0.94 were greatly reduced compared with populations detected at an aw of 0.92; no colonies were formed on NaCl-supplemented YGA at an aw of 0.88. Tolerance to aw values above 0.88 to 0.89 as influenced by solute type was in the order of glucose greater than sorbitol greater than glycerol greater than NaCl. Incubation at 20 degrees generally resulted in an increase in recoverable aleuriospores compared with incubation at 25 degrees C or at 30 degrees C for 14 days followed by 20 degrees C for 10 days. The lethal effect of NaCl on heat-stressed aleuriospores was enhanced at 30 degrees C. The retention of viability of aleuriospores held in sucrose-peptone water diluent (aw, 0.936) for 20 min was essentially the same as that observed when aleuriospores were held in peptone water (aw, 0.997).(ABSTRACT TRUNCATED AT 250 WORDS)     

Colony formation by sublethally heat-injured Zygosaccharomyces rouxii as affected by solutes in the recovery medium and procedure for sterilizing medium.

Recovery and colony formation by healthy and sublethally heat-injured cells of Zygosaccharomyces rouxii as influenced by the procedure for sterilizing recovery media (YM agar [YMA], wort agar, cornmeal agar, and oatmeal agar) were investigated. Media were supplemented with various concentrations of glucose, sucrose, glycerol, or sorbitol and sterilized by autoclaving (110 degrees C, 15 min) and by repeated treatment with steam (100 degrees C). An increase in sensitivity was observed when heat-injured cells were plated on glucose-supplemented YMA at an aw of 0.880 compared with aws of 0.933 and 0.998. Colonies which developed from unheated and heated cells on YMA at aws of 0.998 and 0.933 generally exceeded 0.5 mm in diameter within 3.5 to 4 days of incubation at 25 degrees C, whereas colonies formed on YMA at an aw of 0.880 typically did not exceed 0.5 mm in diameter until after 5.5 to 6.5 days of incubation. The number of colonies exceeding 0.5 mm in diameter which were formed by heat-injured cells on YMA at an aw of 0.880 was 2 to 3 logs less than the total number of colonies detected, i.e., on YMA at an aw of 0.933 and using no limits of exclusion based on colony diameter. A substantial portion of cells which survived heat treatment were sublethally injured as evidenced by increased sensitivity to a suboptimum aw (0.880). In no instance was recovery of Z. rouxii significantly affected by medium sterilization procedure when glucose or sorbitol was used as the aw-suppressing solute.(ABSTRACT TRUNCATED AT 250 WORDS)     

Colony formation by sublethally heat-injured Zygosaccharomyces rouxii as affected by solutes in the recovery medium and procedure for sterilizing medium

Recovery and colony formation by healthy and sublethally heat-injured cells of Zygosaccharomyces rouxii as influenced by the procedure for sterilizing recovery media (YM agar [YMA], wort agar, cornmeal agar, and oatmeal agar) were investigated. Media were supplemented with various concentrations of glucose, sucrose, glycerol, or sorbitol and sterilized by autoclaving (110 degrees C, 15 min) and by repeated treatment with steam (100 degrees C). An increase in sensitivity was observed when heat-injured cells were plated on glucose-supplemented YMA at an aw of 0.880 compared with aws of 0.933 and 0.998. Colonies which developed from unheated and heated cells on YMA at aws of 0.998 and 0.933 generally exceeded 0.5 mm in diameter within 3.5 to 4 days of incubation at 25 degrees C, whereas colonies formed on YMA at an aw of 0.880 typically did not exceed 0.5 mm in diameter until after 5.5 to 6.5 days of incubation. The number of colonies exceeding 0.5 mm in diameter which were formed by heat-injured cells on YMA at an aw of 0.880 was 2 to 3 logs less than the total number of colonies detected, i.e., on YMA at an aw of 0.933 and using no limits of exclusion based on colony diameter. A substantial portion of cells which survived heat treatment were sublethally injured as evidenced by increased sensitivity to a suboptimum aw (0.880). In no instance was recovery of Z. rouxii significantly affected by medium sterilization procedure when glucose or sorbitol was used as the aw-suppressing solute.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interactive effects of solutes, potassium sorbate and incubation temperature on growth, heat resistance and tolerance to freezing of Zygosaccharomyces rouxii.

The interactive effects of solutes, potassium sorbate and incubation temperature on growth, heat resistance and tolerance to freezing of Zygosaccharomyces rouxii were investigated. Growth rates in media supplemented with glucose, sucrose or NaCl to aw 0.93 were more rapid than in unsupplemented media (aw 0.99). Although growth in unsupplemented medium was lower at 35 degrees C, incubation at 21 degrees C or 35 degrees C had little effect on growth in media supplemented with glucose and sucrose. The addition of 300 micrograms potassium sorbate/ml to media resulted in reduced growth rates, particularly at 35 degrees C. Heat resistance of Z. rouxii was substantially greater in cultures previously incubated at 35 degrees C than in cultures incubated at 21 degrees C in media both with and without 300 micrograms potassium sorbate/ml. Zygosaccharomyces rouxii was tolerant to freezing at -18 degrees C for up to 120 d in all test media supplemented with glucose, sucrose or NaCl. The addition of 300 micrograms potassium sorbate/ml to sucrose-supplemented media resulted in increased resistance to freezing in cultures previously incubated at 21 degrees C. Sensitivity to freezing increased when cultures were incubated at 21 degrees C in media not supplemented with solutes. Glucose and sucrose provided the best protection against inactivation by heating and freezing, regardless of the presence of potassium sorbate in growth media.     

Effects of temperature, water activity, and syrup film composition on the growth of Wallemia sebi: development and assessment of a model predicting growth lags in syrup agar and crystalline sugar

We investigated the effects of temperature, water activity (a(w)), and syrup film composition on the CFU growth of Wallemia sebi in crystalline sugar. At a high a(w) (0.82) at both high (20 degrees C) and low (10 degrees C) temperatures, the CFU growth of W. sebi in both white and extrawhite sugar could be described using a modified Gompertz model. At a low a(w) (0.76), however, the modified Gompertz model could not be fitted to the CFU data obtained with the two sugars due to long CFU growth lags and low maximum specific CFU growth rates of W. sebi at 20 degrees C and due to the fact that growth did not occur at 10 degrees C. At an a(w) of 0.82, regardless of the temperature, the carrying capacity (i.e., the cell concentration at t = infinity) of extrawhite sugar was lower than that of white sugar. Together with the fact that the syrup film of extrawhite sugar contained less amino-nitrogen relative to other macronutrients than the syrup film of white sugar, these results suggest that CFU growth of W. sebi in extrawhite sugar may be nitrogen limited. We developed a secondary growth model which is able to predict colony growth lags of W. sebi on syrup agar as a function of temperature and a(w). The ability of this model to predict CFU growth lags of W. sebi in crystalline sugar was assessed.     

Growth of and fumitremorgin production by Neosartorya fischeri as affected by temperature, light, and water activity

The effects of temperature, light, and water activity (aw) on the growth and fumitremorgin production of a heat-resistant mold, Neosartorya fischeri, cultured on Czapek Yeast Autolysate agar (CYA) were studied for incubation periods of up to 74 days. Colonies were examined visually, and extracts of mycelia and CYA on which the mold was cultured were analyzed for mycotoxin content by high-performance liquid chromatography. Growth always resulted in the production of the tremorgenic mycotoxins verruculogen and fumitremorgins A and C. The optimum temperatures for the production of verruculogen and fumitremorgins A and C on CYA at pH 7.0 were 25, 30, and 37 degrees C, respectively. The production of fumitremorgin C by N. fischeri has not been previously reported. Fumitremorgin production was retarded at 15 degrees C, but an extension of the incubation period resulted in concentrations approaching those observed at 25 degrees C. Light clearly enhanced fumitremorgin production on CYA (pH 7.0, 25 degrees C), but not as dramatically as did the addition of glucose, fructose, or sucrose to CYA growth medium (pH 3.5, 25 degrees C). Growth and fumitremorgin production was greatest at aw of 0.980 on CYA supplemented with glucose or fructose and at aw of 0.990 on CYA supplemented with sucrose. Growth and fumitremorgin production were observed at aw as low as 0.925 on glucose-supplemented CYA but not at aw lower than 0.970 on CYA supplemented with sucrose. Verruculogen was produced in the highest amount on all test media, followed by fumitremorgin A and fumitremorgin C.     

Growth of and fumitremorgin production by Neosartorya fischeri as affected by temperature, light, and water activity.

The effects of temperature, light, and water activity (aw) on the growth and fumitremorgin production of a heat-resistant mold, Neosartorya fischeri, cultured on Czapek Yeast Autolysate agar (CYA) were studied for incubation periods of up to 74 days. Colonies were examined visually, and extracts of mycelia and CYA on which the mold was cultured were analyzed for mycotoxin content by high-performance liquid chromatography. Growth always resulted in the production of the tremorgenic mycotoxins verruculogen and fumitremorgins A and C. The optimum temperatures for the production of verruculogen and fumitremorgins A and C on CYA at pH 7.0 were 25, 30, and 37 degrees C, respectively. The production of fumitremorgin C by N. fischeri has not been previously reported. Fumitremorgin production was retarded at 15 degrees C, but an extension of the incubation period resulted in concentrations approaching those observed at 25 degrees C. Light clearly enhanced fumitremorgin production on CYA (pH 7.0, 25 degrees C), but not as dramatically as did the addition of glucose, fructose, or sucrose to CYA growth medium (pH 3.5, 25 degrees C). Growth and fumitremorgin production was greatest at aw of 0.980 on CYA supplemented with glucose or fructose and at aw of 0.990 on CYA supplemented with sucrose. Growth and fumitremorgin production were observed at aw as low as 0.925 on glucose-supplemented CYA but not at aw lower than 0.970 on CYA supplemented with sucrose. Verruculogen was produced in the highest amount on all test media, followed by fumitremorgin A and fumitremorgin C.     

Two-dimensional environmental profiles of growth,deoxynivalenol and nivalenol production by Fusarium culmorum on a wheat-based substrate

AIMS: To determine the effect of interacting conditions of water activity (aw, 0.99-0.85), temperature (15, 25 degrees C) and time (40 days) on growth and production of the mycotoxins deoxynivalenol (DON) and nivalenol (NIV) by Fusarium culmorum on a wheat-based agar medium. METHODS AND RESULTS: Fusarium culmorum grew optimally at 0.995aw and minimally at 0.90 at both 15 and 25 degrees C. No growth was observed at <0.90aw. Overall, temperature, aw and their interaction had a statistically significant effect on the growth rate of F. culmorum. Production of both DON and NIV were over a much narrower range (0.995-0.95aw) than that for growth. The highest concentrations of DON and NIV levels were produced at 0.995aw and 0.981aw at 25 degrees C, respectively, after 40 days of incubation. Statistically, aw, temperature and incubation time, and aw x temperature and temperature x incubation time had a statistically significant effect on DON/NIV production. CONCLUSIONS: This is the first detailed report on the two-dimensional environmental profiles for DON/NIV production by F. culmorum in the UK. SIGNIFICANCE AND IMPACT OF THE STUDY: As part of a hazard analysis critical control point (HACCP) approach, this type of information is critical in monitoring critical control points for prevention of DON/NIV entering the wheat production chain.     

Solute stresses affect growth patterns, endogenous water potentials and accumulation of sugars and sugar alcohols in cells of the biocontrol yeast Candida sake

AIM: To evaluate the effect of modifications of water activity (aw 0. 996-0.92) of a molasses medium with different solutes (glycerol, glucose, NaCl, proline or sorbitol) on growth, intracellular water potentials (psi(c)) and endogenous accumulation of polyols/sugars in the biocontrol yeast Candida sake. METHODS AND RESULTS: Modification of solute stress significantly influenced growth, psi(c) and accumulation of sugars (glucose/trehalose) and polyols (glycerol, erythritol, arabitol and mannitol) in the yeast cells. Regardless of the solute used to modify aw, growth was always decreased as water stress increased. Candida sake cells grew better in glycerol- and proline-amended media, but were sensitive to NaCl. The psi(c) measured using psychrometry showed a significant effect of solutes, aw and time. Cells from the 0.96 aw NaCl treatment presented the lowest psic value (- 5.20 MPa) while cells from unmodified media (aw = 0. 996) had the highest value (- 0.30 MPa). In unmodified medium, glycerol was the predominant reserve accumulated. Glycerol and arabitol were the major compounds accumulated in media modified with glucose or NaCl. In proline media, the concentration of arabitol increased. In glycerol- and sorbitol-amended media, the concentration of glycerol rose. Some correlations were obtained between compatible solutes and psi(c). CONCLUSIONS AND SIGNIFICANCE: This study demonstrates that subtle changes in physiological parameters significantly affect the endogenous contents of C. sake cells. It may be possible to utilize such physiological information to develop biocontrol inocula with improved quality.     

Improving low water activity and desiccation tolerance of the biocontrol agent Pantoea agglomerans CPA-2 by osmotic treatments

AIMS: To study the improvement of tolerance to low water activity (aw) and desiccation during spray drying in Pantoea agglomerans cells subjected to mild osmotic stress during growth. METHODS AND RESULTS: The micro-organism was cultured in an unmodified liquid (control) or in aw-modified media, and viability of these cells was evaluated on unstressed (0.995) and 0.96 aw stressed solid media, in order to check total viability and aw stress tolerance respectively. Significant improvements in viability on unmodified medium were observed with cells grown for 24 h in NaCl 0.98 aw, glycerol 0.98 aw and 0.97 aw and for 48 h in NaCl 0.98 aw and 0.97 aw modified media. Both yield improvements and water stress tolerance were achieved with low aw media. Cells grown for 24 h in NaCl 0.98 aw or for 48 h in NaCl 0.98 aw, 0.97 aw and 0.96 aw, glucose 0.97 aw and glycerol 0.97 aw showed improved aw stress tolerance in comparison with control cells. The best results were obtained with NaCl treatments (0.98 aw and 0.97 aw) which also exhibited better survival rates than control cells during spray-drying process and maintained their efficacy against postharvest fungal pathogens in apples and oranges. CONCLUSIONS: NaCl treatments are very appropriate for improving P. agglomerans low aw tolerance obtaining high production levels and maintaining biocontrol efficacy. SIGNIFICANCE AND IMPACT OF THE STUDY: Improving stress tolerance of biocontrol agents could be an efficient way to obtain consistency and maintain efficacy of biological control under practical conditions.     

Effect of salinity and temperature on germination, growth and ion relations of Panicum turgidum Forssk

Seed germination of Panicum turgidum was significantly affected by salinity levels, temperature and their interaction. Maximum germination was noted in the lowest saline media (25-50 mM) and distilled water at the temperature of 15-25 degrees C and 20-30 degrees C. Seeds germination was substantially delayed and reduced with an increase in NaCl to levels above 50mM. This trend was much pronounced under high levels of NaCl and incubation temperature. Low levels of NaCl (25-50 mM) stimulated shoot and root dry weights of P. turgidum seedlings. However, the highest NaCl levels (>100 mM) resulted in a significant decrease in shoot, root and total dry weights of seedlings. Intermediate degrees of temperature, 15-25 and 20-30 degrees C, resulted in a significant increase in biomass accumulation. The Na+ concentration in shoots and roots significantly increased as NaCl concentration increased. The K+ concentration in roots and K/Na ratio in shoots and roots was significantly reduced as salinity concentration increased. The K/Na ratio was greatly affected by higher NaCl concentration and incubation temperatures.     

Growth and thermal inactivation of Listeria monocytogenes in cabbage and cabbage juice

Studies were done to determine the interacting effects of pH, NaCl, temperature, and time on growth, survival, and death of two strains of Listeria monocytogenes. Viable population of the organism steadily declined in heat-sterilized cabbage stored at 5 degrees C for 42 days. In contrast, the organism grew on raw cabbage during the first 25 days of a 64-day storage period at 5 degrees C. Growth was observed in heat-sterilized unclarified cabbage juice containing less than or equal to 5% NaCl and tryptic phosphate broth containing less than or equal to 10% NaCl. Rates of thermal inactivation increased as pH of clarified cabbage juice heating medium was decreased from 5.6 to 4.0. At 58 degrees C (pH 5.6), 4 X 10(6) cells/mL were reduced to undetectable levels within 10 min. Thermal inactivation rates in clarified cabbage juice (pH 5.6) were not significantly influenced by the presence of up to 2% NaCl; however, heat-stressed cells had increased sensitivity to NaCl in tryptic soy agar recovery medium. Cold enrichment of heat-stressed cells at 5 degrees C for 21 days enhanced resuscitation. Results indicate that L. monocytogenes can proliferate on refrigerated (5 degrees C) raw cabbage which, in turn, may represent a hazard to health of the consumer. Heat pasteurization treatments normally given to cabbage juice or sauerkraut would be expected to kill any L. monocytogenes cells which may be present.     

Response of Campylobacter jejuni to sodium chloride.

Studies were done to provide more comprehensive information on the response of Campylobacter jejuni and nalidixic acid-resistant, thermophilic Campylobacter (NARTC) to sodium chloride at 4, 25, and 42 degrees C. Three strains of C. jejuni were studies, and all could grow at 42 degrees C in the presence of 1.5% NaCl, but not 2.0% NaCl. At the same temperature, NARTC could grow in 2.0% NaCl and was substantially more tolerant to 2.5 and 4.5% NaCl than was C. jejuni. Both C. jejuni and NARTC grew poorly in the absence of added NaCl and grew best in the presence of 0.5% NaCl at 42 degrees C. At 25 degrees C, NaCl concentrations of 1.0 to 2.5% were protective to NARTC, but the same concentrations of salt generally enhanced the rate of death of C. jejuni. At 4 degrees C, both C. jejuni and NARTC were sensitive to 1.0% or more NaCl; however, the rate of death at this temperature was substantially less than that which occurred at 25 degrees C. A 3 log10 decrease of cells occurred in 4.5% NaCl after 1.2 to 2.1 days at 25 degrees C, and a similar reduction in cells took approximately 2 weeks at the same salt concentration and 4 degrees C. Although C. jejuni grows best in the presence of 0.5% NaCl, the presence of NaCl at concentrations as low as 1.0% may retard growth or increase rate of death; hence, it is advisable that growth media used for recovering or enumerating this organism contain 0.5% NaCl, but not 1.0% or more NaCl.     

Survival of Escherichia coli O157:H7 in broth and processed salami as influenced by pH, water activity, and temperature and suitability of media for its recovery.

The survival of unheated and heat-stressed (52 degrees C, 30 min) cells of Escherichia coli O157:H7 inoculated into tryptic soy broth (TSB) adjusted to various pHs (6.0, 5.4, and 4.8) with lactic acid and various water activities (a(w)s) (0.99, 0.95, and 0.90) with NaCl and incubated at 5, 20, 30, and 37 degrees C was studied. The performance of tryptic soy agar (TSA), modified sorbitol MacConkey agar (MSMA), and modified eosin methylene blue agar in supporting colony development of incubated cells was determined. Unheated cells of E. coli O157:H7 grew to population densities of 10(8) to 10(9) CFU ml-1 in TSB (pHs 6.0 and 5.4) at an a(w) of 0.99. Regardless of the pH and a(w) of TSB, survival of E. coli O157:H7 was better at 5 degrees C than at 20 or 30 degrees C. At 30 degrees C, inactivation or inhibition of growth was enhanced by reduction of the a(w) and pH. A decrease in the a(w) (0.99 to 0.90) of TSB in which the cells were heated at 52 degrees C for 30 min resulted in a 1.5-log10 reduction in the number of E. coli O157:H7 cells recovered on TSA; pH did not significantly affect the viability of cells. Recovery was significantly reduced on MSMA when cells were heated in TSB with reduced pH or a(w) for an increased length of time. With the exception of TSB (a(w), 0.90) incubated at 37 degrees C, heat-stressed cells survived for 24 h in recovery broth. TSB (a(w), 0.99) at pH 6.0 or 5.4 supported growth of E. coli O157:H7 cells at 20 or 37 degrees C, but higher numbers of heated cells survived at 5 or 20 degrees C than at 37 degrees C. The ability of unheated and heat-stressed E. coli O157:H7 cells to survive or grow as affected by the a(w) of processed salami was investigated. Decreases of about 1 to 2 log10 CFU g-1 occurred soon after inoculation of salami (pHs 4.86 and 4.63 at a(w)s of 0.95 and 0.90, respectively). Regardless of the physiological condition of the cells before inoculation into processed salami at an a(w) of either 0.95 or 0.90, decreases in populations occurred during storage at 5 or 20 degrees C for 32 days. If present at < or = 100 CFU g-1, E. coli O157:H7 would unlikely survive storage at 5 degrees C for 32 days. However, contamination of salami with E. coli O157:H7 at 10(4) to 10(5) CFU g-1 after processing would pose a health risk to consumers for more than 32 days if storage were at 5 degrees C. Regardless of the treatment conditions, performance of the media tested for the recovery of E. coli O157:H7 cells followed the order TSA > modified eosin methylene blue agar > MSMA.     

Use of gradient plates to study combined effects of temperature, pH, and NaCl concentration on growth of Monascus ruber van Tieghem, an Ascomycetes fungus isolated from green table olives

The effect of temperature, pH, and sodium chloride concentration on the growth of the Ascomycetes fungus Monascus ruber van Tieghem, the main spoilage microorganism during storage of table olives, was studied by using the gradient plate technique. Gradients of NaCl (3 to 9%, wt/vol) at right angles to gradients of pH (2 to 6.8) were prepared for the plates, which were incubated at 25, 30, and 35 degrees C. Visible fungal growth, expressed in optical density units, was recorded by image analysis and graphically presented in the form of three-dimensional grids. Results obtained from the plates indicated that the fungus was salt and acid tolerant, being able to grow at NaCl concentrations of up to 9% (wt/vol) and pH values of as low as 2.2, depending on the incubation temperature. The inhibitory effect of NaCl increased as the pH decreased progressively at 25 and 30 degrees C but not at 35 degrees C. Growth was better at 30 and 25 degrees C as judged by the larger extent of the plates covered by mycelium compared with that at 35 degrees C, where no growth was observed at pHs below 3.7. Differentiation between vegetative (imperfect-stage) and reproductive (perfect-stage) growth was evident on all plates, providing useful information about the effect of environmental conditions on the form of fungal growth. When the growth/no-growth surface model was obtained by applying linear logistic regression, it was found that all factors (pH, NaCl, and temperature) and their interactions were significant. Plots of growth/no-growth interfaces for P values of 0.1, 0.5, and 0.9 described the results satisfactorily at 25 and 35 degrees C, whereas at 35 degrees C the model predicted lower minimum pH values for growth in the range of 7 to 10% NaCl than those observed on the plates. Overall, it is suggested that the fungus cannot be inhibited by any combination of pH and NaCl within the limits of the brine environment, so further processing is required to ensure product stability in the market.     

Pathogenicity of nonstressed, heat-stressed, and resuscitated Listeria monocytogenes 1A1 cells.

The pathogenicity of nonstressed, heat-stressed, and resuscitated cells of Listeria monocytogenes 1A1 was assayed in immunocompromised mice. Cells were stressed by heating them at 56 degrees C for 20 min and were resuscitated by incubation in tryptic soy broth at 25 degrees C. A dose of 10(2) nonstressed and resuscitated cells per mouse was required for pathogenicity; a dose of 10(4) heat-stressed cells per mouse was considerably less pathogenic. Loss of hemolytic activity accompanied the decreased virulence.     

Pathogenicity of nonstressed, heat-stressed, and resuscitated Listeria monocytogenes 1A1 cells.

The pathogenicity of nonstressed, heat-stressed, and resuscitated cells of Listeria monocytogenes 1A1 was assayed in immunocompromised mice. Cells were stressed by heating them at 56 degrees C for 20 min and were resuscitated by incubation in tryptic soy broth at 25 degrees C. A dose of 10(2) nonstressed and resuscitated cells per mouse was required for pathogenicity; a dose of 10(4) heat-stressed cells per mouse was considerably less pathogenic. Loss of hemolytic activity accompanied the decreased virulence.     

温湿度对安徽虫瘟霉在桃蚜居群中流行的影响

用安徽虫瘟霉(Zoopphthoraanhuiensis)接种桃蚜(Myzuspersicae)无翅成蚜3头和健蚜3头组成新的蚜群若干,在不同温湿组合条件下任其繁殖、发病和传染,以评价温湿度对该菌在蚜群中流行的影响.持续26d观察,在偏低温与各湿度(90～100%RH)的组合中均成功诱发了蚜病流行,其程度受湿度影响较小.在10℃与各湿度组合中,最终感病死亡率为72.9%～98.2%,15℃下为78.7%～94.4%,流行强度极大.而20℃下仅100%RH诱发了高强度流行病,其余湿度下感病死亡率仅为5.1%～12.8%.在25℃,100%RH下死亡率仅27.2%.与所有湿度组合中的对照蚜群相比,10℃下流行病控蚜效果为89.5%～96.9%,15℃下为96.1%～98.2%,20℃下为45.9%～85.7%,25℃下为56.4%～69.7%.逐步回归分析表明,安徽虫瘟霉引发的蚜病流行与温度及其相对湿度和蚜群带菌后天数的互作项密切相关(r2=0.82,a＜0.01),这些变量在很大程度上决定了安徽虫瘟霉是否在蚜群中流行.