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Biomedical subjects

L R Beuchat

Publications and source records attributed to L R Beuchat.

At least 109 records · Page 6Linked to original sources

Evaluation of the MicroScan enzyme-based system for the identification of foodborne yeasts.

Eighty-nine strains representing 36 species of foodborne yeasts isolated from fruit juice concentrates were identified using the Baxter MicroScan enzyme-based kit, conventional tests according to a simplified identification method (SIM), and the API 20C kit. Of the 15 test species included in the MicroScan database, only 40% were correctly identified; 13% gave scores of unacceptably low probabilities, 20% were misidentified, and 27% could not be identified. Of the 21 test species not in the MicroScan database, 38% were misidentified and 62% produced biocodes with between-species differences not larger than differences between strains within species. The reliability of the MicroScan enzyme-based system is questioned, in that different results were sometimes obtained upon retesting the same strains. The MicroScan enzyme-based system is rapid, providing results within 4 h. However, because of its restricted and specific database and unreliability, the system appears to be unsuited for the identification of foodborne yeasts.

Food Microbiology↗

Evaluation of enrichment broths for their ability to recover heat-injured Listeria monocytogenes.

Listeria selective enrichment broth (LEB), University of Vermont (UVM) broth, modified UVM (MUVM) broth and Fraser broth (FB) were compared for their ability to recover cells of L. monocytogenes from heated tryptose phosphate broth. Three strains of L. monocytogenes were heated at 54 degrees C for 30 min, inoculated into enrichment broths supplemented with 400 micrograms catalase ml-1, and incubated for 8 h at 30 degrees C. After incubation for 4 h, the total viable cell populations either decreased or did not change, whereas the number of healthy (non-injured) cells of all strains increased significantly in all broths except FB inoculated with the LCDC strain. With an increase in incubation time to 8 h, the number of healthy cells of all strains increased in all broths. At 8 h, the difference between populations of total (injured plus healthy cells) and healthy cells detected in LEB inoculated with two strains was not significant. Overall, recovery of heat-treated cells was significantly higher in LEB, followed by MUVM broth, UVM broth and FB. The addition of catalase to enrichment broths significantly enhanced recovery of heat-injured cells. A slight reduction of catalase activity of heated cells of all test strains in all enrichment broths except FB was observed by extending the incubation period from 4 to 8 h. A test strain that produces relatively higher catalase activity compared to the other strains exhibited the greatest resistance to exogenous hydrogen peroxide. Enumeration of viable L. monocytogenes cells in heated foods should be done using LEB supplemented with 400 micrograms catalase ml-1 to maximize the recovery of injured cells.

Catalase↗

Death of enterohemorrhagic Escherichia coli O157:H7 in real mayonnaise and reduced-calorie mayonnaise dressing as influenced by initial population and storage temperature.

This study was undertaken to determine the survivability of low-density populations (10(0) and 10(2) CFU/g) of enterohemorrhagic Escherichia coli O157:H7 inoculated into real mayonnaise and reduced-calorie mayonnaise dressing and stored at 20 and 30 degrees C, temperatures within the range used for normal commercial mayonnaise distribution and storage. Inactivation patterns at 5 degrees C and inactivation of high-inoculum populations (10(6) CFU/g) were also determined. The pathogen did not grow in either mayonnaise formulation, regardless of the inoculum level or storage temperature. Increases in storage temperature from 5 to 20 degrees C and from 20 to 30 degrees C resulted in dramatic increases in the rate of inactivation. Populations of E. coli O157:H7 in the reduced-calorie and real formulations inoculated with a population of 0.23 to 0.29 log10 CFU/g and held at 30 degrees C were reduced to undetectable levels within 1 and 2 days, respectively; viable cells were not detected after 1 day at 20 degrees C. In mayonnaise containing an initial population of 2.23 log10 CFU/g, viable cells were not detected after 4 days at 30 degrees C or 7 days at 20 degrees C; tolerance was greater in real mayonnaise than in reduced-calorie mayonnaise dressing stored at 5 degrees C. The tolerance of E. coli O157:H7 inoculated at the highest population density (6.23 log 10 CFU/g) was less in reduced-calorie mayonnaise dressing than in real mayonnaise at all storage temperatures. In reduced-calorie mayonnaise dressing and real mayonnaise initially containing 2.23 log10 CFU/g, levels were undetectable after 28 and 58 days at 5 degrees C, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Colitis↗

Fate of Salmonella montevideo on and in raw tomatoes as affected by temperature and treatment with chlorine.

A study was undertaken to determine the survival patterns of Salmonella montevideo G4639 on and in tomatoes during storage and the efficacy of chlorine treatment on inactivation of the pathogen. The population of S. montevideo on the surfaces of inoculated tomatoes stored at 10 degrees C did not change significantly (P < 0.05) throughout an 18-day storage period. Significant increases in population occurred within 7 days and within 1 day when tomatoes were stored at 20 and 30 degrees C, respectively. A significantly higher number of cells was taken up by the core tissue of tomatoes tempered at 25 degrees C when the tomatoes were dipped in a suspension at 10 degrees C compared with the number taken up when the tomatoes were dipped in cell suspensions tempered at 25 or 37 degrees C. Populations remained constant throughout subsequent storage for 8 days at 10 degrees C, regardless of the temperature differential between tomatoes and the dip suspension. Storage of tomatoes at 20 degrees C, however, resulted in significant increases in populations of S. montevideo. Populations of the pathogen on the surfaces and in the core tissues of tomatoes were significantly reduced by dipping for 2 min in a solution containing 60 or 110 ppm (60 or 110 micrograms/ml) chlorine, respectively; however, treatment in solution containing 320 ppm chlorine did not result in complete inactivation. Populations of S. montevideo remained unchanged in chopped tomatoes stored at 5 degrees C for 216 h (9 days) but increased significantly after storage for 96 or 22 h at 20 or 30 degrees C, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Chlorine↗

Suitability of selective plating media for recovering heat- or freeze-stressed Escherichia coli O157:H7 from tryptic soy broth and ground beef.

The efficacy of tryptic soy agar (TSA), modified sorbitol MacConkey agar (MSMA), modified eosin methylene blue (MEMB) agar, and modified SD-39 (MSD) agar in recovering a five-strain mixture of enterohemorrhagic Escherichia coli O157:H7 and five non-O157 strains of E. coli heated in tryptic soy broth at 52, 54, or 56 degrees C for 10, 20, and 30 min was determined. Nonselective TSA supported the highest recovery of heated cells. Significantly (P < or = 0.05) lower recovery of heat-stressed cells was observed on MSMA than on TSA, MEMB agar, or MSD agar. The suitability of MEMB agar or MSD agar for recovery of E. coli O157:H7 from heated or frozen (-20 degrees C) low- or high-fat ground beef was determined. Recovery of E. coli O157:H7 from heated ground beef was significantly (P < or = 0.05) higher on TSA than on MEMB agar, which in turn supported higher recovery than MSD agar did; MSMA was inferior. Recovery from frozen ground beef was also higher on MEMB and MSD agars than on MSMA. Higher populations were generally recovered from high-fat beef than from low-fat beef, but the relative performance of the recovery media was the same. The inability of MSMA to recover stressed cells of E. coli O157:H7 underscores the need to develop a better selective medium for enumerating E. coli O157:H7.

Agar↗

Changes in fatty acid composition of various lipid components of Zygosaccharomyces rouxii as influenced by solutes, potassium sorbate and incubation temperature.

The effects of water activity (aw), solutes, potassium sorbate and incubation temperature on the fatty acid methyl ester composition of the total extracted lipid, triacylglycerol, phospholipid, free fatty acid and steryl ester components of Zygosaccharomyces rouxii were determined. Oleic (C18:1) and linoleic (C18:2) acids were the major components of the total fatty acid fraction, regardless of culturing conditions. Cells grown at 35 degrees C contained a significantly higher percentage of C18:1 and a lower percentage of C18:2 than did cells grown at 21 degrees C. At both temperatures, cells cultured in low-aw (0.93) media contained a significantly higher percentage of C18:1, with a corresponding decrease in C18:2 content than did cells grown at aw 0.99, regardless of the addition of glucose, sucrose, or NaCl to suppress the aw. At 21 degrees C, cells cultured in high-aw (0.99) media supplemented with 300 micrograms sorbate per ml contained higher percentages of C18:1 than did cells cultured in media not supplemented with sorbate. Cells grown at 35 degrees C in low-aw media supplemented with sorbate contained substantially more C18:1 than did cells cultured without sorbate.

Culture Media↗

Fermentation and sensory characteristics of kimchi containing potassium chloride as a partial replacement for sodium chloride.

A study was designed to determine the effects of substituting up to 50% of the NaCl in kimchi, a fermented Chinese cabbage (Brassica pekinensis) product containing scallions, garlic, ginger and hot red pepper powder, with KCl. Brine water (15% salt) used to soak cabbage contained NaCl:KCl ratios of 1:0 (control), 5:1, 2:1 and 1:1 (wt:wt). Total acidity and pH of kimchi reached acceptable ranges of 0.4-0.6% (as lactic acid) and 4.4-4.7, respectively, after 13 days of incubation at 13 +/- 1 degree C. Kimchi made using brine water containing 5:1 and 2:1 (NaCl:KCl) salt ratios was characterized by faster growth of lactic acid bacteria and total aerobic microorganisms compared to the control formulation. Sensory qualities (saltiness, bitterness, sourness, hotness and texture) of kimchi prepared from cabbage soaked in brine containing NaCl and KCl at all test ratios were judged to be acceptable.

Bacteria↗

Use of indirect conductimetry to predict the growth of spoilage yeasts, with special consideration of Zygosaccharomyces bailii.

In recent years, modeling for the purpose of predicting microbiological spoilage of foods has gained much interest. Predictive modeling requires a concentrated mathematical and experimental approach; to collect data of adequate quality is a technically demanding task when several experimental parameters are involved. Rapid, non-traditional, automated techniques are particularly useful in modeling. Of these, electrometric techniques appear to be most promising. Indirect conductimetry was used to study the effect of temperature, aw, pH and potassium sorbate concentration on the growth of Zygosaccharomyces bailii. The automated Malthus 2000 instrument proved to be convenient for gathering a large amount of data that were then used to develop polynomial models describing the response of the yeast to combinations of experimental factors in terms of conductimetric detection time and maximum rate of change in conductance. Results demonstrated that indirect conductimetry is suitable for monitoring the effect of environmental factors on the growth and activity of Z. bailii and perhaps other food spoilage yeasts.

Conductometry↗

Growth and verotoxin 1 production by Escherichia coli O157:H7 in ground roasted beef.

Production of verotoxin 1 (VT1) by hemorrhagic Escherichia coli O157:H7 in food has received limited research attention. A study was therefore conducted to determine the effect of temperature and pH, as achieved using two different acidulants, on VT1 production in ground roasted beef slurry. Slurries (pH 5.9) containing 33% beef and 67% water were incubated at 21 degrees C or 37 degrees C. Transfers were made at 48-h intervals for up to 18 days to determine if repeated subculturing influenced VT1 production. Populations of E. coli O157:H7 ranged from 8.9 x 10(8) to 1.9 x 10(9) CFU/ml within 48 h of transfer, regardless of incubation temperature. Maximum VT1 concentrations ranged from 61 to 63 ng/ml and 63 to 85 ng/ml of slurries incubated at 21 degrees C and 37 degrees C, respectively. The amount produced within 48 h at 37 degrees C was 15-24% higher than the amount produced at 21 degrees C and did not change after ten successive 48-h transfers. Upon changing the incubation temperature of slurry cultures adapted to 21 degrees C to an incubation temperature of 37 degrees C, VT1 production increased within 48 h to the level of cultures which had been previously adapted to 37 degrees C. A shift in temperature from 37 degrees C to 21 degrees C resulted in an initial reduction of about 55% in the amount of VT1 produced within 48 h. For studies on the combined effect of incubation temperature, pH and acidulant, slurries were adjusted to pH 5.4 with acetic and citric acids. Growth and production of VT1 in slurry acidified with acetic acid was markedly reduced compared to that in the control slurry (pH 5.9). The amount of VT1 detected in slurries receiving the second and third 24-h transfer of culture incubated at 21 degrees C was essentially nil. Growth and VT1 production was reduced in slurry acidified with citric acid compared to that observed in the control slurry but not as drastically as that observed in slurry acidified with acetic acid. VT1 concentration in unacidified beef slurry (pH 5.9) and in beef slurry acidified at pH 5.4 with citric acid reached 21 and 16 ng/ml, respectively, within 24 h at 21 degrees C. Results emphasize the need for proper sanitation procedures in beef processing and preparation facilities to reduce the risk of cross contamination of roasted beef and subsequent growth of E. coli O157:H7 and VT1 production.

Animals↗

Inactivation of Escherichia coli O157:H7, salmonellae, and Campylobacter jejuni in raw ground beef by gamma irradiation.

Raw ground beef patties inoculated with stationary-phase cells of Escherichia coli O157:H7, salmonellae, or Campylobacter jejuni were subjected to gamma irradiation (60Co) treatment, with doses ranging from 0 to 2.52 kGy. The influence of two levels of fat (8 to 14% [low fat] and 27 to 28% [high fat]) and temperature (frozen [-17 to -15 degrees C] and refrigerated [3 to 5 degrees C]) on the inactivation of each pathogen by irradiation was investigated. In ascending order of irradiation resistance, the D10 values ranged from 0.175 to 0.235 kGy (C. jejuni), from 0.241 to 0.307 kGy (E. coli O157:H7), and from 0.618 to 0.800 kGy (salmonellae). Statistical analysis revealed that E. coli O157:H7 had a significantly (P < 0.05) higher D10 value when irradiated at -17 to -15 degrees C than when irradiated at 3 to 5 degrees C. Regardless of the temperature during irradiation, the level of fat did not have a significant effect on the D10 value. Salmonellae behaved like E. coli O157:H7 in low-fat beef, but temperature did not have a significant effect when the pathogen was irradiated in high-fat ground beef. Significantly higher D10 values were calculated for C. jejuni irradiated in frozen than in refrigerated low-fat beef. C. jejuni was more resistant to irradiation in low-fat beef than in high-fat beef when treatment was at -17 to -15 degrees C. Regardless of the fat level and temperature during inactivation, these pathogens were highly sensitive to gamma irradiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

Selective media for detecting and enumerating foodborne yeasts.

No one medium is satisfactory for detecting, isolating and enumerating all yeasts in all foods. Antibiotic-supplemented media such as dichloran rose Bengal chloramphenicol agar, tryptone glucose yeast extract chloramphenicol agar, oxytetracycline glucose yeast extract agar and rose Bengal chloramphenicol agar are superior to acidified potato dextrose agar and other acidified media for enumeration of the vast majority of spoilage yeasts. Dichloran glycerol (18%) agar performs well for enumerating moderately xerotolerant yeasts. Malt extract yeast extract glucose (up to 60%) can be used for detecting and enumerating moderate and extreme xerophiles. These media also support the growth of moulds. Lysine agar, Schwarz differential agar and Lin's wild yeast differential agar are used by the brewing industry to differentiate wild yeasts from brewer's strains. Lysine agar is selective for apiculate yeasts and ethanol sulfite yeast extract agar is selective for Saccharomyces. Both have application in wineries. Modified molybdate agar can be used to selectively isolate yeasts from tropical fruits. Preservative-resistant yeasts can be detected on malt acetic agar. The recommended incubation temperature is 25 degrees C, but incubation time between plating and counting colonies ranges from 5 days for determination of general populations of yeasts to 10 days for more for xerotolerant yeasts. There is need for new and improved media for selectively isolating various groups, genera, species and strains of yeasts capable of growing only under specific environmental conditions in specific types of foods and beverages.

Bacteriological Techniques↗

Survival and growth of Escherichia coli O157:H7 on salad vegetables.

The influence of modified-atmosphere packaging, storage temperature, and time on survival and growth of Escherichia coli O157:H7 inoculated onto shredded lettuce, sliced cucumber, and shredded carrot was determined. Growth of psychotrophic and mesophilic microorganisms and changes in pH and sensory qualities of vegetables, as judged by subjective evaluation, were also monitored. Packaging under an atmosphere containing 3% oxygen and 97% nitrogen had no apparent effect on populations of E. coli O157:H7, psychotrophs, or mesophiles. Populations of viable E. coli O157:H7 declined on vegetables stored at 5 degrees C and increased on vegetables stored at 12 and 21 degrees C for up to 14 days. The most rapid increases in populations of E. coli O157:H7 occurred on lettuce and cucumbers stored at 21 degrees C. These results suggest that an unknown factor(s) associated with carrots may inhibit the growth of E. coli O157:H7. The reduction in pH of vegetables was correlated with initial increases in populations of E. coli O157:H7 and naturally occurring microfloras. Eventual decreases in E. coli O157:H7 in some samples, e.g., those stored at 21 degrees C, are attributed to the toxic effect of accumulated acids. Changes in visual appearance of vegetables were not influenced substantially by growth of E. coli O157:H7. The ability of E. coli O157:H7 to growth on raw salad vegetables subjected to processing and storage conditions simulating those routinely used in commercial practice has been demonstrated.

Escherichia coli↗

Survival and growth of Escherichia coli O157:H7 in ground, roasted beef as affected by pH, acidulants, and temperature.

A study was undertaken to determine the fate of Escherichia coli O157:H7 in ground, roasted beef as influenced by the combined effects of pH, acidulants, temperature, and time. There was essentially no change in the viable population of E. coli O157:H7 when beef salads (pH 5.40 to 6.07) containing up to 40% mayonnaise were incubated at 5 degrees C for up to 72 h. At 21 and 30 degrees C, significant (P < or = 0.05) increases in populations of the organism occurred in salads containing 16 to 32% mayonnaise (pH 5.94 to 5.55) between 10 and 24 h of incubation. Death was more rapid as the pH of acidified beef slurries incubated at 5 degrees C was decreased from 5.98 to 4.70. E. coli O157:H7 grew in control slurries (pH 5.98) and in slurries containing citric and lactic acids (pHs 5.00 and 5.40) incubated at 21 degrees C for 24 h; decreases occurred in slurries acidified to pHs 4.70, 5.00, and 5.40 with acetic acid or pH 4.70 with citric or lactic acid. At 30 degrees C, populations decreased in slurries acidified to pHs 4.70 and 5.00 with acetic acid. Citric and lactic acids failed to prevent significant increases in populations in slurries at pH 4.70 to 5.40 between 10 and 24 h of incubation. The order of effectiveness of acidulants in inhibiting growth was acetic acid > lactic acid > or = citric acid. The same order was observed for inactivation of E. coli O157:H7 in acidified (pH 5.00) beef slurry heated at 54 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Acids↗

Media for detecting and enumerating yeasts and moulds.

Dilution plating techniques are designed to determine populations of viable fungal propagules per unit weight or volume of food. Direct plating techniques, on the other hand, are designed to assess the internal mycoflora of individual pieces of foods, e.g., seeds or dried fruits, and results are expressed as a percentage of infected pieces. Both techniques are used by industry and regulatory agencies to monitor levels of fungal contamination at various stages of food handling, storing, processing and marketing. Peptone (0.1%) water is commonly used as a diluent for samples to be homogenized or blended. Buffered diluents containing up to 30% glucose or 60% sucrose are recommended for enumerating xerophiles. No one medium is satisfactory for detection or enumeration of yeasts and moulds in all foods. Dichloran rose bengal chloramphenicol agar, oxytetracycline glucose yeast extract agar and rose bengal chloramphenicol agar are superior to acidified potato dextrose agar for enumeration of yeasts and moulds. Dichloran 18% glycerol agar performs well for enumerating moderately xerophilic yeasts and moulds. Fastidious xerophiles require media containing high concentrations of sugars and/or sodium chloride. Media have been formulated to detect potentially aflatoxigenic aspergilli and mycotoxigenic strains of penicillia and fusaria, but increased selectivity and specificity of media for detecting mycotoxigenic moulds are needed. Heat-resistant mould ascospores often require heat treatment prior to plating in order to activate the germination process. The spread-plate technique is strongly preferred over the pour-plate technique for enumerating yeasts and moulds. The recommended incubation temperature is 25 degrees C, but incubation time between plating and counting colonies ranges from 5 days for determination of general populations of mycoflora to 4 weeks or more for fastidious xerophiles. There is a need for new and improved media for selectively isolating various groups, genera, species and/or strains of fungi capable of growing only under specific environmental conditions, e.g., low aw or, in the case of sublethally injured cells, under conditions which facilitate resuscitation. Improved media are needed which accurately detect moulds producing specific mycotoxins in a wide range of food types.

Colony Count, Microbial↗

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.

Adaptation, Biological↗

Evaluation of Chemicals for Restricting Colony Spreading by a Xerophilic Mold, Eurotium amstelodami, on Dichloran-18% Glycerol Agar.

Twenty chemicals were screened for their effectiveness in restricting colony spreading by four strains of a xerophilic mold, Eurotium amstelodami, on dichloran-18% glycerol agar. Triton X-100, Triton X-301, Tergitol NP-7, and Tergitol 15-S-3 (each at 200 mug/ml) and 1,000 mug of sodium deoxycholate, 1 mug of iprodione, 0.1 mug of propiconazole, and 0.01 mug of Maxim per ml were judged to be most effective for restricting the rate of colony spreading.

Journal Article↗

Effects of potassium sorbate on growth patterns, morphology, and heat resistance of Zygosaccharomyces rouxii at reduced water activity.

A study was made of the effects of potassium sorbate on growth, morphology, and heat sensitivity of an osmotolerant yeast, Zygosaccharomyces rouxii, grown in media (water activity (aw) 0.93) supplemented with glucose and sucrose. Growth patterns of Z. rouxii in YM broth supplemented with glucose (YMBG) and sucrose (YMBS) were similar, although increased potassium sorbate concentration in both media resulted in decreased growth rates. Growth in YMBS containing potassium sorbate was not as prolific as that in YMBG containing potassium sorbate. Inhibition of growth was indicated by decreased absorbance (at 600 nm) of cells grown in YMBS and in YMBG and YMBS supplemented with potassium sorbate at 600 or 1000 micrograms/mL. Slight decreases in cell size and alteration of cellular morphology were associated with increased potassium sorbate concentration. Plasmolysis increased as potassium sorbate concentration was elevated in YMBS but not in YMBG. Tolerance of Z. rouxii to potassium sorbate was enhanced by previous adaptation of cells in media with elevated potassium sorbate concentrations. Heat resistance of cells unadapted to potassium sorbate showed little or no increase regardless of culture age, but increased substantially in cells grown in media containing potassium sorbate, particularly YMBS.

Culture Media↗

Behavior of Aeromonas species at refrigeration temperatures.

The ability of many strains of Aeromonas hydrophila and A. sobria to produce several types of virulence factors has been documented. The presence of Aeromonas in drinking water, as well as in river and saline waters and on various finfish and shellfish taken from them, has caused some concern relative to the role this bacterium plays as a causative agent of human gastroenteritis. The fairly common occurrence of Aeromonas on red meats, poultry and fresh produce and its ability to grow at 4 degrees C gives rise to further concern over public health risks which may be associated with consumption of these foods. A brief overview of the behavior of Aeromonas species at refrigeration temperatures is presented.

Aeromonas↗