Survival of Salmonella typhimurium and Escherichia coli O157:H7 in cheese brines

Survival of Salmonella typhimurium and Escherichia coli O157:H7 was studied in model brines and brine from three cheese plants. Three strain mixtures of S. typhimurium and E. coli O157:H7 (10(6) CFU/ml) were inoculated separately into 23% model brine with or without added pasteurized whey (2%) and as a combined inoculum into the commercial brines. The model brines were incubated at 8 and 15 degrees C for 28 days, and the commercial brines at 4 and 13 degrees C for 35 days. Populations of both pathogens in the model brine + whey decreased slowly over 28 days (1.0-2.0 log CFU/ml) with greater survival at 8 degrees C than at 15 degrees C. Corresponding decreases in model brine without whey were 1.9-3.0 log CFU/ml, with greater survival at 8 degrees C than at 15 degrees C. Both S. typhimurium and E. coli O157:H7 survived significantly better (P < 0.05) at 4 degrees C than at 13 degrees C in two of the commercial brines. The survival of each pathogen in the commercial brines at 13 degrees C was significantly influenced by brine pH. Both pathogen populations decreased most rapidly in commercial brines during the first week of storage (2.5-4.0 and 2.3-2.8 log CFU/ml for S. typhimurium and E. coli O157:H7, respectively) with significant recovery (ca. 0.5 log CFU/ml increase) often occurring in the second week of storage. Counts changed little thereafter. Overall, E. coli O157:H7 survived better than S. typhimurium, with differences of 0.1-1.2 log CFU/ml between the two pathogens. Results of this study show that cheese brine could support the survival of contaminating S. typhimurium and E. coli O157:H7 for several weeks under typical brining conditions.     

Survival of Salmonella and Escherichia coli O157:H7 in chorizos

Mexican-style raw meat sausages (chorizos) are not regulated in California when they are produced in small ethnic food markets. These sausages are sold uncooked, but their formulation imparts a color that may lead the consumer to assume that they are already cooked, and thus the chorizos may sometimes be eaten without proper cooking. If pathogens are present in such cases, illness may result. Survival of Salmonella and Escherichia coli O157:H7 in chorizos was evaluated under different storage conditions selected based on an initial survey of uninspected chorizos in California. Chorizos were formulated with five different initial water activity (aw) values (0.85, 0.90, 0.93, 0.95, and 0.97), stored under four conditions (refrigeration at 6 to 8 degrees C, room temperature at 24 to 26 degrees C, under a hood at 24 to 26 degrees C with forced air circulation, and incubation at 30 to 31 degrees C with convective air circulation), and sampled after 1, 2, 4, and 7 days. The initial pH was 4.8 and remained near 5.0 from day 1 of the sampling period. Two separate studies of packs inoculated with five-strain cocktails of Salmonella and of E. coli O157:H7 were performed twice for each initial aw. The three lowest aw values (0.85, 0.90, and 0.93) and the incubation and hood storage conditions were more effective (P < or = 0.05) at reducing the target pathogen levels in chorizos than were the two highest aw values (0.95 and 0.97) and the refrigeration storage condition, regardless of storage time. These results provide a scientific basis for guidelines given to producers of uninspected chorizo and should reduce the probability of foodborne illness associated with these products.     

大肠杆菌O157:H7的冷冻损伤及解冻存活

为探讨冷冻后残存的大肠杆菌O157:H7（Escherichia coli O157:H7）在解冻后的存活情况,本研究首先比较4 株E. coli O157:H7冷冻后的死亡和损伤情况,进而采用无营养的磷酸盐缓冲液作为基质研究冷冻后不同解冻方式对E. coli O157:H7存活的影响。结果表明：4 株E. coli O157:H7 －20 ℃冷冻24、48、72 h后均发生了一定程度的死亡和损伤,冷冻时间越长细菌致死和致伤程度越明显,且存在菌株差异,冷冻72?h时菌株CICC21530的损伤率最高,为87.70%。采用混合菌株进行解冻实验,4?株E.?coli?O157:H7磷酸盐缓冲液菌液冷冻后立即置于20、30、37?℃解冻,细菌发生了进一步的死亡,解冻温度越高死亡越明显,3?个温度组在解冻48?h时菌落数均显著低于冷冻72?h时菌落数（P＜0.05）。进一步探讨缓慢解冻方式对菌体存活的影响,菌液冷冻后先置于4?℃一定时间（0、2、6、12?h）,再置于37?℃不同时间（5、10、30?min）观察菌株存活情况,结果表明4?℃缓慢解冻时间越长,越有利于细菌的存活,4?℃、12?h/37?℃、5～30?min解冻方式下改良山梨醇麦康凯琼脂上菌落数仍显著低于胰蛋白胨大豆琼脂上的菌落数（P＜0.05）,表明仍有损伤菌的存在。本实验提示采用缓慢解冻反而有利于残存菌的存活,冷冻食品风险评估时应重视残存菌尤其是损伤菌的检测和控制。     

Survival of a five-strain cocktail of Escherichia coli O157:H7 during the 60-day aging period of cheddar cheese made from unpasteurized milk

The U.S. Food and Drug Administration Standard of Identity for Cheddar cheeses requires pasteurization of the milk, or as an alternative treatment, a minimum 60-day aging at > or =2 degrees C for cheeses made from unpasteurized milk, to reduce the number of viable pathogens that may be present to an acceptable risk. The objective of this study was to investigate the adequacy of the 60-day minimum aging to reduce the numbers of viable pathogens and evaluate milk subpasteurization heat treatment as a process to improve the safety of Cheddar cheeses made from unpasteurized milk. Cheddar cheese was made from unpasteurized milk inoculated with 10(1) to 10(5) CFU/ml of a five-strain cocktail of acid-tolerant Escherichia coli O157:H7. Samples were collected during the cheese manufacturing process. After pressing, the cheese blocks were packaged into plastic bags, vacuum sealed, and aged at 7 degrees C. After 1 week, the cheese blocks were cut into smaller-size uniform pieces and then vacuum sealed in clear plastic pouches. Samples were plated and enumerated for E. coli O157:H7. Populations of E. coli O157:H7 increased during the cheese-making operations. Population of E. coli O157:H7 in cheese aged for 60 and 120 days at 7 degrees C decreased less than 1 and 2 log, respectively. These studies confirm previous reports that show 60-day aging is inadequate to eliminate E. coli O157:H7 during cheese ripening. Subpasteurization heat-treatment runs were conducted at 148 degrees F (64.4 degrees C) for 17.5 s on milk inoculated with E. coli O157:H7 at 10(5) CFU/ml. These heat-treatment runs resulted in a 5-log E. coli O157: H7 reduction.     

Survival of Escherichia coli O157:H7 in Galotyri cheese stored at 4 and 12 degrees C

Post-process contamination of fresh acid-curd cheeses with Escherichia coli O157:H7 may pose a risk considering the low infectious dose and the ability of the pathogen to survive in acidic foods. To evaluate its survival in Galotyri, a traditional Greek acid-curd cheese, portions (0.5 kg) of two commercial fresh products, one artisan (pH 3.9+/-0.1) and the other industrial (pH 3.7+/-0.1), were inoculated with approximately 3.0 or 6.5 log cfu g(-1) of a five-strain cocktail of E. coli O157:H7, including rifampicin-resistant derivatives of the strains ATCC 43895 and ATCC 51657, and stored aerobically at 4 and 12 degrees C. Survival was monitored for 28 days by plating cheese samples on tryptic soy agar with 100 mg l(-1) rifampicin (TSA+Rif), SMAC and Fluorocult E. coli O157:H7 agar media. The pathogen declined much faster (P<0.05) in the industrial as compared to the artisan cheeses at both temperatures. Thus, while E. coli O157:H7 became undetectable by culture enrichment after 14 days at 4 degrees C in industrial samples, irrespective of the inoculation level, populations of 1.4-1.9 and 4.2-5.1 log cfu g(-1) survived after 28 days in the corresponding artisan cheeses with the low and high inocula, respectively. Survival was longer and greater (P<0.05) on TSA+Rif than on SMAC and Fluorocult, indicating the presence of acid-injured cells. Interestingly, survival of E. coli O157:H7 after 14-28 days in cheeses was better at 12 degrees C than at 4 degrees C, probably due to yeasts which grew on the surface of temperature-abused cheeses. The large difference in the pathogen's inactivation between the industrial and artisan cheeses at 4 degrees C could not be associated with major differences in pH or type/concentration of organic acids, suggesting another anti-E. coli O157:H7 activity by the industrial starter. The high survival of the pathogen in artisan Galotyri under conditions simulating commercial storage should be of concern.     

Survival of Escherichia coli O157:H7 in cucumber fermentation brines

Abstract: Bacterial pathogens have been reported on fresh cucumbers and other vegetables used for commercial fermentation. The Food and Drug Administration currently has a 5‐log reduction standard for E. coli O157:H7 and other vegetative pathogens in acidified pickle products. For fermented vegetables, which are acid foods, there is little data documenting the conditions needed to kill acid resistant pathogens. To address this knowledge gap, we obtained 10 different cucumber fermentation brines at different stages of fermentation from 5 domestic commercial plants. Cucumber brines were used to represent vegetable fermentations because cabbage and other vegetables may have inhibitory compounds that may affect survival. The 5‐log reduction times for E. coli O157:H7 strains in the commercial brines were found to be positively correlated with brine pH, and ranged from 3 to 24 d for pH values of 3.2 to 4.6, respectively. In a laboratory cucumber juice medium that had been previously fermented with Lactobacillus plantarum or Leuconostoc mesenteroides (pH 3.9), a 5‐log reduction was achieved within 1 to 16 d depending on pH, acid concentration, and temperature. During competitive growth at 30 °C in the presence of L. plantarum or L. mesenteroides in cucumber juice, E. coli O157:H7 cell numbers were reduced to below the level of detection within 2 to 3 d. These data may be used to aid manufacturers of fermented vegetable products determine safe production practices based on fermentation pH and temperature. Practical Application: Disease causing strains of the bacterium E. coli may be present on fresh vegetables. Our investigation determined the time needed to kill E. coli in cucumber fermentation brines and how E. coli strains are killed in competition with naturally present lactic acid bacteria. Our results showed how brine pH and other brine conditions affected the killing of E. coli strains. These data can be used by producers of fermented vegetable products to help assure consumer safety.     

Survival of enterohemorrhagic Escherichia coli O157:H7 in retail mustard

Escherichia coli O157:H7 survival in acid foods such as unpasteurized apple cider and fermented sausage is well documented. Researchers have determined that E. coli O157:H7 can survive in refrigerated acid foods for weeks. The potential of acid foods to serve as a vector of E. coli O157:H7 foodborne illness prompted this study to determine the fate of this organism in retail mustard containing acetic acid when stored at room and refrigerated temperatures. Various retail brands of dijon, yellow, and deli style mustard, pH ranging from 3.17 to 3.63, were inoculated individually with three test strains of E. coli O157:H7. Samples were inoculated with approximately 1.0 x 10(6) CFU/g, incubated at room (25+/-2.5 degrees C) and refrigerated (5+/-3 degrees C) temperatures, and assayed for surviving test strains at predetermined time intervals. An aliquot was appropriately diluted and plated using sorbitol MacConkey agar (SMAC). When the test strain was not recoverable by direct plating, the sample was assayed by enrichment in modified tryptic soy broth and recovered using SMAC. Growth of E. coli O157:H7 test strains was inhibited in all retail mustard styles. E. coli O157:H7 was not detected in dijon style mustard beyond 3 h at room and 2 days at refrigerated temperatures. Survival in yellow and deli style mustard was not detected beyond 1 h. Overall, test strain survival was greater at refrigerated than room temperature. Retail mustard demonstrated the ability to eliminate effectively any chance contamination by this organism within hours to days, suggesting that these products are not a likely factor in E. coli O157:H7 foodborne illness.     

Efficacy of ultraviolet light for reducing Escherichia coli O157:H7 in unpasteurized apple cider

This study examined the efficacy of UV light for reducing Escherichia coli O157:H7 in unpasteurized cider. Cider containing a mixture of acid-resistant E. coli O157:H7 (6.3 log CFU/ml) was treated using a thin-film UV disinfection unit at 254 nm. Dosages ranged from 9,402 to 61,005 microW-s/cm2. Treatment significantly reduced E. coli O157:H7 (P < or = 0.0001). Mean reduction for all treated samples was 3.81 log CFU/ml. Reduction was also affected by the level of background microflora in cider. Results indicate that UV light is effective for reducing this pathogen in cider. However, with the dosages used in this experiment, additional reduction measures are necessary to achieve the required 5-log reduction.     

Survival of Escherichia coli O157:H7 in traditional African yoghurt fermentation

Growth and survival of a nontoxigenic strain of Escherichia coli O157:H7 (ATCC 43888) was determined in traditionally fermented pasteurized milk. Preheated milk was inoculated with 1% (v/v) of a mixed culture of Lactobacillus delbrueckii ssp. bulgaricus (NCIMB 11778) and Streptococcus salivarius ssp. thermophilus (NCIMB 110368) and incubated at 25, 30, 37 or 43 degrees C for 24 h. E. coli O157:H7 (10(5) CFU/ml) were introduced into the milk pre- and post-fermentation. Fermented milk samples were subsequently stored at either 4 degrees C (refrigerator temperature) or 25 degrees C (to mimic African ambient temperature) for 5 days. After 24 h of fermentation, the pH of the samples fermented at the higher temperatures of 37-43 degrees C decreased from 6.8 to 4.4-4.0 ( +/- 0.2) whereas at the lower temperature of 25 degrees C, the pH decreased to pH 5.0 +/- 0.1. During this period, viable counts for E. coli O157:H7 increased from 10(5) to 10(8) - 10(9) CFU/ml except in milk fermented at 43 degrees C wherein viability declined to 10(4) CFU/ml. In fermented (25-30 degrees C) milk stored at 4 degrees C for 5 days, E. coli O157:H7 viability decreased from 10(8-9) to 10(6-7) CFU/ml whereas milk fermented at 43 degrees C resulted in loss of detectable cells. In contrast, storage of fermented milk samples at 25 degrees C for 5 days eventually resulted in complete loss of viability irrespective of fermentation temperature. Stationary phase E. coli O157:H7 inoculated post-fermentation (25 and 43 degrees C) survived during 4 degrees C storage, but not 25 degrees C storage. Fermentation temperature and subsequent storage temperature are critical to the growth and survival of E. coli O157:H7 in traditional fermented products involving yoghurt starter cultures.     

Survival and growth of Listeria monocytogenes and enterohemorrhagic Escherichia coli O157:H7 in minimally processed artichokes

The ability of Listeria monocytogenes and Escherichia coli O157:H7 inoculated by immersion (at 4.6 and 5.5 log CFU/ g, respectively) to survive on artichokes during various stages of preparation was determined. Peeling, cutting, and disinfecting operations (immersion in 50 ppm of a free chlorine solution at 4 degrees C for 5 min) reduced populations of L. monocytogenes and E. coli O157:H7 by only 1.6 and 0.8 log units, respectively. An organic acid rinse (0.02% citric acid and 0.2% ascorbic acid) was more effective than a tap water rinse in removing these pathogens. Given the possibility of both pathogens being present on artichokes at the packaging stage, their behavior during the storage of minimally processed artichokes was investigated. For this purpose, batches of artichokes inoculated with L. monocytogenes or E. coli O157:H7 (at 5.5 and 5.2 log CFU/g, respectively) were packaged in P-Plus film bags and stored at 4 degrees C for 16 days. During this period, the equilibrium atmosphere composition and natural background microflora (mesophiles, psychrotrophs, anaerobes, and fecal coliforms) were also analyzed. For the two studied pathogens, the inoculum did not have any effect on the final atmospheric composition (10% O2, 13% CO2) or on the survival of the natural background microflora of the artichokes. L. monocytogenes was able to survive during the entire storage period in the inoculated batches, while the E. coli O157:H7 level increased by 1.5 log units in the inoculated batch during the storage period. The modified atmosphere was unable to control the behavior of either pathogen.     

Survival of Listeria monocytogenes and Escherichia coli O157:H7 during sauerkraut fermentation

Sauerkraut was produced from shredded cabbage, as is typical in the United States, and from whole head cabbages, which is a traditional process in parts of Eastern Europe. The sauerkraut was inoculated with five strain mixtures of Escherichia coli O157:H7 and Listeria monocytogenes, and the populations of these bacteria, as well as lactic acid bacteria, pH, and titratable acidity, were monitored over the course of fermentation. Fermentation variables were temperature (18 and 22 degrees C) and salt concentration (1.8, 2.25, and 3%). For most of the analyses, the type of cabbage processing was a significant factor, although within cabbage type, neither salt nor fermentation temperature had significant effects. The final pH of the whole-head sauerkraut was lower than the shredded sauerkraut, but the titratable acidity was significantly higher in the shredded sauerkraut. E. coli O157:H7 and L. monocytogenes persisted in the brines for most of the fermentation, although at the end of the fermentations (15 days for shredded, 28 days for whole head), neither pathogen had detectable populations. E. coli populations decreased more rapidly in the shredded sauerkraut even though the pH was higher because of the higher total acidity in the shredded sauerkraut. Acid-tolerant strains of E. coli and L. monocytogenes were isolated from both shredded and whole-head sauerkraut at different salt concentrations and temperatures after 15 days of fermentation and could be detected at 35 days in the wholehead sauerkraut.     

Survival of Escherichia coli O157:H7 in meat product brines containing antimicrobials

Brine solution injection of beef contaminated with Escherichia coli O157:H7 on its surface may lead to internalization of pathogen cells and/or cross-contamination of the brine, which when recirculated, may serve as a source of new product contamination. This study evaluated survival of E. coli O157:H7 in brines formulated without or with antimicrobials. The brines were formulated in sterile distilled water (simulating the composition of freshly prepared brines) or in a nonsterile 3% meat homogenate (simulating the composition of recirculating brines) at concentrations used to moisture-enhance meat to 110% of initial weight, as follows: sodium chloride (NaCl, 5.5%) + sodium tripolyphosphate (STP, 2.75%), NaCl + sodium pyrophosphate (2.75%), or NaCl + STP combined with potassium lactate (PL, 22%), sodium diacetate (SD, 1.65%), PL + SD, lactic acid (3.3%), acetic acid (3.3%), citric acid (3.3%), nisin (0.0165%) + ethylenediamine tetraacetic acid (EDTA, 200 mM), pediocin (11000 AU/mL) + EDTA, sodium metasilicate (2.2%), cetylpyridinium chloride (CPC, 5.5%), or hops beta acids (0.0055%). The brines were inoculated (3 to 4 log CFU/mL) with rifampicin-resistant E. coli O157:H7 (8-strain composite) and stored at 4 or 15 °C (24 to 48 h). Immediate (0 h) pathogen reductions (P < 0.05) of 1.8 to ≥ 2.4 log CFU/mL were observed in brines containing CPC or sodium metasilicate. Furthermore, brines formulated with lactic acid, acetic acid, citric acid, nisin + EDTA, pediocin + EDTA, CPC, sodium metasilicate, or hops beta acids had reductions (P < 0.05) in pathogen levels during storage; however, the extent of pathogen reduction (0.4 to > 2.4 log CFU/mL) depended on the antimicrobial, brine type, and storage temperature and time. These data should be useful in development or improvement of brine formulations for control of E. coli O157:H7 in moisture-enhanced meat products. PRACTICAL APPLICATION: Results of this study should be useful to the meat industry for developing or modifying brine formulations to reduce the risk of E. coli O157:H7 in moisture-enhanced meat products.     

Risk assessment for Escherichia coli O157:H7 in marketed unpasteurized milk in selected East African countries

We carried out a study to assess the risk associated with the presence of Shiga toxigenic Escherichia coli (STEC) in informally marketed unpasteurized milk in urban East Africa. Data for the risk models were obtained from on-going and recently completed studies in Kenya and Uganda. Inputs for the model were complemented with data from published literature in similar populations. A fault-tree scenario pathway and modular process risk model approach were used for exposure assessment. Hazard characterization was based on a socioeconomic study with dose-responses derived from the literature. We used a probabilistic approach with Monte Carlo simulation and inputs from farm and household surveys. The qualitative analysis suggested a low to moderate risk of infection from consuming milk and that the widespread consumer practice of boiling milk before consumption was an important risk mitigator. Quantitative analysis revealed that two to three symptomatic STEC infections could be expected for every 10,000 unpasteurized milk portions consumed, with a possible range of 0 to 22 symptomatic cases. Sensitivity analyses to assess the uncertainty and variability associated with the model revealed that the factor with the greatest influence on disease incidence was the prevalence of STEC in dairy cattle. Risk assessment is a potentially useful method for managing food safety in informal markets.     

Destruction of Escherichia coli O157:H7 by vanillic acid in unpasteurized juice from six apple cultivars

The behavior of Escherichia coli O157:H7 in Granny Smith, Gala, Empire, McIntosh, Red Delicious, and Golden Delicious apple juice with or without supplementation with 5 or 10 mM vanillic acid was examined over a storage period of 7 days at 4 and 15 degrees C. The consequences of supplementation on sensory difference and preference were also determined by triangle testing. Juices made from the six apple cultivars had pH values ranging between pH 3.13 and 3.92. Vanillic acid exerted a concentration, pH, and time-dependent lethal effect toward E. coli O157:H7 in unpasteurized apple juice. Supplementation with 10 mM vanillic acid led to a 5-logarithm reduction in populations after 7 days at both temperatures, but sensory analysis revealed significant differences from and preference for unsupplemented juices. Supplementation with 5 mM vanillic acid accelerated death of E. coli O157:H7, but population reductions ranged from 5 log CFU/ml in low pH juices to none in high pH juices, particularly at 4 degrees C. No sensory difference or preference was detected in two of the six juices at this level of supplementation.     

Colicin concentrations inhibit growth of Escherichia coli O157:H7 in vitro

Escherichia coli O157:H7 is a virulent foodborne pathogen that causes severe human illness and inhabits the intestinal tract of food animals. Colicins are antimicrobial proteins produced by E. coli strains that inhibit or kill other E. coli. In the present Study, the efficacy of three pore-forming colicins (El, N, and A) were quantified in vitro against E. coli O157:H7 strains 86-24 and 933. Colicins E1 and N reduced the growth of E. coli O157:H7 strains, but the efficacy of each colicin varied among strains. Colicin E1 was more effective against both strains of E. coli O157:H7 than colicins A and N and reduced (P < 0.05) populations of E. coli O157:H7 at concentrations <0.1 microg/ml. These potent antimicrobial proteins may potentially provide an effective and environmentally sound preharvest strategy to reduce E. coli O157:H7 in food animals.     

Survival of Escherichia coli O157:H7 during storage in pressure-treated orange juice.

The effect of a high-pressure treatment on the survival of a pressure-resistant strain of Escherichia coli O157:H7 (NCTC 12079) in orange juice during storage at 3 degrees C was investigated over the pH range of 3.4 to 5.0. The pH of shelf-stable orange juice was adjusted to 3.4, 3.6, 3.9, 4.5, and 5.0 and inoculated with 10(8) CFU ml(-1) of E. coli O157:H7. The orange juice was then pressure treated at 400 MPa for 1 min at 10 degrees C or was held at ambient pressure (as a control). Surviving E. coli O157:H7 cells were enumerated at 1-day intervals during a storage period of 25 days at 3 degrees C. Survival of E. coli O157:H7 during storage was dependent on the pH of the orange juice. The application of high pressure prior to storage significantly increased the susceptibility of E. coli O157:H7 to high acidity. For example, after pressure treatment, the time required for a 5-log decrease in cell numbers was reduced from 13 to 3 days at pH 3.4, from 16 to 6 days at pH 3.6, and from >25 to 8 days at pH 3.9. It is evident that the use of high-pressure processing of orange juice in order to increase the juice's shelf-life and to inactivate pathogens has the added advantage that it sensitizes E. coli O157:H7 to the high acid conditions found in orange juice, which results in the survival of significantly fewer E. coli O157:H7 during subsequent refrigerated storage.     

Escherichia coli O157:H7 and its significance in foods

Escherichia coli O157:H7 was conclusively identified as a pathogen in 1982 following its association with two food-related outbreaks of an unusual gastrointestinal illness. The organism is now recognized as an important cause of foodborne disease, with outbreaks reported in the U.S.A., Canada, and the United Kingdom. Illness is generally quite severe, and can include three different syndromes, i.e., hemorrhagic colitis, hemolytic uremic syndrome, and thrombotic thrombocytopenic purpura. Most outbreaks have been associated with eating undercooked ground beef or, less frequently, drinking raw milk. Surveys of retail raw meats and poultry revealed E. coli O157:H7 in 1.5 to 3.5% of ground beef, pork, poultry, and lamb. Dairy cattle, especially young animals, have been identified as a reservoir. The organism is typical of most E. coli, but does possess distinguishing characteristics. For example, E. coli O157:H7 does not ferment sorbitol within 24 h, does not possess beta-glucuronidase activity, and does not grow well or at all at 44-45.5 degrees C. The organism has no unusual heat resistance; heating ground beef sufficiently to kill typical strains of salmonellae will also kill E. coli O157:H7. The mechanism of pathogenicity has not been fully elucidated, but clinical isolates produce one or more verotoxins which are believed to be important virulence factors. Little is known about the significance of pre-formed verotoxins in foods. The use of proper hygienic practices in handling foods of animal origin and proper heating of such foods before consumption are important control measures for the prevention of E. coli O157:H7 infections.     

Survival of Escherichia coli O157:H7 during Manufacture and Storage of White Brined Cheese

Escherichia coli O157:H7 is a major foodborne pathogen that causes severe disease in humans. Survival of E. coli O157:H7 during processing and storage of white brined cheese was investigated. Cheeses were prepared using pasteurized milk inoculated with a 4 strain E. coli O157:H7 cocktail (7 log₁₀ CFU/g) with or without yogurt starter culture (Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus salivarius ssp. thermophilus) and stored in 10% or 15% NaCl brine at 10 and 21 ºC for 28 d. NaCl concentration, water activity (a_w), pH, and numbers of E. coli O157:H7 and lactic acid bacteria (LAB) were determined in cheese and brine. E. coli O157:H7 was able to survive in cheese stored in both brines at 10 and 21 ºC regardless of the presence of starter LAB, although the latter significantly enhanced E. coli O157:H7 reduction in cheese or its brine at 10 ºC. E. coli O157:H7 numbers were reduced by 2.6 and 3.4 log₁₀ CFU/g in cheese stored in 10% and 15% NaCl brine, respectively, in the presence of starter LAB and by 1.4 and 2.3 log₁₀ CFU/g, respectively, in the absence of starter LAB at 10 ºC. The pathogen survived, but at lower numbers in the brines. The salt concentration of cheese stored in 10% brine remained about 5% during ripening, but in 15% brine, the NaCl level increased 1.6% to 8.1% (w/w) by 28 d. Values of pH and a_w slightly decreased 1 d after exposure to brine and reached 5.5 to 6.6 and 0.88 to 0.94, respectively, in all treatments.     

失活／存活模型在大肠杆菌O157：H7风险分析中的应用

目的研究出口分割鸡肉中大肠杆菌O157：H7的控制措施。方法建立了出口分割鸡肉中大肠杆菌O157：H7在速冻过程中的失活模型和在-18℃下的存活模型,通过风险分析提出控制措施。结果利用所建模型定量描述了出口分割鸡肉从成品到消费过程中大肠杆菌O157：H7带菌量的变化,得出摄入1份鸡肉感染大肠杆菌O157：H7病的风险。结论本研究提出的控制措施可提高出口分割鸡肉的安全性。