Food safety and inspection service regulatory testing program for Escherichia coli O157:H7 in raw ground beef

We analyzed raw ground beef testing data to determine whether a decrease in the rate of Escherichia coli O157:H7-positive raw ground beef samples has occurred since the inception of Food Safety and Inspection Service (U.S. Department of Agriculture) regulatory actions and microbiological testing concerning this commodity and pathogen. A main effects log-linear Poisson regression model was constructed to evaluate the association between fiscal year and the rate of E. coli O157:H7-positive raw ground beef samples while controlling for the effect of season for the subset of test results obtained from fiscal year (FY)2000 through FY2003. Rate ratios were used to compare the rate of E. coli O157:H7-positive raw ground beef samples between sequential years to identify year-to-year differences. Of the 26,521 raw ground beef samples tested from FY2000 through FY2003, 189 (0.71%) tested positive for E. coli O157:H7. Year-to-year comparisons identified a 50% reduction in the rate of positive ground beef samples from FY2002 to FY2003 when controlling for season (95% CI, 10 to 72% decrease; P = 0.02). This decrease was the only significant year-to-year change in the rate of E. coli O157:H7-positive raw ground beef samples but was consistent in samples obtained from both federally inspected establishments and retail outlets. We believe this decrease is attributed to specific regulatory actions by Food Safety and Inspection Service and subsequent actions implemented by the industry, with the goal of reducing E. coli O157:H7 adulteration of raw ground beef. Continued monitoring is necessary to confirm that the decrease in the rate of E. coli O157:H7 in raw ground beef samples we observed here represents the beginning of a sustained trend.     

Growth of Escherichia coli O157:H7 in raw ground beef stored at 10 degrees C and the influence of competitive bacterial flora,strain variation,and fat level

Pure-culture broth-based models of the growth of Escherichia coli O157:H7 have been used to estimate its behavior in ground beef, even though these models have not been adequately validated for this food product. This situation limits accurate estimates of the behavior of E. coli O157:H7 in ground beef and introduces uncertainties in risk assessments. In the present study, the growth of single and multiple strains of E. coli O157:H7 were measured in retail ground beef stored at 10 degrees C for up to 12 days, and the results were compared with estimates generated by the U.S. Department of Agriculture's Pathogen Modeling Program (PMP; version 5.1). At pH 5.9, the PMP predicted a maximum population density (MPD) of 9.13 log10 CFU/g, an exponential growth rate (EGR) of 0.052 log10 CFU/h, and a lag time of 56.3 h. Similar parameter values were observed for sterilized ground beef; however, no lag phase was observed. In contrast, the mean MPD and EGR for retail ground beef were 5.09 log10 CFU/g and 0.019 log10 CFU/h, respectively, and no lag phase was observed. Both the EGR and the MPD increased with decreasing fat levels. There was low variation in the MPD and EGR parameters for the nine E. coli O157:H7 ground beef isolates. Two isolates of competitive native flora were separately added to sterilized ground beef, and the EGR and MPD decreased as the ratio of competitive flora to E. coli O157:H7 increased. For one strain, at ratios of 1:1, 10:1, and 100:1, the EGRs were 0.033, 0.025, and 0.018 log10 CFU/h, respectively, and the MPDs were 6.14, 5.08, and 4.84 log10 CFU/g, respectively. These results demonstrate that existing broth-based models for E coli O157:H7 must be validated for food and that models should consider the effects of the food matrix, the competitive microflora, and potential pathogen strain variation.     

Distribution patterns of Escherichia coli O157:H7 in ground beef produced by a laboratory-scale grinder

This study determined the distribution patterns of Escherichia coli O157:1H7 in ground beef when a contaminated beef trim was introduced into a batch of uncontaminated beef trims prior to grinding in a small-scale laboratory grinder. A beef trim (15.3 +/- 2 g) was inoculated with a rifampicin-resistant strain of E. coli O157:H7 (E. coli O157:H7rif) and introduced into a stream of noncontaminated beef (322 +/- 33 g) prior to grinding. Seven inoculum levels (6, 5, and 4 total log CFU [high]; and 3, 2, 1, and 0 total log CFU [low]) were studied in triplicate. E. coli O157:H7rif was not detected in 3.1 to 43% of the ground beef inoculated with the high levels or in 3.4 to 96.9% of the ground beef inoculated with the low levels. For all inoculum levels studied, the five ground beef fractions (each 7.8 +/- 0.6 g) with the highest pathogen levels accounted for 59 to 100% of the total pathogens detected. For all inoculum levels, there was a linear relationship between the quantity of ground beef containing E. coli O157:H7rif and the inoculum level. The quantity of E. coli O157:H7rif in the beef remaining in the grinder was proportional to the inoculum level and was related to the location in the grinder. Different components of the grinder accumulated E. coli O157:H7rif in different quantities, with the most significant accumulation being in the nut (collar) that attaches the die to the blade. This study determined specific distribution patterns of E. coli O157:H7rif after the grinding of a contaminated beef trim along with uncontaminated trims, and the results indicate that the grinding operation should be regarded as a means of distribution of microbial contamination in risk analyses of ground beef operations.     

Evaluating lethality of beef roast cooking treatments against Escherichia coli O157:H7

Added salt, seasonings, and phosphates, along with slow- and/or low-temperature cooking impart desirable characteristics to whole-muscle beef, but might enhance Escherichia coli O157:H7 survival. We investigated the effects of added salt, seasoning, and phosphates on E. coli O157:H7 thermotolerance in ground beef, compared E. coli O157:H7 thermotolerance in seasoned roasts and ground beef, and evaluated ground beef-derived D- and z-values for predicting destruction of E. coli O157:H7 in whole-muscle beef cooking. Inoculated seasoned and unseasoned ground beef was heated at constant temperatures of 54.4, 60.0, and 65.5°C to determine D- and z-values, and E. coli O157:H7 survival was monitored in seasoned ground beef during simulated slow cooking. Inoculated, seasoned whole-muscle beef roasts were slow cooked in a commercial smokehouse, and experimentally determined lethality was compared with predicted process lethality. Adding 5% seasoning significantly decreased E. coli O157:H7 thermotolerance in ground beef at 54.4°C, but not at 60 or 65.5°C. Under nonisothermal conditions, E. coli O157:H7 thermotolerance was greater in seasoned whole-muscle beef than in seasoned ground beef. Meeting U.S. Government (U.S. Department of Agriculture, Food Safety and Inspection Service, 1999, Appendix A) whole-muscle beef cooking guidance, which targets Salmonella destruction, would not ensure ≥6.5-log CFU/g reduction of E. coli O157:H7 in ground beef systems, but generally ensured $ 6.5-log CFU/g reduction of this pathogen in seasoned whole-muscle beef. Calculations based on D- and z-values obtained from isothermal ground beef studies increasingly overestimated destruction of E. coli O157:H7 in commercially cooked whole-muscle beef as process severity increased, with a regression line equation of observed reduction = 0.299 (predicted reduction) + 1.4373.     

Rapid detection of Escherichia coli O157:H7 by immunomagnetic separation and real-time PCR

A method combining immunomagnetic separation (IMS) and real-time (5'-nuclease) PCR was developed to detect Escherichia coli O157:H7. Monoclonal antibody specific for the E. coli O157 antigen was added to protein A-coated magnetic particles to create antibody-coated beads. The beads specifically captured E. coli O157:H7 from bacterial suspensions. The cells were eluted from the beads and lysed by heating; the eluate was then assayed by real-time PCR, using primers and probe specifically targeting the eaeA gene of E. coli O157:H7. Approximately 50% of the cells in suspension were captured by the beads and detected by real-time PCR. No cross-reactivity was detected when other strains of E. coli were tested. This method was applied to detect E. coli O157:H7 from ground beef. Both cell capture efficiency and real-time PCR efficiency were reduced by meat-associated inhibitors. However, we were still able to detect up to 8% of E. coli O157:H7 from inoculated ground beef samples. The detection sensitivity varied among ground beef samples. The minimum detection limit was <5x10(2) cells ml(-1) for suspensions of E. coli O157:H7 in buffer and 1.3x10(4) cells g(-1) for E. coli O157:H7 in ground beef. The combination of IMS and real-time PCR results in rapid, specific and quantitative detection of E. coli O157:H7 without the need for an enrichment culture step.     

Effects of using reduced volumes of nonselective enrichment medium in methods for the detection of Escherichia coli O157:H7 from raw beef

Recent work from our laboratory revealed that tryptic soy broth (TSB) was a superior enrichment medium for use in test-and-hold Escherichia coli O157:H7 methods at levels down to a ratio of three volumes of medium to one volume of sample. Lower ratios were examined for their effect on results obtained from culture isolation, the BAX E. coli O157:H7 MP assay, and the Assurance GDS E. coli O157:H7 assay. Ground beef and boneless beef trim were inoculated with a high level (170 CFU/65 g of ground beef and 43 CFU/65 g of trim) and a low level (17 CFU/65 g of ground beef and 4 CFU/65 g of trim) of E. coli O157:H7 and enriched in 3, 1, 0.5, and 0 volumes of TSB. The volume of TSB used did not affect E. coli O157:H7 detection by culture isolation, Assurance GDS detection in ground beef or trim, or the BAX MP assay detection in ground beef. However, BAX MP assay detection of E. coli O157:H7 in beef trim was 50, 42, and 33% positive when enrichment volumes of 0.5x, 1x, and 3x, respectively, were used. Optimum results with all methods were obtained using 1 volume of TSB. We concluded that detection test results can be considered valid as long as enrichment medium is used, even when it is less than the specified 3 or 10 volumes.     

Empirical Distribution Models for Escherichia coli 57:H7 in Ground Beef Produced by a Mid-size Commercial Grinder

ABSTRACT: The purpose of this research was to develop empirical models that describe the amount and distribution of ground beef contaminated with Escherichia coli O157:H7 when a contaminated beef trim is introduced into a batch of uncontaminated beef before processing in a mid-size commercial grinder (34 g/s). A beef trim was inoculated with a rifampacin-resistant strain of E. coli O157:H7 and added to a batch of noncontaminated trims at the grinding step. To study the distribution of the E. coli O157:H7^rif in the ground beef, 6 treatments with different inoculum levels (1 to 6 log₁₀ colony-forming units [CFU]) were tested. Removal or pick up of the residual contamination with E. coli O157:H7^rif left in the grinder was evaluated. E. coli O157:H7^rif was detected in 9% to 86% of the total ground beef for the 1 to 6 log₁₀ CFU inoculum levels, respectively. E. coli O157:H7^rif contamination was detected in the collar that fixes the grinder's die and blade to the hub. An exponential algorithm described the relationship between the quantities of ground beef containing E. coli O157:H7^rif and the inoculum level ( R ²= 0.82). Distribution models based on a Chi-squared algorithm were developed for each inoculum level describing the contamination level as a function of the batch fraction processed ( R ²= 0.81 to 0.99). The results of this study corroborate that when beef processors test for pathogenic contamination in a mid-scale grinder, they should test the beef residues in the collar that fixes the grinder's die and blade to the hub.     

Survival of Escherichia coli O157:H7 in ground-beef patties during storage at 2, -2, 15 and then -2 degrees C, and -20 degrees C

The survival of Escherichia coli O157:H7 and of a nonpathogenic control strain of E. coli was monitored in raw ground beef that was stored at 2 degrees C for 4 weeks, -2 degrees C for 4 weeks, 15 degrees C for 4 h and then -2 degrees C for 4 weeks, and -20 degrees C. Irradiated ground beef was inoculated with one E. coli control strain or with a four-strain cocktail of E. coli O157:H7 (ca. 10(5) CFU/g), formed into patties (30 to 45 g), and stored at the appropriate temperature. The numbers of the E. coli control strain decreased by 1.4 log 10 CFU/g, and pathogen numbers declined 1.9 log 10 CFU/g when patties were stored for 4 weeks at 20 degrees C. When patties were stored at -2 degrees C for 4 weeks, the numbers of the E. coli control strain and the serotype O157:H7 strains decreased 2.8 and 1.5 log 10 CFU/g, respectively. Patties stored at 15 degrees C for 4 h prior to storage at -2 degrees C for 4 weeks resulted in 1.6 and 2.7 log 10-CFU/g reduction in the numbers of E. coli and E. coli O157:H7, respectively. Storage of retail ground beef at 15 degrees C for 4 h (tempering) did not result in increased numbers of colony forming units per gram, as determined with violet red bile, MRS lactobacilli, and plate-count agars. Frozen storage (-20 degrees C) of ground-beef patties that had been inoculated with a single strain of E. coli resulted in approximately a 1 to 2 log 10-CFU/g reduction in the numbers of the control strain and individual serotype O157:H7 strains after 1 year. There was no significant difference between the survival of the control strain and the O157:H7 strains, nor was there a difference between O157:H7 strains. These data demonstrate that tempering of ground-beef patties prior to low-temperature storage accelerated the decline in the numbers of E. coli O157:H7.     

Influence of freezing and freezing plus acidic calcium sulfate and lactic acid addition on thermal inactivation of Escherichia coli O157:H7 in ground beef

Undercooked ground beef is a leading vehicle for acquiring Escherichia coli O157:H7 infections through consumption of foods. Studies have been performed to determine the effect of freezing and the combined effect of freezing and addition of a mixture of 20% acidic calcium sulfate (final concentration of 0.4% in ground beef) and 10% lactic acid (final concentration of 0.2% in ground beef) (ACS-LA) on the thermal sensitivity of E. coli O157:H7 in ground beef. Five strains of E. coli O157: H7 were separately inoculated into ground beef and held at 5 degrees C for up to 10 days or -20 degrees C for up to 3 weeks then heated at 57, 60, 62.8, 64.3, and 68.3 degrees C to determine rates of thermal inactivation. Results revealed that D-values (decimal reduction times) at equivalent temperatures for four of five E. coli O157:H7 strains were less in the previously frozen than in the refrigerated ground beef and that strains isolated from ground beef in 1993 and 1994 were generally more sensitive to thermal inactivation than those isolated in 1999 and 2000. Only one strain of E. coli O157:H7 was used to determine the effect of ACS-LA in previously frozen or refrigerated ground beef on rates of thermal inactivation. The addition of ACS-LA to ground beef at 20 ml/kg increased the thermal sensitivity of E. coli O157:H7 in both previously frozen and refrigerated ground beef, with greatest rates of inactivation occurring in previously frozen ground beef containing ACS-LA. D-values at 57 degrees C obtained for E. coli O157:H7 in previously refrigerated and frozen ground beef containing ACS-LA and ACS-LA diluted by half were significantly (P < 0.05) less than those obtained in ground beef with no ACS-LA added. D-values at 60 and 62.8 degrees C were consistently less in ACS-LA treated ground beef, but for most treatments the results were not significantly (P > 0.05) different than the controls. Results revealed that the addition of ACS-LA to ground beef, whether frozen or refrigerated, can reduce the temperature or time required to kill E. coli O157:H7 during heating.     

Comparative evaluation of different chromogenic/fluorogenic media for detecting Escherichia coli O157:H7 in food.

Escherichia coli O157:H7 is a serious and common human pathogen that can cause diarrhoea, haemorrhagic colitis, and haemolytic uraemic syndrome (HUS). This study evaluated the enrichment, detection and confirmation procedures for the isolation of E. coli O157:H7 from raw ground beef and raw drinking milk. The purpose of this investigation was to compare Rainbow Agar O157 (RB; Biolog, Hayward, USA), Biosynth Culture Medium O157:H7 (BCM O157:H7; Biosynth, Staad, Switzerland) and Fluorocult HC (HC; Merck, Darmstadt, Germany) with the conventional Sorbitol MacConkey Agar (SMAC, Merck) using mEC + n (raw ground beef) and mTSB + n (raw milk) enrichment media. Single-path GLISA test (Gold Labeled Immuno Sorbent Assay; Merck) was used as the confirmation test. Growth of 466 strains of gram-negative rods isolated from food samples and 46 known E. coli strains from type culture and other collections (34 E. coli O157:H7 strains and 12 serotypes other than E. coli O157:H7) was examined on the agar media. The E. coli O157:H7 strains could readily be isolated and recognized uniquely by their typical black/grey colonies on RB and blue/black colonies on BCM O157:H7. Examination of the 46 known strains of E. coli reference strains showed false negative results on BCM O157:H7 (3.0%), RB (8.8%), HC (5.9%) and SMAC (5.9%) agars. On BCM O157:H7 no false negative results were found with the typical E. coli O157:H7 (beta-D-glucuronidase and sorbitol negative strains). One of two atypical E. coli O157:H7 strains (beta-D-glucuronidase positive) showed similar colouration to the typical strains and was mis-identified by each of the three media (RB, BCM O157:H7, and SMAC agar media). None of the 60 food samples tested yielded E. coli O157:H7. Examination of the food samples, showed that RB gave the lowest number of false positives. The percentages were RB (2.1%), BCM O157:H7 (3.3%), HC (6.2%), and SMAC (57.3%).     

Antibacterial effect of lactoferricin B on Escherichia coli O157:H7 in ground beef.

The antibacterial activity of lactoferricin B on enterohemorrhagic Escherichia coli O157:H7 in 1% peptone medium and ground beef was studied at 4 and 10 degrees C. In 1% peptone medium, 50 and 100 microg of lactoferricin B per ml reduced E. coli O157:H7 populations by approximately 0.7 and 2.0 log CFU/ml, respectively. Studies comparing the antibacterial effect of lactoferricin B on E. coli O157:H7 in 1% peptone at pH 5.5 and 7.2 did not reveal any significant difference (P > 0.5) at the two pH values. Lactoferricin B (100 microg/g) reduced E. coli O157:H7 population in ground beef by about 0.8 log CFU/g (P < 0.05). No significant difference (P > 0.5) was observed in the total plate count between treatment and control ground beef samples stored at 4 and 10 degrees C. The antibacterial effect of lactoferricin B on E. coli O157:H7 observed in this study is not of sufficient magnitude to merit its use in ground beef for controlling the pathogen.     

High levels of background flora inhibits growth of Escherichia coli O157:H7 in ground beef

The influence of natural background flora under aerobic and anaerobic incubation on the growth of Escherichia coli O157:H7 in ground beef was investigated. The background flora from eight different commercial ground beef were added to ground beef spiked with E. coli O157:H7 and stored either aerobically or anaerobically at 12 degrees C. The results showed that the presence of a large number of background bacteria in the ground meat inhibited the growth of E. coli O157:H7 both aerobically and anaerobically. Inhibition was more pronounced under anaerobic conditions. The background floras consisted mainly of lactic acid bacteria of which approximately 80% were Lactobacillus sakei. These results show the importance of the natural background flora in meat for inhibition of growth of E. coli O157:H7.     

Effects of salt, sodium pyrophosphate, and sodium lactate on the probability of growth of Escherichia coli O157:H7 in ground beef

Ground beef products are susceptible to contamination with Escherichia coli O157:H7. The objective of this study was to examine the effect of salt, sodium pyrophosphate (SPP), and sodium lactate on the probability of growth of E. coli O157:H7 in ground beef under a temperature abuse condition. Ground beef containing 0 to 2.25% salt, 0 to 0.5% SPP, and 0 to 3% lactate was inoculated with a four-strain mixture of E. coli O157:H7, vacuum packaged, and stored at 10°C for 15 days. A total of 25 combinations of the three additives, each with 20 samples, were tested. A logistic regression was used to model the probability of growth of E. coli O157:H7 (with a 1.0-log CFU/g increase during storage) as a function of salt, SPP, and lactate. The resultant probability model indicated that lactate at higher concentrations decreased the probability of growth of E. coli O157:H7 in ground beef, and the effect was more pronounced at higher salt concentrations. At salt concentrations below 1.3%, the increase of SPP concentration marginally increased the growth probabilities of E. coli O157:H7. The model illustrated the effect of salt, SPP, and lactate on the growth probabilities and growth or no-growth behavior of E. coli O157:H7 in ground beef and can be used to improve the microbial food safety of ground beef products.     

Thermal process validation for Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes in ground turkey and beef products

At 55 to 70 degrees C, thermal inactivation D-values for Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes were 19.05 to 0.038, 43.10 to 0.096, and 33.11 to 0.12 min, respectively, in ground turkey and 21.55 to 0.055, 37.04 to 0.066, and 36.90 to 0.063 min, respectively, in ground beef. The z-values were 5.73, 5.54, and 6.13 degrees C, respectively, in ground turkey and 5.43, 5.74, and 6.01 degrees C, respectively, in ground beef. In both ground turkey and beef, significant (P < 0.05) differences were found in the D-values between E. coli O157:H7 and Salmonella or between E. coli O157:H7 and L. monocytogenes. At 65 to 70 degrees C, D-values for E. coli O157:H7, Salmonella, and L. monocytogenes were also significantly (P < 0.05) different between turkey and beef. The obtained D- and z-values were used in predicting process lethality of the pathogens in ground turkey and beef patties cooked in an air impingement oven and confirmed by inoculation studies for a 7-log (CFU/g) reduction of E. coli O157:H7, Salmonella, and L. monocytogenes.     

Bactericidal effects of Lactobacillus reuteri and allyl isothiocyanate on Escherichia coli O157:H7 in refrigerated ground beef

Two naturally occurring antimicrobial agents were tested in packages of refrigerated ground beef for their ability to reduce the viability of Escherichia coli O157:H7 during storage. Allyl isothiocyanate (AITC) and Lactobacillus reuteri were tested separately and together for their action against a cocktail of five strains of E. coli O157:H7 in ground beef held at 4 degrees C for 25 days. Ground beef prepared from whole, raw inside round beef roasts was inoculated with low (3 log CFU/g) or high (6 log CFU/g) levels of the E. coli O157:H7 mixture. The beef was treated with AITC (about 1,300 ppm), L. reuteri, or both, along with 250 mM of glycerol per kg of meat at two levels (3 and 6 log CFU/g) and according to a design that yielded 8 controls plus 10 different treatments. Samples were analyzed for E. coli O157:H7 survivors, numbers of total bacteria, and lactic acid bacteria on days 0 to 25 at 5-day intervals. L. reuteri at both input levels with glycerol killed E. coli O157:H7 at both inoculated levels before day 20. AITC completely eliminated E. coli O157:H7 at the low-inoculum level (3 log CFU/g) and reduced viability >4.5 log CFU/g at the high-inoculum level (6 log CFU/g) by the end of the storage period. The combination of L. reuteri and AITC did not yield an additive effect against E. coli O157:H7 viability. L. reuteri in the presence of glycerol was highly effective against E. coli O157:H7 in ground beef during refrigerated storage (4 degrees C) in modified atmosphere packages. Sensory testing is planned to evaluate effects of treatments.     

Distribution of Escherichia coli O157:H7 in beef processed in a table-top bowl cutter

Beef-processing equipment can be contaminated with pathogens such as Escherichia coli O157:H7 and Salmonella spp. The bowl cutter has wide application in particle-size reduction and blending of meat products. This study was undertaken to determine (i) the distribution patterns of E. coli O157:H7 in equipment components and ground beef produced with a table-top bowl cutter under different operational conditions and (ii) the likelihood that pathogen contamination can be transferred to subsequent batches after a batch of beef contaminated with E. coli O157:H7 has been processed in the same bowl cutter. A beef trim (44.6 +/- 29.5 g) inoculated with 2 log CFU of an E. coli O157:H7 mutant strain resistant to rifampicin (E. coli O157:H7rif) was fed by hand into an uncontaminated beef-trim batch under two different batch sizes (2 and 4 kg), three processing times (60, 120, and 240 s), and two feeding modes (running and stoppage fed). There were no significant differences (P > or = 0.05) among all the treatments for the averages of the counts of E. coli O157:H7rif distributed in the ground beef. Regardless of the processing time and the method used to feed the beef trims into the bowl cutter, the whole batch and the following subsequent batch became contaminated when previously contaminated beef was processed. Areas of the bowl cutter most likely to be contaminated with E. coli O157:H7 were (i) the material left on the top of the comb/knife guard and (ii) the knife. Material that overflowed the bowl cutter, when processing the batch with E. coli O157:H7rif, contaminated the equipment surroundings. A Pearson V probability distribution function was determined to describe the distribution of pathogenic organisms in the ground beef, a distribution that can also be applied when conducting process risk analyses on mixing-particle reduction operations for beef trims.     

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.     

Magnetic nanoparticle-antibody conjugates for the separation of Escherichia coli O157:H7 in ground beef

The immunomagnetic separation with magnetic nanoparticle-antibody conjugates (MNCs) was investigated and evaluated for the detection of Escherichia coli O157:H7 in ground beef samples. MNCs were prepared by immobilizing biotin-labeled polyclonal goat anti-E. coli antibodies onto streptavidin-coated magnetic nanoparticles. For bacterial separation, MNCs were mixed with inoculated ground beef samples, then nanoparticle-antibody-E. coli O157:H7 complexes were separated from food matrix with a magnet, washed, and surface plated for microbial enumeration. The capture efficiency was determined by plating cells bound to nanoparticles and unbound cells in the supernatant onto sorbitol MacConkey agar. Key parameters, including the amount of nanoparticles and immunoreaction time, were optimized with different concentrations of E. coli O157:H7 in phosphate-buffered saline. MNCs presented a minimum capture efficiency of 94% for E. coli O157:H7 ranging from 1.6 x 10(1) to 7.2 x 10(7) CFU/ml with an immunoreaction time of 15 min without any enrichment. Capture of E. coli O157:H7 by MNCs did not interfere with other bacteria, including Salmonella enteritidis, Citrobacter freundii, and Listeria monocytogenes. The capture efficiency values of MNCs increased from 69 to 94.5% as E. coli O157:H7 decreased from 3.4 x 10(7) to 8.0 x 10(0) CFU/ml in the ground beef samples prepared with minimal steps (without filtration and centrifugation). An enrichment of 6 h was done for 8.0 x 10(0) and 8.0 x 10(1) CFU/ml of E. coli O157:H7 in ground beef to increase the number of cells in the sample to a detectable level. The results also indicated that capture efficiencies of MNCs for E. coli O157:H7 with and without mechanical mixing during immunoreaction were not significantly different (P > 0.05). Compared with microbeads based immunomagnetic separation, the magnetic nanoparticles showed their advantages in terms of higher capture efficiency, no need for mechanical mixing, and minimal sample preparation.     

Draft risk assessment of the public health impact of Escherichia coli O157:H7 in ground beef

An assessment of the risk of illness associated with Escherichia coli O157:H7 in ground beef was drafted in 2001. The exposure assessment considers farm, slaughter, and preparation factors that influence the likelihood of humans consuming ground beef servings containing E. coli O157:H7 and the number of cells in a contaminated serving. Apparent seasonal differences in prevalence of cattle infected with E. coli O157:H7 corresponded to seasonal differences in human exposure. The model predicts that on average 0.018% of servings consumed during June through September and 0.007% of servings consumed during the remainder of the year are contaminated with one or more E. coli O157:H7 cells. This exposure risk is combined with the probability of illness given exposure (i.e., dose response) to estimate a U.S. population risk of illness of nearly one illness in each 1 million (9.6 x 10(-7)) servings of ground beef consumed. Uncertainty about this risk ranges from about 0.33 illness in every 1 million ground beef servings at the 5th percentile to about two illnesses in every 1 million ground beef servings at the 95th percentile.