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A longitudinal study was conducted to investigate the nature of Escherichia coli O157:H7 colonization of feedlot cattle over the final 100 to 110 days of finishing. Rectal fecal grab samples were collected from an initial sample population of 788 steers every 20 to 22 days and microbiologically analyzed to detect E. coli O157:H7. The identities of presumptive colonies were confirmed using a multiplex PCR assay that screened for gene fragments unique to E. coli O157:H7 (rfbE and fliC(h7)) and other key virulence genes (eae, stx(1), and stx(2)). Animals were classified as having persistent shedding (PS), transient shedding (TS), or nonshedding (NS) status if they consecutively shed the same E. coli O157:H7 genotype (based on the multiplex PCR profile), exhibited variable E. coli O157 shedding, or never shed morphologically typical E. coli O157, respectively. Overall, 1.0% and 1.4% of steers were classified as PS and NS animals, respectively. Characterization of 132 E. coli O157:H7 isolates from PS and TS animals by pulsed-field gel electrophoresis (PFGE) typing yielded 32 unique PFGE types. One predominant PFGE type accounted for 53% of all isolates characterized and persisted in cattle throughout the study. Isolates belonging to this predominant and persistent PFGE type demonstrated an enhanced (P < 0.0001) ability to adhere to Caco-2 human intestinal epithelial cells compared to isolates belonging to less common PFGE types but exhibited equal virulence expression. Interestingly, the attachment efficacy decreased as the genetic divergence from the predominant and persistent subtype increased. Our data support the hypothesis that certain E. coli O157:H7 strains persist in feedlot cattle, which may be partially explained by an enhanced ability to colonize the intestinal epithelium.

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Previous work using a large data set (no. 1, n = 5355) of carcass sponge samples from three large-volume beef abattoirs highlighted the potential use of binary (present or absent) Enterobacteriaceae results for predicting the absence of Salmonella on carcasses. Specifically, the absence of Enterobacteriaceae was associated with the absence of Salmonella. We tested the accuracy of this predictive approach by using another large data set (no. 2, n = 2,163 carcasses sampled before or after interventions) from the same three data set no. 1 abattoirs over a later 7-month period. Similarly, the predictive approach was tested on smaller subsets from data set no. 2 (n = 1,087, and n = 405) and on a much smaller data set (no. 3, n = 100 postintervention carcasses) collected at a small-volume abattoir over 4 months. Of Enterobacteriaceae-negative data set no. 2 carcasses, > 98% were Salmonella negative. Similarly accurate predictions were obtained in the two data subsets obtained from data set no. 2 and in data set no. 3. Of final postintervention carcass samples in data set nos. 2 and 3, 9 and 70%, respectively, were Enterobacteriaceae positive; mean Enterobacteriaceae values for the two data sets were -0.375, and 0.169 log CFU/100 cm2 (detection limit = -0.204, and Enterobacteriaceae negative assigned a value of -0.505 log CFU/100 cm2). Salmonella contamination rates for final postintervention beef carcasses in data set nos. 2 and 3 were 1.1 and 7.0%, respectively. Binary Enterobacteriaceae results may be useful in evaluating beef abattoir hygiene and intervention treatment efficacy.

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The efficacy of adding presumptive Lactobacillus sakei (LS) strain 10-EGR-a, the most inhibitory from among 12 ground beef Lactobacillus isolates, to inhibit growth by Escherichia coli O157:H7 and multidrug-resistant (MDR) Salmonella (serovars Newport and Typhimurium) was evaluated in a beef-derived broth medium at 10 degrees C and in fresh raw ground beef at 10 and 5 degrees C. Pathogen inhibition was observed in the broth medium at both high (10(8):10(5) to 10(7):10(5)) and low (10(6):10(5) to 10(5):10(5)) LS:pathogen ratios. After 9 days at 10 degrees C, in broth medium with high LS:pathogen ratios, growth of E. coli O157:H7 and MDR Salmonella was inhibited by an average of 2.6 and 3.2 log CFU/ml, respectively, whereas in broth medium with low LS:pathogen ratios, E. coli O157:H7 and MDR Salmonella growth was inhibited by an average of 2.8 and 1.8 log CFU/ml, respectively. However, in raw ground beef no significant inhibition was seen with LS:pathogen ratios of 10(5):10(2) to 10(5):10(3). Significant inhibition was seen at very high LS:pathogen ratios (10(6) to 10(7):10(2) to 10(3)), but gross spoilage of the product occurred by day 6. Although presumptive LS 10-EGR-a can inhibit growth of E. coli O157:H7 and MDR Salmonella in a beef-derived broth medium, the inability to produce similar results in ground beef without deleteriously affecting the quality of the product is a limitation that needs further investigation.

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The U.S. Department of Agriculture (USDA) requires beef abattoir operators to periodically analyze beef carcass sponge samples for levels of Escherichia coli. Additional beef carcass sponge sampling is commonly used by processors to evaluate the efficacy of beef abattoir antimicrobial intervention systems. The USDA sample preparation procedure requires that beef carcass sponge samples be mechanically stomached for 2 min before the sample fluid is squeezed out for analysis. When a large number of sponge samples must be analyzed, the stomaching step can limit throughput. In this study, we compared the USDA sample preparation procedure with repeatedly squeezing the sponge during a 10-s interval to expel the sample fluid. Separate sponge samples were obtained from each half of 100 chilled postintervention beef carcasses from a large-volume abattoir during a 4-month period. The USDA and squeezing treatments were randomly assigned to the halves of each carcass. All sponge samples were analyzed for E. coli, coliforms, Enterobacteriaceae, and aerobic mesophilic bacteria using Petrifilm methods. The sample preparation method had no significant effect (signed rank value > 0.05) on the results of any analytical test, although aerobic mesophilic bacteria counts tended to be higher after the USDA method than after manual squeezing alone. These results suggest that manual squeezing may be a simple and rapid alternative sample preparation method when gram-negative bacteria such as E. coli, coliforms, or Enterobacteriaceae are being enumerated from beef carcass sponge samples used to monitor operational abattoir hygiene.

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To develop a process for predicting the likelihood of Salmonella contamination on beef carcasses, we evaluated the influence of several possible causative factors (i.e., year, abattoir, day of week, month, and intervention system components) on the risk of Salmonella and indicator organism contamination. Hide and carcass sponge samples were collected in 2005 to 2006 in six steps at three abattoirs in the East (A), Midwest (B), and Southwest (C) United States. Each abattoir used the same intervention system. Samples were analyzed for aerobic plate counts (APCs; n = 18,990) and Enterobacteriaceae counts (EBCs; n = 18,989) and the presence or absence of Salmonella (n = 5,355). Our results demonstrated that many factors play a significant role in the level of microbial contamination of beef carcasses. Overall, Salmonella prevalence and EBC levels were significantly higher in 2006 than in 2005. APCs and EBCs were highest in abattoirs A (3.57 log CFU/100 cm2) and B (1.31 log CFU/100 cm2). The odds of detecting a positive Salmonella isolate were greatest in abattoir C and lowest in abattoir A. Across the three abattoirs, the overall intervention process effectively reduced microbiological contamination. Salmonella prevalence fell from 45% (preevisceration) to 0.47% (postchilled-lactic acid), and there were APC and EBC reductions of 5.43 and 5.28 log CFU/100 cm2, respectively, from hide-on to postchilled-lactic acid samples. At each abattoir, composites of three individual EBC-negative carcass samples yielded Salmonella-negative results 97 to 99% of the time. These results suggest the possibility of using indicator test results to accurately predict the absence of Salmonella in a beef carcass sample.

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