Simulating Escherichia coli O157:H7 transmission to assess effectiveness of interventions in Dutch dairy-beef slaughterhouses

Biofilm-Forming Capacity of Escherichia coli Isolated from Cattle and Beef Packing Plants: Relation to Virulence Attributes, Stage of Processing, Antimicrobial Interventions, and Heat Tolerance

Despite the importance of biofilm formation in the contamination of meat by pathogenic Escherichia coli at slaughter plants, drivers for biofilm remain unclear. To identify selection pressures for biofilm, we evaluated 745 isolates from cattle and 700 generic E. coli isolates from two beef slaughter plants for motility, the expression of curli and cellulose, and biofilm-forming potential. Cattle isolates were also screened for serogroup, stx₁, stx₂, eae, and rpoS. Generic E. coli isolates were compared by source (hide of carcass, hide-off carcass, and processing equipment) before and after the implementation of antimicrobial hurdles. The proportion of E. coli isolates capable of forming biofilms was lowest (7.1%; P < 0.05) for cattle isolates and highest (87.3%; P < 0.05) from equipment. Only one enterohemorrhagic E. coli (EHEC) isolate was an extremely strong biofilm former, in contrast to 73.4% of E. coli isolates from equipment. Isolates from equipment after sanitation had a greater biofilm-forming capacity (P < 0.001) than those before sanitation. Most cattle isolates were motile and expressed curli, although these traits along with the expression of cellulose and the detection of rpoS were not necessary for biofilm formation. In contrast, isolates capable of forming biofilms on equipment were almost exclusively motile and able to express curli. The results of the present study indicate that cattle rarely carry EHEC capable of making strong biofilms in slaughter plants. However, if biofilm-forming EHEC contaminates equipment, current sanitation procedures may not eliminate the most robust biofilm-forming strains. Accordingly, new and effective antibiofilm hurdles for meat-processing equipment are required to reduce future instances of foodborne disease. IMPORTANCE As the majority of enterohemorrhagic E. coli (EHEC) isolates are not capable of forming biofilms, sources were undetermined for biofilm-forming EHEC isolated from "high-event periods" in beef slaughter plants. This study demonstrated that sanitation procedures used on beef-processing equipment may inadvertently lead to the survival of robust biofilm-forming strains of E. coli. Cattle only rarely carry EHEC capable of forming strong biofilms (1/745 isolates evaluated), but isolates with greater biofilm-forming capacity were more likely (P < 0.001) to survive equipment sanitation. In contrast, chilling carcasses for 3 days at 0°C reduced (P < 0.05) the proportion of biofilm-forming E. coli. Consequently, an additional antibiofilm hurdle for meat-processing equipment, perhaps involving cold exposure, is necessary to further reduce the risk of foodborne disease.

Characterization of Escherichia coli O157:H7 strains from contaminated raw beef trim during "high event periods"

The development and implementation of effective antimicrobial interventions by the beef processing industry in the United States have dramatically reduced the incidence of beef trim contamination by Escherichia coli O157:H7. However, individual processing plants still experience sporadic peaks in contamination rates where multiple E. coli O157:H7-positive lots are clustered in a short time frame. These peaks have been referred to as "high event periods" (HEP) of contamination. The results reported here detail the characterization of E. coli O157:H7 isolates from 21 HEP across multiple companies and processing plants to gain insight regarding the mechanisms causing these incidents. Strain genotypes were determined by pulsed-field gel electrophoresis, and isolates were investigated for characteristics linking them to human illness. Through these analyses, it was determined that individual HEP show little to no diversity in strain genotypes. Hence, each HEP has one strain type that makes up most, if not all, of the contamination. This is shown to differ from the genotypic diversity of E. coli O157:H7 found on the hides of cattle entering processing plants. In addition, it was found that a large proportion (81%) of HEP are caused by strain types associated with human illness. These results pose a potential challenge to the current model for finished product contamination during beef processing.

Modelling the contamination of lettuce with Escherichia coli O157:H7 from manure-amended soil and the effect of intervention strategies

AIMS

A growing number of foodborne illnesses has been associated with the consumption of fresh produce. In this study, the probability of lettuce contamination with Escherichia coli O157:H7 from manure-amended soil and the effect of intervention strategies was determined.

METHODS AND RESULTS

Pathogen prevalence and densities were modelled probabilistically through the primary production chain of lettuce (manure, manure-amended soil and lettuce). The model estimated an average of 0.34 contaminated heads per hectare. A minimum manure storage time of 30 days and a minimum fertilization-to-planting interval of 60 days was most successful in reducing the risk. Some specific organic farming practices concerning manure and soil management were found to be risk reducing.

CONCLUSIONS

Certain specific organic farming practices reduced the likelihood of contamination. This cannot be generalized to organic production as a whole. However, the conclusion is relevant for areas like the Netherlands where there is high use of manure in both organic and conventional vegetable production.

SIGNIFICANCE AND IMPACT OF THE STUDY

Recent vegetable-associated disease outbreaks stress the importance of a safe vegetable production chain. The present study contributed to this by providing a first estimate of the likelihood of lettuce contamination with E. coli O157:H7 and the effectiveness of risk mitigation strategies.

Modelling a national programme for the control of foodborne pathogens in livestock: the case of Salmonella Dublin in the Danish cattle industry

A 'virtual hierarchy' model is described for studying the spread of pathogens between herds of livestock. This novel approach to simulating disease has animals, herds, and geographic regions in a national livestock industry arranged as a hierarchy of objects in computer memory. Superimposed on all objects is an infection-recovery cycle, a control programme, and surveillance based on test results and animal movement. The model was applied to predicting progress in the control of Salmonella Dublin in the Danish dairy cattle industry over a 10-year period. More frequent testing of bulk tank milk for antibodies to S. Dublin was less effective than improved herd biosecurity. Restricting cattle movement between regions provided a strong benefit to those regions initially with a low prevalence of infection. Enhanced control within infected herds was of intermediate benefit. A combination of strategies was highly effective although cost and feasibility of this option needs further exploration.

Stochastic modelling to assess economic effects of treatment of chronic subclinical mastitis caused by Streptococcus uberis

Chronic subclinical mastitis is usually not treated during the lactation. However, some veterinarians regard treatment of some types of subclinical mastitis to be effective. The goal of this research was to develop a stochastic Monte Carlo simulation model to support decisions around treatment of chronic subclinical mastitis caused by Streptococcus uberis. Factors in the model included the probability of cure after treatment, probability of the cow becoming clinically diseased, transmission of infection to other cows, and physiological effects of the infection. Using basic input parameters for Dutch circumstances, the average economic costs per cow of an untreated chronic subclinical mastitis case caused by Str. uberis in a single quarter from day of diagnosis onwards was €109. With treatment, the average costs were higher (€120). Thus, for the average cow, treatment was not efficient economically. However, the risk of high costs was much higher when cows with chronic subclinical mastitis were not treated. A sensitivity analysis showed that profitability of treatment of chronic subclinical Str. uberis mastitis depended on farm-specific factors (such as economic value of discarded milk) and cow-specific factors (such as day of diagnosis, duration of infection, amount of transmission to other cows and cure rate). Therefore, herd level protocols are not sufficient and decision support should be cow specific. Given the importance of cow-specific factors, information from the current model could be applied to automatic decision support systems.

Food as a vehicle for transmission of Shiga toxin-producing Escherichia coli

Contaminated food continues to be the principal vehicle for transmission of Escherichia coli O157:H7 and other Shiga toxin-producing E. coli (STEC) to humans. A large number of foods, including those associated with outbreaks (alfalfa sprouts, fresh produce, beef, and unpasteurized juices), have been the focus of intensive research studies in the past few years (2003 to 2006) to assess the prevalence and identify effective intervention and inactivation treatments for these pathogens. Recent analyses of retail foods in the United States revealed E. coli O157:H7 was present in 1.5% of alfalfa sprouts and 0.17% of ground beef but not in some other foods examined. Differences in virulence patterns (presence of both stx1 and stx2 genes versus one stx gene) have been observed among isolates from beef samples obtained at the processing plant compared with retail outlets. Research has continued to examine survival and growth of STEC in foods, with several models being developed to predict the behavior of the pathogen under a wide range of environmental conditions. In an effort to develop effective strategies to minimize contamination, several influential factors are being addressed, including elucidating the underlying mechanism for attachment and penetration of STEC into foods and determining the role of handling practices and processing operations on cross-contamination between foods. Reports of some alternative nonthermal processing treatments (high pressure, pulsed-electric field, ionizing radiation, UV radiation, and ultrasound) indicate potential for inactivating STEC with minimal alteration to sensory and nutrient characteristics. Antimicrobials (e.g., organic acids, oxidizing agents, cetylpyridinium chloride, bacteriocins, acidified sodium chlorite, natural extracts) have varying degrees of efficacy as preservatives or sanitizing agents on produce, meat, and unpasteurized juices. Multiple-hurdle or sequential intervention treatments have the greatest potential to minimize transmission of STEC in foods.