SEARCH

SEARCH BY CITATION

Keywords:

  • anti-microbial;
  • E. coli O157:H7;
  • foodborne pathogens;
  • Ionophores;
  • Salmonella

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Ionophores
  6. Bacterial cultures
  7. Pure culture experiments
  8. Mixed culture ruminal fluid experiments
  9. Reagents and supplies
  10. Statistical analysis
  11. Results
  12. Pure culture experiments
  13. Mixed culture experiments
  14. Discussion
  15. Acknowledgements
  16. References

Aims: To examine the effects of ionophores on Salmonella and Escherichia coli O157:H7 in pure and mixed ruminal fluid cultures.

Methods and Results: Four Salmonella serotypes (Dublin, Derby, Typhimurium, and Enteriditis) and two strains of E. coli O157:H7 (ATCC 43895 and FDIU 6058) were cultured in the presence of varying concentrations of ionophores (monensin, lasalocid, laidlomycin propionate, and bambermycin) in pure and mixed ruminal fluid cultures. Bacterial growth rates in pure culture were not affected (P > 0·10) by ionophores at concentrations up to 10 times the approximate rumen ionophore concentration under normal feeding regimens. Likewise, ionophores had no effect (P > 0·10) on Salmonella or E. coli CFU plated from 24-h ruminal fluid incubations. Ionophore treatment decreased (P < 0·01) the acetate : propionate ratio in ruminal fluid cultures as expected.

Conclusions: Ionophores had no effect on the foodborne pathogens Salmonella and E. coli O157:H7 in vitro.

Significance and Impact of the Study: The results suggest that ionophore feeding would have little or no effect on Salmonella or E. coli populations in the ruminant.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Ionophores
  6. Bacterial cultures
  7. Pure culture experiments
  8. Mixed culture ruminal fluid experiments
  9. Reagents and supplies
  10. Statistical analysis
  11. Results
  12. Pure culture experiments
  13. Mixed culture experiments
  14. Discussion
  15. Acknowledgements
  16. References

Two of the most important etiologic agents of foodborne illness in humans are Salmonella and Escherichia coli O157:H7 (Buzby et al. 1996). Escherichia coli O157:H7, first recognized in 1982, is considered an important agent of foodborne disease with worldwide distribution. The hallmark of infection is a distinctive syndrome characterized by painful, bloody diarrhea with little or no fever, termed hemorrhagic colitis (Griffin and Tauxe 1991; Besser et al. 1999). Although outbreaks of E. coli in humans have been linked to water, vegetables, fruit juice, and venison, the majority of the cases of human illness for which a source has been determined have resulted from foods that originated from cattle, usually ground beef (Besser et al. 1993; Gansheroff and O'Brien 2000; Gage 2001). Escherichia coli O157:H7 has been isolated from beef and dairy cattle at all stages of production, and although shedding is intermittent and can be difficult to detect, it appears to be fairly widespread throughout the cattle population (Hancock et al. 1998; Elder et al. 2000).

In humans, Salmonella spp. are the most commonly reported and costly cause of foodborne disease in humans with foodborne transmission accounting for approximately 95% of all salmonellosis cases in the United States (Mead et al. 1999). Salmonella populate the intestinal tracts of various animal species, including beef and dairy cattle, which represent a major reservoir for human foodborne salmonellosis (Fedorka-Cray et al. 1998). Beef and dairy products have been identified as important vectors in outbreaks of Salmonella in humans (Bean and Griffin 1990).

Ionophores were approved by the United States Food and Drug Administration in the mid-1970s as feed additives for livestock, and since then their use has become routine in the feeding of growing ruminants. The use of ionophores has attracted interest, given the temporal relationship between initial ionophore use in the United States cattle industry and the increase in E. coli O157:H7 cases (Griffin and Tauxe 1991; Rasmussen et al. 1999). Researchers have suggested that because E. coli is a gram-negative bacterium, ionophores might promote the incidence of E. coli in cattle by inhibiting competitive gram-positive species (Dennis et al. 1981; Henderson et al. 1981; Schelling, 1984). However, survey data and experimentation in cattle has yielded conflicting results (Garber et al. 1995; Dargatz et al. 1997; Herriott et al. 1998). In spite of this, the ability of ionophores to alter the gut microbiota may give E. coli and/or Salmonella a selective advantage and warrants further research. Therefore, a series of experiments were conducted to evaluate the effects of the ionophores monensin, lasalocid, laidlomycin propionate, and bambermycin on four Salmonella serotypes and two strains of E. coli O157:H7 in both pure and mixed ruminal fluid culture.

Ionophores

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Ionophores
  6. Bacterial cultures
  7. Pure culture experiments
  8. Mixed culture ruminal fluid experiments
  9. Reagents and supplies
  10. Statistical analysis
  11. Results
  12. Pure culture experiments
  13. Mixed culture experiments
  14. Discussion
  15. Acknowledgements
  16. References

Lasalocid (Bovatec®) and laidlomycin propionate (Cattlyst®) were generously provided by Alpharma, (Chicago Heights, IL, USA). Bambermycin (Gainpro®) was provided by Hoechst Roussel Vet (Warren, NJ, USA). Monensin (Rumensin®) was from Elanco (Greenfield, IN, USA). Stock ionophore solutions were prepared by adding each ionophore to autoclaved 95% (v/v) ethanol in sealed tubes to achieve concentrations of 20, 36, 15 and 2 mg ml−1 of monensin, lasalocid, laidlomycin propionate, and bambermycin, respectively. Dilutions from these stocks were made as above to achieve a dose that would approximate ruminal concentrations under normal feeding regimens (0·04, 0·007, 0·003, and 0·0004 mg ml−1 rumen fluid of monensin, lasalocid, laidlomycin propionate, and bambermycin, respectively). Multiples of these expected ruminal concentrations (0·125, 0·25, 0·50, 1, 2, 5, and 10×) were made from the stock solutions.

Bacterial cultures

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Ionophores
  6. Bacterial cultures
  7. Pure culture experiments
  8. Mixed culture ruminal fluid experiments
  9. Reagents and supplies
  10. Statistical analysis
  11. Results
  12. Pure culture experiments
  13. Mixed culture experiments
  14. Discussion
  15. Acknowledgements
  16. References

Escherichia coli O157:H7 strain 933 (ATCC 43895) and Salmonella serotypes Derby (ATCC 6960) and Dublin (ATCC 15480) were obtained from the American Type Culture Collection (Manassas, VA, USA). Escherichia coli O157:H7 strain 6058 (isolated from ground beef following a fatal outbreak of hemorrhagic colitis) was provided by Dr Dan Rice of the Field Disease Investigation Unit at Washington State University in Pullman. Salmonella isolates Typhimurium and Enteriditis were obtained from the National Veterinary Services Laboratory (Ames, IA, USA). Strains not naturally resistant to novobiocin (NO) and nalidixic acid (NA) were made resistant to 25 μg ml−1 NO and 20 μg ml−1 NA. Bacterial strains were cultured in anoxic tryptic soy broth (TSB) medium at 39°C and bacteria that were stable through three successive overnight transfers were utilized in the following experiments.

Pure culture experiments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Ionophores
  6. Bacterial cultures
  7. Pure culture experiments
  8. Mixed culture ruminal fluid experiments
  9. Reagents and supplies
  10. Statistical analysis
  11. Results
  12. Pure culture experiments
  13. Mixed culture experiments
  14. Discussion
  15. Acknowledgements
  16. References

Pure cultures of E. coli O157:H7 strains 933 and 6058 and Salmonella serotypes Derby, Dublin, Enteriditis, and Typhimurium were individually added (0·5 ml) to 9 ml of autoclaved TSB. Ionophores were then added (0·2 ml of above solutions) to achieve the desired final concentration. The tubes were sealed, vortexed, and incubated at 39°C. Growth rates were estimated via measurement of optical density (O.D.) at 600 nm using a Spectronic 20D spectrophotometer (Rochester, NY, USA). Cell density readings were taken every 30 min until the O.D. reached 0·6. Each experimental series was replicated three times on separate days.

Mixed culture ruminal fluid experiments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Ionophores
  6. Bacterial cultures
  7. Pure culture experiments
  8. Mixed culture ruminal fluid experiments
  9. Reagents and supplies
  10. Statistical analysis
  11. Results
  12. Pure culture experiments
  13. Mixed culture experiments
  14. Discussion
  15. Acknowledgements
  16. References

Ruminal fluid was obtained from fistulated Holstein and Jersey cows maintained on a Bermuda grass hay (70%) and concentrate diet. Care, use, and handling of the cattle was pre-approved by the Animal Care and Use Committee of the Food and Feed Safety Research Laboratory, USDA. Ruminal contents were strained via a fine mesh nylon strainer (Reaves and Co., Durham, NC, USA), pooled and transported to the laboratory. Ruminal fluid was transferred anerobically to a medium containing (l−1): 292 mg of K2HPO4.3H2O, 240 mg of KH2PO4, 120 mg of (NH4)2SO4, 480 mg of NaCl, 100 mg of MgSO4.7 H2O, 64 mg of CaCl2.2H2O, 600 mg of cysteine hydrochloride, 4 g of Na2CO3 and 1 g each of sigmacell, glucose, xylose, and cellobiose. The final concentration of ruminal fluid was 33% (vol/vol). To this ruminal fluid medium, 2 ml of a 1 : 10 dilution of the bacteria was added (initial bacterial concentrations were approximately 105 and 104 CFU ml−1 of E. coli and Salmonella, respectively).

One millilitre samples of this inoculated ruminal fluid were serially diluted (in 10-fold increments) in phosphate buffered saline (PBS, pH 7·0), plated, and incubated overnight at 37°C for direct counting to determine intitial bacterial concentrations. Escherichia coli O157:H7 strains 6058 and 933 were plated on MacConkey's agar supplemented with 25 μg ml−1 NO and 20 μg ml−1 NA. Salmonella serotypes were plated on brilliant green agar (BGA) supplemented with 20 μg ml−1 NO.

Ten millilitre of the above medium was added to anoxic tubes flushed with O2 free CO2 and the tubes sealed with butyl rubber stoppers and aluminum crimps. The respective ionophores were added (0·2 ml) and tubes incubated for 24 h at 37°C. Following incubation, 1 ml was removed from each sample for serial dilution and plating as described above. Bacterial colonies were directly counted the following day. After plating, diluted samples (1 : 10) were centrifuged, filtered and stored at −20°C until analyzed for volatile fatty acids (VFA) by gas-liquid chromatography as previously described (Corrier et al. 1990). This experiment was replicated twice with each bacteria, for all concentrations of each ionophore.

Reagents and supplies

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Ionophores
  6. Bacterial cultures
  7. Pure culture experiments
  8. Mixed culture ruminal fluid experiments
  9. Reagents and supplies
  10. Statistical analysis
  11. Results
  12. Pure culture experiments
  13. Mixed culture experiments
  14. Discussion
  15. Acknowledgements
  16. References

Unless otherwise noted, all media and agar were obtained from Difco Laboratories (Detroit, MI, USA). Reagents and antibiotics were obtained from Sigma Chemical Co. (St. Louis, MO, USA).

Statistical analysis

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Ionophores
  6. Bacterial cultures
  7. Pure culture experiments
  8. Mixed culture ruminal fluid experiments
  9. Reagents and supplies
  10. Statistical analysis
  11. Results
  12. Pure culture experiments
  13. Mixed culture experiments
  14. Discussion
  15. Acknowledgements
  16. References

Data were analyzed using the GLM procedure of SAS (SAS Inst. Inc., Cary, NC, USA). Data from multiple repetitions were pooled prior to analysis. Growth rates, CFU, and VFA data were subjected to analysis of variance appropriate for a completely randomized design. Analysis of VFA data showed no differences within individual bacterial strains or serotypes; therefore, the two E. coli strains and the four Salmonella serotypes were pooled and analyzed collectively as E. coli or Salmonella. Differences among mean values were considered significant at a 5% (P < 0·05) level of significance.

Pure culture experiments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Ionophores
  6. Bacterial cultures
  7. Pure culture experiments
  8. Mixed culture ruminal fluid experiments
  9. Reagents and supplies
  10. Statistical analysis
  11. Results
  12. Pure culture experiments
  13. Mixed culture experiments
  14. Discussion
  15. Acknowledgements
  16. References

Growth rates for E. coli O157:H7 strains 933 and 6058 and for Salmonella serotypes Derby, Dublin, Enteriditis, and Typhimurium are presented in Fig. 1. No differences (P > 0·10) were found in growth rates for either strain of E. coli O157:H7 when comparing increasing concentrations of the same ionophore or when comparing different ionophores. Similarly, the type of ionophore or ionophore concentration had no effect (P > 0·10) on growth rate of any of the Salmonella serotypes examined. Laidlomycin propionate appeared to increase the growth rate of Salmonella dublin compared with other ionophore treatments; however, this effect was not statistically significant.

image

Figure 1. Specific maximal growth rates (h−1) of E. coli O157:H7 strains 933 (a) and 6058 (b) and Salmonella serotypes Derby (c), Dublin (d), Enteriditis (e), and Typhimurium (f) after exposure to increasing concentrations (0, 0·125, 0·25, 0·50, 1, 2, 5, and 10×) of lasalocid (▪), laidlomycin propionate (□), bambermycin (•) and monensin (○) in pure culture. Base concentration approximates ruminal concentrations under normal feeding regimens (1X = 0·004, 0·007, 0·003, and 0·0004 mg ml−1 rumen fluid of monensin, lasalocid, laidlomycin propionate, and bambermycin, respectively)

Download figure to PowerPoint

Mixed culture experiments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Ionophores
  6. Bacterial cultures
  7. Pure culture experiments
  8. Mixed culture ruminal fluid experiments
  9. Reagents and supplies
  10. Statistical analysis
  11. Results
  12. Pure culture experiments
  13. Mixed culture experiments
  14. Discussion
  15. Acknowledgements
  16. References

Colony forming units of E. coli O157:H7 and Salmonella after incubation with ruminal fluid are presented in Fig. 2. Increasing ionophore concentrations had no effect (P > 0·10) on E. coli O157:H7 strain 933 and 6058. Likewise, no differences (P > 0·10) were observed when different ionophores were compared. Salmonella serotypes were unaffected (P > 0·10) by ionophore concentration or type. The highest lasalocid concentration appeared to decrease CFU of E. coli strain 933, however this was not statistically significant. In general, Salmonella serotypes did not appear to survive as well in rumen fluid incubations, as suggested by lower CFU when compared with E.coli, however initial concentrations of inoculated Salmonella were one  log10 CFU lower. Laidlomycin propionate appeared to decrease CFU of Salmonella Derby but this was not significantly different from other ionophore treatments.

image

Figure 2. Colony forming units ( log10) of E. coli O157:H7 strains 933 (a) and 6058 (b) and Salmonella serotypes Derby (c), Dublin (d), Enteritidis (e), and Typhimurium (f) after exposure to increasing concentrations (0, 0·125, 0·25, 0·50, 1, 2, 5, and 10×) of lasalocid (▪), laidlomycin propionate (□), bambermycin (•) and monensin (○) in mixed ruminal fluid culture. Base concentration approximates ruminal concentrations under normal feeding regimens (1X = 0·004, 0·007, 0·003, and 0·0004 mg ml−1 rumen fluid of monensin, lasalocid, laidlomycin propionate, and bambermycin, respectively)

Download figure to PowerPoint

Individual analysis of bacterial strains indicating monensin treatment resulted in a numerical, but not significant (P > 0·10), decrease in the acetate : propionate ratio (data not shown). When strains 933 and 6058 were pooled and analyzed collectively as E. coli, the acetate : propionate ratio decreased (P < 0·05) in monensin treatments (Fig. 3). Similarly, when Salmonella serotypes were pooled, acetate:propionate ratio decreased in monensin (P < 0·01) and laidolomycin propionate (P < 0·05) treatments, and tended to decrease (P = 0·07) in lasalocid treatments. Bambermycin had no effect (P > 0·10) on acetate : propionate ratios (Fig. 3).

image

Figure 3. Pooled acetate : propionate ratios for two strains of E. coli O157:H7 (a) and four Salmonella serotypes (b) after exposure to increasing concentrations (0, 0·125, 0·25, 0·50, 1, 2, 5, and 10×) of lasalocid (▪), laidlomycin propionate (□), bambermycin (•) and monensin (○) in mixed ruminal fluid culture. Base concentration approximates ruminal concentrations under normal feeding regimens (1X = 0·004, 0·007, 0·003, and 0·0004 mg ml−1 rumen fluid of monensin, lasalocid, laidlomycin propionate, and bambermycin, respectively)

Download figure to PowerPoint

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Ionophores
  6. Bacterial cultures
  7. Pure culture experiments
  8. Mixed culture ruminal fluid experiments
  9. Reagents and supplies
  10. Statistical analysis
  11. Results
  12. Pure culture experiments
  13. Mixed culture experiments
  14. Discussion
  15. Acknowledgements
  16. References

Ruminant animals are asymptomatic carriers of E. coli O157:H7 and other enterohemorrhagic E. coli (Beutin et al. 1993; Rasmussen et al. 1993; Bielaszewska et al. 2000; Cornick et al. 2000) with the majority of human outbreaks linked to contact with ruminant animals or to products derived from ruminants (Gage 2001). Ninety-five percent of an estimated 1·4 million non-typhoidal Salmonella cases in the United States are estimated to be foodborne (Tauxe 1991; Mead et al. 1999) with beef, lamb, and dairy products listed as major sources of infection (Holmberg et al. 1984; Bean and Griffin 1990; Cullor 1995). In the United States, ionophores are widely used in the feeding of growing beef and dairy cattle, sheep and goats and the benefits to growing ruminants and the subsequent effects of ruminal fermentation are well-documented (Russell and Strobel 1989). The use of ionophores has attracted interest because of the increase in human E. coli O157:H7 cases and the corresponding timeframe of widespread ionophore use (Griffin and Tauxe 1991; Rasmussen et al. 1999). Furthermore, it is hypothesized that the increasing number of antimicrobial-resistant Salmonella strains isolated from human salmonellosis cases are because of widespread use of antimicrobial agents in food animal production and that these resistant strains originate from animals (Cohen and Tauxe 1986). However, to our knowledge, no direct link has been established between ionophore use in ruminants and the development of antimicrobial resistance in foodborne pathogens. In fact, research conducted in our laboratory showed no increase in antimicrobial susceptibility of E. coli O157:H7 or Salmonella Typhimurium in lambs fed ionophores (T.S. Edrington unpublished data). Bambermycin has been reported to actually decrease antimicrobial resistance of E. coli and Salmonella in calves and swine (Federic and Sokol 1973; Sokol et al. 1973; Dealy and Moeller 1976, 1977).

In our experiments, the ionophores rumensin, lasalocid, and laidlomycin propionate decreased the acetate : propionate ratio in mixed ruminal fluid incubations as expected, a typical response in animals fed ionophores (Russell and Strobel 1989). Overall, we saw no effects of ionophore treatment on E. coli O157:H7 or Salmonella with respect to growth rates in pure culture or CFU in mixed ruminal fluid incubations. In contrast, the prevalence of E. coli O157 was higher in dairy herds that used monensin, lasalocid, and or decoquinate in their heifer rations compared with herds not using these additives (Herriott et al. 1998). Supporting our findings, Garber et al. (1995) reported no association between fecal shedding of E. coli O157:H7 and ionophore use in dairy calves. Dargatz et al. (1997) likewise reported no relationship between ionophore use and E. coli O157:H7 in feedlot cattle. In a survey of 100 United States feedlots, Losinger et al. (1997) found no difference in the number of fecal samples positive for Salmonella when ionophores were included in the diet. In further support of our results, Dealy and Moeller (1977) reported calves supplemented with bambermycin in their feed had similar intestinal E. coli populations compared with control calves. Bambermycin supplemented feed reduced the duration and prevalence of Salmonella Typhimurium shedding in experimentally infected calves and swine (Dealy and Moeller 1976, 1977).

Research examining the effects of ionophores on E. coli and Salmonella are conflicting and highlight the complexity of the ruminant animal. We initiated our research in pure culture to eliminate the confounding effects of the ruminal microflora. The lack of any ionophore effect may be attributed to the double membrane present on gram-negative bacteria that is capable of excluding a variety of compounds (Ahmed and Booth 1981; Brock et al. 1994). However, because it is generally accepted that ionophores inhibit gram-positive bacteria favoring gram-negative species (Dennis et al. 1981; Henderson et al. 1981; Schelling 1984), ionophore feeding could theoretically increase populations of E. coli by inhibiting competing gram-positive species. Ionophore exposure may have inhibited gram-positive species in these mixed ruminal culture experiments, however based on our results, it did not benefit Salmonella or E. coli populations. Disseminating the factors involved in the carriage and shedding of foodborne pathogens in ruminants is a complex, but necessary task, as long as there is opportunity for contamination of our food supply. Incorporating knowledge from this work and others, along with pre- and post-harvest intervention strategies may help to improve the safety of ruminant-derived foods.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Ionophores
  6. Bacterial cultures
  7. Pure culture experiments
  8. Mixed culture ruminal fluid experiments
  9. Reagents and supplies
  10. Statistical analysis
  11. Results
  12. Pure culture experiments
  13. Mixed culture experiments
  14. Discussion
  15. Acknowledgements
  16. References
  • Ahmed, S. and Booth, I.R. (1981) Quantitative measurements of the proton motive force and its relation to steady state lactose accumulation in Escherichia coli. Biochemistry Journal 200, 573581.
  • Bean, N.H. and Griffin, P.M. (1990) Foodborne disease outbreaks in the United States, 1973–1987: pathogens, vehicles and trends. Journal of Food Protection 53, 804810.
  • Besser, R.E., Lett, S.M., Weber, J.T., Doyle, M.P., Barrett, T.J., Wells, J.G. and Griffin, P.M. (1993) An outbreak of diarrhea and hemolytic uremic syndrome from Escherichia coli O157:H7 in fresh-pressed apple cider. Journal of the American Medical Association 269, 22172220.
  • Besser, R.E., Griffin, P.M. and Slutsker, L. (1999) Escherichia coli O157:H7 gastroenteritis and the hemolytic uremic syndrome: an emerging infectious disease. Annual Reviews in Medicine 50, 355367.
  • Beutin, L., Geier, D., Steinruck, H., Zimmerman, S. and Scheutz, F. (1993) Prevalence and some properties of verotoxin (Shiga-like toxin) producing Escherichia coli in seven different species of healthy domestic animals. Journal of Clinical Microbiology 31, 24832488.
  • Bielaszewska, M., Schmidt, H., Liesegang, A., Prager, R., Rabsch, W., Tschape, H., Cizek, A., Janda, J., Blahova, K. and Karch, H. (2000) Cattle can be a reservoir of sorbitol fermenting shiga toxin producing Escherichia coli O157:H- strains and a source of human diseases. Journal of Clinical Microbiology 38, 34703473.
  • Brock, T.D., Madigan, M.T., Martinko, J.M. and Parker, J. (1994) Biology of Microorganisms, 7th edn. Englewood Cliffs, NJ, USA: Prentice Hall, Inc.
  • Buzby, J.C., Roberts, T., Jordan Lin, C.T. and MacDonald, J.M. (1996) Bacterial Foodborne Disease – Medical Costs and Productivity Losses. USDA-ERS Report 741. Washington D.C., USA.
  • Cohen, M.L. and Tauxe, R.V. (1986) Drug-resistant Salmonella in the United States: an epidemiologic perspective. Science 234, 964969.
  • Cornick, N.A., Booher, S.L., Casey, T.A. and Moon, H.W. (2000) Persistant colonization of sheep by Escherichia coli O157:H7 and other E. coli pathotypes. Applied and Environmental Microbiology 66, 49264934.
  • Corrier, D.E., Hinton, A., Ziprin, R.L. and DeLoach, J.L. (1990) Effect of dietary lactose on Salmonella colonization of market age broiler chickens. Avian Diseases 34, 668676.
  • Cullor, J.S. (1995) Common pathogens that cause foodborne disease: can they be controlled on the dairy. Veterinary Medicine 90, 185194.
  • Dargatz, D.A., Wells, S.J., Thomas, L.A., Hancock, D.D. and Garber, L.P. (1997) Factors associated with the presence of Escherichia coli O157 in feces of feedlot cattle. Journal of Food Protection 60, 466470.
  • Dealy, J. and Moeller, M.W. (1976) Influence of bambermycins on Salmonella infection and antibiotic resistance in swine. Journal of Animal Science 42, 13311336.
  • Dealy, J. and Moeller, M.W. (1977) Influence of bambermycins on Salmonella infection and antibiotic resistance in calves. Journal of Animal Science 44, 734738.
  • Dennis, S.M., Nagaraja, T.G. and Bartley, E.E. (1981) Effects of lasalocid or monensin on lactate-producing or -using rumen bacteria. Journal of Animal Science 52, 418426.
  • Elder, R.O., Keen, J.E., Siragusa, G.R., Barkocy-Gallagher, G.A., Koohmaraie, M. and Lagreid, W.W. (2000) Correlation of enterohemorrhagic Escherichia coli O157 prevalence in feces, hides, and carcasses of beef cattle during processing. Proceedings of the National Academy of Science, USA , 97, 29993003.
  • Federic, F. and Sokol, A. (1973) Effect of peroral application of a colicinogenic strain of Escherichia coli on the incidence of different categories of plasmids in Escherichia coli isolated from weaned piglets fed on flavomycin. Folia Microbiologica 18, 65.
  • Fedorka-Cray, P.J., Dargatz, D.A., Thomas, L.A. and Gray, J.T. (1998) Survey of Salmonella serotypes in feedlot cattle. Journal of Food Protection 61, 525530.
  • Gage, R. (2001) Outbreaks of Escherichia coli O157:H7 infections among children associated with farm visits – Pennsylvania and Washington, 2000. CDC's Morbidity and Mortality Weekly Report 50, 293297.
  • Gansheroff, L.J. and O'Brien, A.D. (2000) Escherichia coli O157:H7 in beef cattle presented for slaughter in the U.S.: higher prevalence rates than previously estimated. Proceedings of the National Academy of Science, USA , 97, 29592961.
  • Garber, L.P., Wells, S.J., Hancock, D.D., Doyle, M.P., Tuttle, J., Shere, J.A. and Zhoa, T. (1995) Risk factors for fecal shedding of Escherichia coli O157:H7 in dairy calves. Journal of the American Veterinary Medical Association 207, 4649.
  • Griffin, P.M. and Tauxe, R.V. (1991) The epidemiology of infection caused by Escherichia coli O157:H7, other enterohaemorrhagic E. coli, and the associated hemolytic uremic syndrome. Epidemiology Reviews 13, 6097.
  • Hancock, D.D., Besser, T.E. and Rice, D.H. (1998) Ecology of Escherichia coli O157:H7 in cattle and impact of management practices. In ed. Kaper, J.B. and O'BrienA.D. Escherichia coli O157:H7 and Other Shiga Toxin-Producing E. coli Strains. pp. 8591. Washington, DC: American Society of Microbiology Press.
  • Henderson, C., Stewart, C.S. and Nekrep, F.V. (1981) The effect of monensin on pure and mixed cultures of rumen bacteria. Journal of Applied Bacteriology 51, 159169.
  • Herriott, D.E., Hancock, D.D., Ebel, E.D., Carpenter, L.V., Rice, D.H. and Besser, T.E. (1998) Association of herd management factors with colonization of dairy cattle by shiga toxin-positive Escherichia coli O157. Journal of Food Protection 61, 802807.
  • Holmberg, S.D., Wells, J.G. and Cohen, M.L. (1984) Animal-to-man transmission of antimicrobial-resistant Salmonella: investigations of U.S. outbreaks, 1971–1983. Science 225, 833835.
  • Losinger, W.C., Garber, L.P., Smith, M.A., Hurd, H.S., Biehl, L.G., Fedorka-Cray, P.J., Thomas, L.A. and Ferris, K. (1997) Management and nutritional factors associated with the detection of Salmonella sp. from cattle fecal specimens from feedlot operation in the United States. Preventative Veterinary Medicine 31, 231244.
  • Mead, P.S., Slutsker, L., Dietz, V., McCraig, L.F., Bresee, J.S., Shapiro, C., Griffin, P.M., and Tauxe, R.V. (1999) Food-related illness and death in the United States. Emerging Infectious Diseases 5(5), 139.
  • Rasmussen, M.A., Cray, W.C., Casey, T.A. and Whipp, S.C. (1993) Rumen contents as a reservoir of enterohemorrhagic Escherichia coli. FEMS Microbiology Letters 114, 7984.
  • Rasmussen, M.A., Wickman, T.L., Cray Jr., W.C. and Casey, T.A. (1999) Escherichia coli O157:H7 and the rumen environment. In E. coli O157 in Farm Animals, ed. Stewart, C.S. and Flint, H.J. pp. 39NY, USA: CAB International.
  • Russell, J.B. and Strobel, H.J. (1989) Effect of ionophores on ruminal fermentation. Applied and Environmental Microbiology 55, 16.
  • Schelling, G.T. (1984) Monensin mode of action in the rumen. Journal of Animal Science 58, 15181527.
  • Sokol, A., Kremery, V., Federic, F., Fajtar, V. and Janouskova, J. (1973) The influence of flavomycin on the elimination of R-factors of Escherichia coli in vitro. Folia Microbiologica 18, 176.
  • Tauxe, R.V. (1991) Salmonella: a postmodern pathogen. Journal of Food Protection 54, 563568.