In reacting to the statement, “Meat from Grass Fed Cattle is Safer than Meat from Cattle that are Corn Fed”, the first question that comes to mind is “safer how”? Most consumers think of safety in terms of microbial contamination, hormone and antibiotic residues.

Commercially, cattle are grazed on grass or pasture for some or all of their lives. Many go to feed lots at 12-18 mo of age where they are fed a high carbohydrate grain diet for 4-6 months to increase their growth rate and bring them to market more quickly. In the latter situation, they live in higher density environments and may be treated with antibiotics to treat infections or hormones to increase growth rate. Because their use is under our control, the FDA and USDA have set stringent standards based on exhaustive safety and efficacy testing for antibiotics and hormones in the livestock industry. Because of consumer concerns about antibiotics and hormones, the USDA has certain label claims relative to beef.

No Hormones (beef): Sufficient documentation to show that the animal was raised without hormones.

No Antibiotics (beef): Sufficient documentation to show that the animal was raised without antibiotics.

Certified Organic: Sufficient documentation to show that animals are fed 100% certified organic feed. Some vitamin and mineral supplements are permitted but hormones and antibiotics are not. Animals must have access to pasture but may be finished in a feedlot.

Grass Fed: The Ag Marketing Service established a voluntary claim for grass-fed livestock. It stipulates that animals receive at least 99% of their lifetime energy from a grass- or forage-based diet. Hormones, antibiotics and long-term confinement use are allowed. Verification of the label claim is voluntary. This standard is under consideration by the USDA.

If we consider that the antibiotic and hormone issues are under federal control, and that feeding regimen precludes neither, then the remaining primary concern with beef is microbiological safety. Cattle are asymptomatic natural reservoirs of both harmless bacteria and harmful bacteria (Escherichia coli O157:H7, Salmonella spp., Campylobacter spp.). These pathogens can enter the food supply from cattle via fecal contamination of carcasses at slaughter. In beef cattle, the prevalence of E. coli O157 ranges from <1 to 20% in feedlots and from <1 to 27% on pasture. Hussein (2007). Calves (<3 mo) are more likely to have higher levels of E. coli O157 and Campylobacter spp. in their waste (15). Stocking density has no direct effect on pathogen level.

Rapidly growing (feedlot) and high-producing dairy cattle are fed large grain rations as a calorie source to increase growth rate and milk production. Attempts to correlate the incidence of E. coli O157:H7 (and other pathogens) with specific diets or feeding management practices have resulted in inconsistent findings. It is unclear if diet can influence the survival of E. coli O157:H7 in the gastrointestinal system or in feces in the environment, and if so, whether that has any direct impact on meat derived from animals receiving different diets.

All cattle eat grass for some portion of their lives. Some are fed grains in the months immediately preceding slaughter. Cattle have bacteria in the rumen and gut that break down the cellulose in plant materials. They shed these bacteria continually in their waste. Starch from a corn based diet can escape degradation in the rumen and pass into the colon where it is broken down to sugars. E. coli can ferment these sugars increasing the acidity (lowering pH) in the colon (12). The numbers of acid-resistant E. coli, which are more likely to survive the gastric acidity of the human stomach, can then increase (18). This acid-tolerance could increase the capacity of E. coli to cause human disease. In some studies, E. coli numbers have been higher in grain fed cattle (7) while in others. Feeding corn, with the resultant change in pH, had no relationship to E. coli O157 (8). Some studies report differences due to type of grain in the diet. E. coli O157:H7 prevalence and fecal excretion has been shown to be higher in barley-fed than in corn-fed cattle (5). Other studies have shown that E. coli in feces and ruminal fluid are unaffected by diet (hay or corn silage) during in the growing period, however switch to a corn-based finishing diet increases the concentration in both locations (6). When switched from a feedlot ration to a forage-based diet, fewer cattle naturally infected with E. coli Ol57:H7 shed the pathogen (18). Some reports indicate that fecal shedding of E. coli can be reduced by feeding forage (1 month) prior to slaughter (20, 7, 18, 10) while other studies have found that fecal shedding of pathogenic bacteria was unaffected (3, 11).

E. coli may be suppressed by specific acids produced when forages (hay, rye grass, silage) are digested (16). Prior to transport for slaughter, feeding pasture-fed cattle forage, rather than fasting, can reduce the risk of carcass contamination with bacteria of digesta or fecal origin. Specific feed components may impact the frequency and magnitude of fecal excretion of E. coli O157:H7. Some phenolic acids found in forage plants can decrease viable counts of E. coli O157:H7 in animals finished on corn (20).

Low numbers of beef cattle shed Salmonella spp. at slaughter (7%). This varies little as a function of the pre-slaughter production system (grass or lot feeding) (11). Feedlot cattle are more likely to have Campylobacter spp. in their intestinal tracts and on their carcasses than are pasture-fed cattle (13). However, post-slaughter carcass chilling reduces the incidence by 10 fold in 20 hr. More rumen (78%) and fecal (94%) samples from pasture fed cattle have been shown to be positive for Campylobacter spp. than those from concentrate (grain) fed cattle (50% rumen, 72% fecal) (17).

This brings us to the second question, “do higher levels of fecal pathogen loads and excretion rates have an effect on the meat derived from their carcasses”?

Muscle tissue is sterile unless it is contaminated by an external source. Livestock hides, which can become a significant source of microbial contamination, can occur at the feedlot, in transport trailers, and in contaminated lairage pens (9). Long transport times and time off feed prior to slaughter also increases hide contamination (2, 3). E. coli and Salmonella spp. transfer onto cattle hides that occurs in the holding environments of U. S beef processing plants accounts for a larger proportion of hide and carcass contamination than does the initial bacterial population found on the cattle exiting the feedlot (1). Hide wash cabinets, carcass trimming, spraying with sanitizers (4), hot-water washing, irradiation, and using dips (14) all reduce microbial load.

Diet per se appears to have some, if small, effects on pathogens in the rumen, gut and feces of cattle. However, once they are slaughtered, whether those bacteria end up on the carcass or on the meat cut from it is largely a function of sanitary practices during transport, lairage, slaughter, chilling, and processing. Therefore, the statement, “Meat from Grass Fed Cattle is Safer than Meat from Cattle that are Corn Fed” is between “Unknown” and “Misguided” because if we don’t have good repeatable scientific evidence that changes in microbe distributions in the rumen, hindgut, feces and hides that result from dietary alterations do, in fact carry over onto the meat, we would be remiss in saying that the diet is the (one and only) cause of the problem.

_References

_{Arthur, TM, Bosilevac, JM, Brichta-Harhay, DM, Kalchayanand, N, King, DA, Shackelford,-SD, Wheeler, TL, Koohmaraie, M. 2008. Source tracking of Escherichia coli O157:H7 and Salmonella contamination in the lairage environment at commercial U.S. beef processing plants and identification of an effective intervention. J. Food Protect. 71(9): 1752-1760}

_{Arthur,TM, Bosilevac, JM, Brichta-Harhay, DM, Guerini, MN, Kalchayanand, N, Shackelford, SD, Wheeler, TL, Koohmaraie, M. 2007. Transportation and lairage environment effects on prevalence, numbers, and diversity of Escherichia coli O157:H7 on hides and carcasses of beef cattle at processing. J. Food Protect. 70(2): 280-286}

_{Beach, JC, Murano, EA, Acuff, GR. 2002. Prevalence of Salmonella and Campylobacter in beef cattle from transport to slaughter. J. Food Protect. 65(11): 1687-1693}

_{Bell, KY, Cutter, CN, Sumner, SS. 1997. Reduction of foodborne micro-organisms on beef carcass tissue using acetic acid, sodium bicarbonate, and hydrogen peroxide spray washes. Food Microbio. 14(5): 439-448}

_{Berg, J, McAllister, T, Bach, S, Stilborn, R, Hancock, D, LeJeune, J. 2004. Escherichia coli O157:H7 excretion by commercial feedlot cattle fed either barley- or corn-based finishing diets. J. Food Protect. 67(4): 666-671}

_{Berry, ED, Wells, JE, Archibeque, SL, Ferrell, CL, Freetly, HC, Miller,DN. 2006. Influence of genotype and diet on steer performance, manure odor, and carriage of pathogenic and other fecal bacteria. II. Pathogenic and other fecal bacteria. J. An. Sci. 84(9): 2523-2532}

_{Callaway, TR, Elder, RO, Keen, JE, Anderson, RC, Nisbet, DJ 2003. Forage feeding to reduce preharvest Escherichia coli populations in cattle, a review. J. Dairy Sci. 86(3): 852-860}

_{Depenbusch, BE, Nagaraja, TG, Sargeant, JM, Drouillard, JS, Loe, ER, Corrigan, ME. 2008. Influence of processed grains on fecal pH, starch concentration, and shedding of Escherichia coli O157 in feedlot cattle. J. An. Sci. 86(3): 632-639}

_{Dewell, GA, Simpson, CA, Dewell,RD, Hyatt, DR, Belk, KE, Scanga, JA, Morley, PS; Grandin,-T; Smith,-GC; Dargatz,-DA; Wagner,-BA; Salman,-MD. 2008. Risk associated with transportation and lairage on hide contamination with Salmonella enterica in finished beef cattle at slaughter. J. Food Protect. 71(11): 2228-2232}

_{Diez-Gonzalez, F, Callaway, TR, Kizoulis, MG, Russell, JB . 1998. Grain feeding and the dissemination of acid-resistant Escherichia coli from cattle. Sci. 281(5383), 1666-166}

_{Fegan, N, Vanderlinde, P, Higgs, G, Desmarchelier, P. 2004. Quantification and prevalence of Salmonella in beef cattle presenting at slaughter. J. App. Microbio. 97(5), 892-89}

_{Gilbert, RA, Denman, SE, Padmanabha, J, Fegan, N, Al-Ajmi, D, McSweeney, CS. 2008. Effect of diet on the concentration of complex Shiga toxin-producing Escherichia coli and EHEC virulence genes in bovine faeces, hide and carcass. Internat. J. Food Microbio.. 121(2): 208-216}

_{Grau, FH. 2008. Campylobacter jejuni and Campylobacter hyointestinalis in the intestinal tract and on the carcasses of calves and cattle. J. Food Protect. 51(11) p. 857-861}

_{Hussein, H S, Lake, SL, Ringkob, TP. 2001. Cattle as a reservoir of Shiga-like toxin-producing Escherichia coli including O157:H7 – pre- and post-harvest control measures to assure beef safety. Prof. An. Sci. 17(1), 1-16}

_{Hutchison, ML, Walters, LD, Avery, SM, Munro, F, Moore, A. 2005. Analyses of livestock production, waste storage, and pathogen levels and prevalences in farm manures. App. Environ. Microbiol. 71(3): 1231-1236}

_{Jacobson, LH, Nagle, TA, Gregory, NG, Bell, RG, Le-Roux, G, Haines, JM. 2002. Effect of feeding pasture-finished cattle different conserved forages on Escherichia coli in the rumen and faeces. Meat Sci. 62(1): 93-106}

_{Krueger, NA, Anderson, RC, Krueger, WK, Horne, WJ, Riley, DG, Loneragan,GH, Phillips,WA, Gray, JT, Fedorka-Cray, PJ. 2008. Fecal shedding of foodborne pathogens by Florida-born heifers and steers in U.S. beef production segments. J. Food Protect. 71(4): 807-81}

_{Russell, JB, Diez-Gonzalez, F, Jarvis, GN. 2000. Invited review: effects of diet shifts on Escherichia coli in cattle. J. Dairy Sci. 83(4): 863-873}

_{Wesley, IV, Callaway,TR, Edrington, TS, Carstens, GE, Harvey,RB, Nisbet, DJ. 2008. Prevalence and concentration of Campylobacter in rumen contents and feces in pasture and feedlot-fed cattle. Foodborne Path. Disease. 5(5): 571-577}

_{Wells,JE, Berry,ED, Varel,VH, 2005. Effects of common forage phenolic acids on Escherichia coli O157:H7 viability in bovine feces. App. Environ. Microbiol. 71(12): 7974-7979}

Introduction
The global population is predicted to increase to 9.5 billion people in the year 2050 (U.S. Census Bureau, 2008). This is predicted to increase total food requirements by 100% (Tilman et al., 2002) and will be further complicated by demand for milk and meat protein as formerly-low-income populations become more affluent (Keyzer et al., 2005). The challenge to the beef industry is to reconcile the need for sufficient safe, affordable, high-quality meat for the growing population, with consumer quality and safety perceptions related to different feeding or production systems.

The US beef production cycle
The majority of US beef systems can be summarized into three stages: 1) cow-calf operations where calves are born, suckle and graze until weaning at 6-10 months old; 2) weanling/stocker operations where animals graze until 12-18 months of age and; 3) feedlots where stock are group-housed and fed a corn-based diet until slaughter (18-22 months; IBISWorld, 2009). Most of a ‘corn-fed’ animal’s life is therefore spent grazing on pasture: high-corn diets are only fed in the final finishing period to ensure animals meet processor weight and condition specifications. Feedlots have been a major contributor to increased production efficiency over the past 30 years: in 1975, 24.0 billion lb of beef was produced in 1975 from 40.9 million beef animals (USDA, 1976), compared to 26.7 billion lb from 34.5 million head in 2008 (USDA, 2009). This is equivalent to an extra 187 lb of beef being produced per animal.

The term ‘grass-fed’ is used to refer to cattle that are finished by grazing on pasture. The USDA certification process for grass-fed livestock specifies that animals must be fed only grass and forage after weaning, with no grain or grain byproducts in the diet (USDA/AMS, 2007). Nonetheless, it is important to note that unlike organic certification, this process is strictly voluntary and any producer may label their product as ‘grass-fed’ without third-party verification. Furthermore, ‘grass-fed’ certification does not contain any provision re: the use of antimicrobials or hormones. Therefore although the label may imply a quality attribute for the product, it refers only to feeding practices and should not be considered as a guarantee of other management, safety or quality considerations. Grass-fed beef systems are demonstrably less efficient than conventional production, with greater resource use and environmental impact per pound of beef produced (Avery and Avery, 2008). Although grass-fed beef often commands a premium price for the producer, this may be beyond the purchasing capacity of many consumers; therefore the economic and social sustainability of the system is questionable.

Beef product safety
The USDA Food Safety and Inspection Service (FSIS; http://www.fsis.usda.gov/) acts to ensure a safe and wholesome US meat supply via a variety of regulations, policies and inspection programs. Nonetheless, media coverage of potential human health risks associated with beef, combined with readily-available (but not necessarily scientifically-verified) information from the internet, may lead to its safety being questioned by the consumer.

Beef product safety was seriously questioned by the consumer after a link was discovered between the UK outbreak of bovine spongiform encephalopathy (BSE) and the incidence of variant Creutzfeldt–Jakob (vCJD) disease in humans. This led to consumer concern that was exacerbated further when the first US BSE case was noted in 2003 (CDC, 2008). Nonetheless, removing animal protein from ruminant diets, surveillance testing and policies to prevent at-risk cattle from entering the human food chain have prevented BSE from becoming endemic in the USA (CDC, 2008) and the risk of contracting vCJD from US beef, regardless of production system, is extremely low (Donnelly, 2004).

Aggressive publicity campaigns by activist groups opposed to conventional agriculture have led to feedlots being labeled as ‘factory farms’ (Nierenberg, 2005), claiming that corn-fed beef is not ‘natural’ (Pollan, 2006) and may threaten human health (O’Brien, 2001). This leads to the misconception that foodborne diseases are the prevalence of feedlots alone and that meat from grass-fed animals is intrinsically safe. For example, the bacteria E. coli 0157:H7 is found naturally in animal and human intestines, but via ingestion of insufficiently-cooked contaminated meat, is responsible for ~73,000 cases of foodborne disease per year (Mead et al., 1999). Indeed, E. coli 0157:H7 contamination led to the recall of 21.7 million lb of beef in 2007 (USDA, 2007). Contamination of meat by intestinal contents or fecal matter at slaughter is a major factor in E. coli 0157:H7 transmission (National Academy of Sciences, 2002), but this risk is reduced to almost zero by effective slaughterhouse management policies (Bacon et al., 2000). Despite activists’ claims, studies show similar prevalence of E. coli 0157:H7 between pasture and feedlot systems (Renter et al., 2004; Rasmussen & Casey, 2001) and there is no scientific evidence to suggest that beef from grass-fed cattle may be microbiologically safer than from cattle that are fed corn.

Absence claims (e.g. ‘no hormones’, ‘no antibiotics’) are increasingly used as marketing tools within the livestock industry to differentiate between nutritionally-identical products. Two classes of pharmaceuticals used in beef production are highlighted as areas of consumer concern: antimicrobials and hormones (Brewer & Rojas, 2008). Antimicrobials are used within the beef production system for therapeutic and feed additive purposes. Animal welfare is paramount within animal agriculture; therefore rigorously-tested therapeutic drugs approved by the U.S. Food and Drug Administration (FDA) are used to treat disease in both corn- and grass-fed beef animals (FDA/CVM, 2000). Use of therapeutic antimicrobials only when necessary and under the guidance of a veterinarian, ensures optimum animal health, welfare and the production of safe food for human consumption. Feed antimicrobials improve the efficiency by which feed is converted into weight gain, thus reducing resource use (NRC, 1999). The majority (83%) of feedlots added antimicrobial drugs to feed or water in 1999 (USDA, 2000) and although similar data is not available for pasture-based systems, the ‘grass-fed’ label does not automatically infer their absence. Nevertheless, bacterial resistance to antimicrobial drugs is a growing concern. The FDA (2000) considers the sporadic nature of therapeutic drug use unlikely to contribute to antimicrobial resistance; but, following a European ban on feed antimicrobial use in swine and poultry, evidence suggests that these drugs may also be removed from US production systems in future (Fajt, 2007). One often-repeated myth is that antimicrobials from feedlot systems are concentrated in meat, whereas grass-fed beef is ‘natural’ and therefore healthy. Regardless of feeding system, strict regulations govern the minimum time period that must elapse between therapeutic and feed antimicrobial use and slaughter to prevent antimicrobial residues being in meat. Products that exceed residue limits do not enter the food supply and confer significant financial penalties to the producer (FSIS, 2008). Despite marketing claims by individual producers, there is no scientific evidence to suggest that antimicrobial use differs between corn-fed or grass-fed systems, and the regulatory, monitoring and testing procedures in place ensure that beef products from all systems are safe for human consumption.

Growth-promoting steroid hormones are integral to US beef production, facilitating increased meat production using fewer resources and with a lower environmental impact (Avery & Avery, 2007). A constant supply of hormones is provided by a subcutaneous implant in the ear, which is discarded at slaughter (Preston, 1999). Despite the rigorous animal and human safety certification programs in place, some organizations suggest that supplemental hormones given to beef animals may lead to undesirable human health effects including early puberty and cancer (Balter, 1999). However, the hormones used in implants are natural or synthetic equivalents of steroids naturally produced by the animal and the quantities found in meat are extremely low (Preston, 1999). For example, an individual would have to consume >13 lb of beef from an implanted steer to equal the amount of estradiol naturally found in a single egg (Foreign Agricultural Service, 1999). As with antimicrobial drugs, the ‘grass-fed’ label is no indication of whether growth-promoting hormones are used in a beef production system and there is no published scientific data to imply a difference in hormone use between pasture and corn-based feeding systems.

Conclusion
Opportunities exist for all food production systems within US agriculture and individual producers may gain economic benefits from niche marketing strategies. However, the safety and quality of all beef products is assured by federal regulatory, monitoring and testing procedures, therefore marketing claims solely relating to feeding practices should not be considered to confer any quality or safety benefits above other production systems.

_{References
Avery, A. and D. Avery (2007) The Environmental Safety and Benefits of Pharmaceutical Technologies in Beef Production. Hudson Institute, Center for Global Food Issues, Washington, DC.
Bacon, R. T., K. E. Belk, J. N. Sofos and G. C. Smith (2000) Executive summary: Incidence of Escherichia coli O157:H7 on hide, carcass and beef trimmings samples collected from United States packing plants. 53rd Annual Reciprocal Meat Conference, Ohio State University, Columbus, OH.
Balter, M. (1999) Scientific cross-claims fly in continuing beef war. Science 284:1453-1455.
Brewer, M. S. and M. Rojas (2008) Consumer attitudes towards issues in food safety J. Food Saf. 28:1-22.
CDC (2008) BSE (Bovine Spongiform Encephalopathy, or Mad Cow Disease)http://www.cdc.gov/ncidod/dvrd/bse/
Donnelly, C. A. (2004) Bovine Spongiform Encephalopathy in the United States — An Epidemiologist’s View. New Enlg. J. Med. 350:539-542.
Fajt, V. R. (2007) Regulation of drugs used in feedlot diets. Vet. Clin. North Am. 23:299-307.
FDA (2000). HHS Response to House Report 106-157- Agriculture, Rural Development, Food and Drug Administration, and Related Agencies, Appropriations Bill, 2000. Human-Use Antibiotics in Livestock Production. U.S. FDA, Washington, DC.
FDA/CVM (2000) Judicious Use of Antimicrobials for Beef Cattle Veterinarians. FDA/CVM, Washington, DC.
FSIS (2008) 2008 FSIS National Residue Program Scheduled Sampling Plans. USDA/FSIS, Washington, DC
Foreign Agricultural Service (1999) A Primer on Beef Hormones.http://stockholm.usembassy.gov/Agriculture/hormone.html
IbisWorld (2009) Beef Cattle Production in the US #11211. IBISWorld, Santa Monica, CA.
Keyzer, M. A., M. D. Merbis, I. F. P. W. Pavel, and C. F. A. van Wesenbeeck (2005) Diet shifts towards meat and the effects on cereal use: can we feed the animals in 2030? Ecological Economics 55:187-202.
Mead, P. S., L. Slutsker, V. Dietz, L. F. McCaig, J. S. Bresee, C. Shapiro, P. M. Griffin, and R. V. Tauxe (1999) Food-Related Illness and Death in the United States. Centers for Disease Control and Prevention, Atlanta, GA.
National Academy of Sciences (2002) Escherichia coli O157:H7 in Ground Beef: Review of a Draft Risk Assessment. Committee on the Review of the USDA E. coli O157:H7, Farm-to-Table Process Risk Assessment, Washington, DC.
Nierenberg, D. (2005) Happier Meals: Rethinking the Global Meat Industry. Worldwatch paper # 171. Worldwatch Institute.http://www.wellfedworld.org/PDF/WorldWatch%20Happier%20Meals.pdf
NRC (1999) The Use of Drugs in Food Animals: Benefits and Risks. National Academy Press, Washington, DC.
O’Brien, T. (2001) Factory farms and human health. The Ecologist. 31:5 30-34, 58-59.
Pollan, M. (2006) The Omnivore’s Dilemma. The Penguin Group (USA) Inc, New York, NY.
Preston, R. L. (1999) Hormone containing growth promoting implants in farmed livestock. Adv. Drug Del. Rev. 38:123-138.
Rasmussen, M. A. and T. A. Casey (2001) Environmental and food safety aspects of Escherichia coli O157:H7 infections in cattle. Crit. Rev. Microbiol. 27:57-73.
Renter, D. G., J. M. Sargeant and L. L. Hungerford (2004) Distribution of Escherichia coli O157:H7 within and among cattle operations in pasture-based agricultural areas. Am. J. Vet. Res. 65:1367-1376.
Tilman, D., K. G. Cassman, P. A. Matson, R. Naylor and S. Polasky (2002) Agricultural sustainability and intensive production practices. Nature 418:671-677
U.S. Census Bureau (2008) Total Midyear Population for the World: 1950-2050.http://www.census.gov/ipc/www/idb/worldpop.html
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USDA (2009) Data and Statistics.http://www.nass.usda.gov/Data_and_Statistics/Quick_Stats/index.asp
USDA/AMS (2007) United States Standards for Livestock and Meat Marketing Claims, Grass (Forage) Fed Claim for Ruminant Livestock and the Meat Products Derived From Such Livestock. Docket # AMS–LS–07–0113;LS–05–09. USDA/AMS, Washington, DC.}

Introduction: What do we mean by safe?
To begin this discussion, we need to understand what “safe” really means. From the standpoint of meat and poultry products, the U.S. Department of Agriculture, Food Safety Inspection Service (USDA-FSIS) classifies hazards as either physical, chemical or biological. The risks associated with either physical or chemical hazards in live cattle are primarily related to animal husbandry practices, and are generally unrelated to the diet of the animals. There is nothing inherent in the production of either grass or grain fed cattle that would be likely to change the risk associated with either physical or chemical hazards. This leaves biological hazards, which in the United States are commonly thought of as bacterial contaminants. These would include various species of Salmonella and the pathogenic Escherichia coli species, especially E. coli O157:H7. This document will focus exclusively on the potential differences in biological hazards between grass fed and grain fed cattle.

What the Science says: Live Animal
Live cattle may harbor E. coli O157:H7 with no external signs of illness. This makes detection in the live animal difficult, as the bacterium may be not only in the intestinal tract and feces, but also on the hide, hair and hooves of the animal. Since the early 1990â€s there has been a growing body of scientific evidence on the impact of cattle diets on the presence of bacteria of public health significance in or on the live animal. In particular, feeding large amounts of corn has been reported to increase the populations of Escherichia coli (both pathogenic and non-pathogenic) in the cowâ€s digestive tract. These bacteria are also more resistant to acid conditions. Switching the diet of the live cow from grain to grass has been reported to result in a decrease in the overall population as well as a decrease in the acid resistance of the E. coli population.
When humans consume food, the food collects in the gastric fluid in the stomach, which is very acidic. This exposure to acid may kill many potential human pathogens, preventing the human host from becoming ill. Increasing the acid resistance of any group of bacteria may increase the probability of survival, and therefore the likelihood that the human host may become ill. It is biologically plausible that bacteria which are more acid-tolerant may be more likely to survive passage through the stomach and therefore be more likely to cause illness in humans. However, many enteric bacteria have a natural occurring acid tolerance response which allows significant numbers of bacteria to survive exposure to the low pH of gastric fluid. E. coli O157:H7 in particular has a very low infectious dose for humans, which suggests that it has the innate mechanisms to survive passage through the human stomach already, without any increase in acid resistance.

What the Science says: Processing
E. coli O157:H7 follows a seasonal pattern both in terms of occurrence on cattle and in numbers of human illnesses, with the peak times for both during the summer months. In some studies, up to 75% of the cattle entering slaughter establishments have been reported to carry E. coli O157:H7 on their hides. However, in those same studies, the bacterium is rarely isolated from the finished carcasses in the holding coolers. This is directly attributable to the hygienic slaughter and dressing of the carcasses, as well as the widespread use of a variety of antimicrobial interventions, which have been developed over the last twenty years. In addition, the USDA-FSIS made a substantial change in the meat inspection regulations in 1996, with the focus of inspection now on the prevention of contamination during processing.
E. coli O157:H7 does not infect cattle in the sense that it does not enter the edible muscle tissue. It is confined to the gastro-intestinal tract or the external surface of the animal, and contamination occurs as accidental contact between the hide or intestinal contents with the edible muscle tissue. Modern slaughter establishments focus on reducing microbial contamination by both physical means (avoidance of contamination) and direct interventions. Although no system is perfect, as evidenced by the continuing recalls and human illnesses linked to meat, it is overall remarkably effective in controlling contamination in an industry that slaughters over 30 million cattle every year.

What are the practical implications:
The science suggests that cattle fed grass in place of grain may have lower populations of E. coli in their intestinal contents and that these bacteria may be less resistant to acid. However, whether these lower numbers of E. coli relate to less contamination of beef carcasses is at best hypothetical. In spite of popular opinion, contamination of beef carcasses with bacteria of public health significance is a relatively rare occurrence, although it is certainly regrettable when it occurs. Whether an increase in acid resistance of E. coli O157:H7 results in a greater risk of infection would appear unlikely, as naturally occurring E. coli O157:H7 already seem to possess the mechanisms necessary to survive passage through the human stomach.

Is meat from grass-fed cattle is safer than meat from cattle that are fed corn?

Probably not from a biological point of view. The practical implications of lower E. coli populations to carcass contamination have not been demonstrated, and it is unlikely that an increase in acid resistance will materially affect the number of human infections from E. coli O157:H7.

No matter which diet the animals are fed, there is still the need for the consumer to exercise reasonable care in handling any raw foods. Much like driving an automobile where the driver takes reasonable precautions, such as wearing a seat belt and driving defensively, consumers need to handle and cook food safely. The USDA-FSIS has many consumer resources for handling and cooking foods, but the pamphlet “Basics for handling food safely” probably summarizes these practices best. The basics are:

• Clean – Wash hands and surfaces often
• Separate – Keep raw and cooked foods separate
• Cook – Cook to proper temperatures
• Chill – Refrigerate foods promptly

_References

_{Arthur, T.M., J. M. Bosilevac, X. Nou, S. D. Shackelford, T. L. Wheeler, M. P. Kent, D. Jaroni, B. Pauling, D. M. Allen, and M. Koohmaraie. 2004. Escherichia coli O157 Prevalence and Enumeration of Aerobic Bacteria, Enterobacteriaceae, and Escherichia coli O157 at Various Steps in Commercial Beef Processing Plants. J. Food Protect. 67: 658–665}

_{Diez-Gonzalez, F., T. R. Callaway, M. G. Kizoulis, J. B. Russell 1998. Grain Feeding and the Dissemination of Acid-Resistant Escherichia coli from Cattle. Science 281, 1666-1668.}

_{Martz, F. 2000. Pasture-based finishing of cattle and eating quality of beef.
http://aes.missouri.edu/fsrc/research/pasture.stm (accessed 21 May 2009)}

_{Russell, J.B. F. Diez-Gonzalez and G.N. Jarvis. 2000. Potential effect of cattle diets on the transmission of pathogenic Escherichia coli to humans. Microbes and Infection, 2: 45−53.}

_{U.S. Department of Agriculture – Food Safety and Inspection Service. 2009. Safe Food Handling.http://www.fsis.usda.gov/factsheets/Safe_Food_Handling_Fact_Sheets/index.asp(accessed 21 May 2009)}

_{U.S. Department of Agriculture – Food Safety and Inspection Service. 2009. Basics for handling food safely.
http://www.fsis.usda.gov/PDF/Basics_for_Safe_Food_Handling.pdf (accessed 21 May 2009)}