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Hay, Grain, and E. coli Revisited
In September 1998, we published a paper entitled ``Grain-Feeding and the Dissemination of Acid-Resistant Escherichia coli from Cattle'' (Science 281:1666-1668). Results indicated that grain feeding, by enhancing colonic fermentation and decreasing pH, could induce the extreme acid resistance of E. coli, but a brief period of hay feeding could reverse this trend. Because acid-resistant E. coli are more likely to survive the low pH of the human gastric stomach, we felt that these findings were important.
Beginning in October 1998, commentaries by D. D. Hancock, T. E. Besser, C. Gill, and C. Hovde Bohach were widely distributed on the Internet and in magazines, and in April 1999 they sent a letter to Science (284:51-52) criticizing our work. In our reply, we demonstrated that nearly all of the references cited by Hancock et al. had quotations that failed to support their arguments (Science 284:52-53).
In July 1999, Hovde et al. published a note entitled ``Effect of Cattle Diet on Escherichia coli O157:H7 Acid-Resistance'' (Appl. Environ. Microbiol. 65:3233-3235). Grain-fed cattle had lower colonic pH values than cattle fed hay (5.5 versus 7.2), but the difference in acid-resistant E. coli was <1 log. They indicated that there could have been ``differences in methods of acid exposure or bacterial enumeration,'' but the idea that acid resistance was important or influenced by cattle diet was rebuffed.
In November 1999, scientists from the University of Nebraska published a cattle study (Scott et al., 2000 Neb. Beef Report, 39-41), and they examined the effect of three grain sources. Pooled results indicated that grain to hay diet changes increased colonic pH (6.5 to 7.9) and acid-resistant E. coli decreased from 10,000 to 20 viable cells per g feces. They concluded that ``this study confirms Diez-Gonzalez (1998) report that feeding hay for a short duration can reduce acid-resistant E. coli populations.''
The July 1999 note of Hovde et al. also examined the effect of grain and hay on cattle (n = 8) that were inoculated with a large doses (1010 cells) of E. coli O157:H7 that were grown aerobically in Luria broth. Because hay-fed cattle shed E. coli O157:H7 longer than grain-fed cattle, the authors concluded that ``feeding cattle hay may increase human infections with E. coli O157:H7.''
In November 1999, scientists from The U.S. Meat Animal Research Center described experiments that used cattle naturally infected with E. coli O157:H7 (Keen et al., 80th Conference of Research Workers in Animal Diseases, Abstract #86, Nov. 7-9, 1999, Chicago, Ill.). When beef cattle (n = 200 animals) were fed rations rich in grain, 52% of the animals were E. coli O157:H7 positive. Cattle (n =100) maintained on grain continued to shed E. coli O157:H7 (53% positive), but only 18% of the cattle (n = 100) that were switched to hay for 7 days were E. coli O157:H7 positive (P < 0.05). Cattle fed hay had colonic pH values that were higher than those fed grain (7.0 versus 6.0), but acid-resistant E. coli were not enumerated.
Given results of Scott et al. and Keen et al., the conclusion of our original Science article still seems appropriate. However, further research is needed to see if other dietary changes can also decrease the acid-resistance of E. coli and E. coli O157:H7 shedding from cattle.
James B. Russell
ARS/USDA and Department of Microbiology
Cornell University, Ithaca, N.Y.
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