The number and proportion of CTX-M positive Escherichia coli organisms were determined in feces from cattle, chickens, and pigs in the United Kingdom to provide a better understanding of the risk of the dissemination of extended-spectrum -lactamase (ESBL) bacteria to humans from food animal sources. Samples of bovine (n ؍ 35) and swine (n ؍ 20) feces were collected from farms, and chicken cecal contents (n ؍ 32) were collected from abattoirs. There was wide variation in the number of CTX-M-positive E. coli organisms detected; the median (range) CFU/g were 100 (100 ؋ 10 6 to 1 ؋ 10 6 ), 5,350 (100 ؋ 10 6 to 3.1 ؋ 10 6 ), and 2,800 (100 ؋ 10 5 to 4.7 ؋ 10 5 ) for cattle, chickens, and pigs, respectively. The percentages of E. coli isolates that were CTX-M positive also varied widely; median (range) values were 0.013% (0.001 to 1%) for cattle, 0.0197% (0.00001 to 28.18%) for chickens, and 0.121% (0.0002 to 5.88%) for pigs. The proportion of animals designated high-density shedders (>1 ؋ 10 4 CFU/g) of CTX-M E. coli was 3/35, 15/32, and 8/20 for cattle, chickens, and pigs, respectively. We postulate that high levels of CTX-M E. coli in feces facilitate the dissemination of bla CTX-M genes during the rearing of animals for food, and that the absolute numbers of CTX-M bacteria should be given greater consideration in epidemiological studies when assessing the risks of food-borne transmission.The emergence and spread of extended-spectrum beta-lactamases (ESBLs), particularly among Enterobacteriaceae, is well recognized as a threat to the efficacy of extended-spectrum cephalosporins for the treatment of serious infections (8,11,14,31,35). During the last 5 years, ESBLs belonging to the CTX-M family of enzymes have been reported from many countries from a variety of different food-producing animals, including cattle, chickens, and pigs (17,21,22,25), and these animals are recognized as reservoirs of extended-spectrum -lactamase producers (12). The presence of CTX-M ESBLs in United Kingdom cattle was first reported in 2005 and 2006 (20, 37). More recently, CTX-M ESBL-positive Escherichia coli has also been detected in United Kingdom poultry flocks (32).Epidemiological studies often use selective agar to ascertain and report the presence or absence of CTX-M-producing bacteria in samples; however, the number of bacteria present in samples is usually not reported. Fecal carriage is an important factor for the spread of CTX-M ESBL bacteria among both human communities (34) and animals (1, 12, 15, 24) and will be influenced by factors such as previous treatment with antimicrobials (2, 4, 6). The dissemination of CTX-M E. coli in food production units may occur via fecal cross-contamination between groups of animals (or individuals), and the contamination of food derived from animals may occur during processing in the abattoir. Previous studies have shown that the number of E. coli O157 organisms shed in feces is an important factor for dissemination during slaughter and carcass processing (29). Therefore, the aim of t...