TABLE 1.

Acid resistance of E. coli O157:H7 from grain-fed and hay-fed cattle

Animal no.	% E. coli survival in cattle feda:
	Grain	Hay
1	75	29
2	34	100
3	12	100
4	10	15
5	82	23
6	22	29
7	34	19
8	23	20
Mean	37	42

^aThe grain diet contained 90% corn–10% alfalfa silage. The hay diet contained 100% timothy. The average pH of the colonic digesta from animals fed grain was 5.5, and the average pH of the colonic digesta from animals fed hay was 7.2; both are consistent with the digesta pH values reported in reference 4. Percents survival were calculated from cultures on differential sorbitol MacConkey agar supplemented with 4-methylumbelliferyl-β-d-glucuronide (100 mg/liter) before and after acid shock at pH 2.0 for 1 h at 37°C. Presumptive E. coli O157:H7 isolates were confirmed serologically. The inoculum was E. coli O157:H7 strain ATCC 43895 (American Type Culture Collection, Manassas, Va.). All animals were tested on three separate days; data from one typical sampling day are shown.