Gastrointestinal Acidosis and Its Effect on Escherichia coli in Cattle

Abstract

Escherichia coli is a normal inhabitant of the gastrointestinal (GI) tract of warm blooded animals, and most E. coli strains are harmless. However, gram-negative bacteria can release lipopolysaccharide complexes from their cell walls (including lipid A) when they lyse.¹⁴ These endotoxins can cause fever, and even death, but only if the bacterium migrates from the gut to the blood. Some strains of E. coli produce an enterotoxin that resembles cholera toxin, and this protein causes acute diarrhea even if the bacterium never crosses the intestinal epithelium.¹⁷

In 1982, an E. coli strain designated as O157:H7 was isolated from the bloody feces of people that had consumed contaminated hamburgers, and these strains produced toxins that could diffuse into intestinal cells.^14,20 Genetic analyses of E. coli O157:H7 indicated that it carried two toxin genes that were homologous to the ones borne by Shigella.¹⁷ E.coli O157:H7 has a powerful hemolysin and an intestinal adherence factor known as Intimin.¹⁷ Humans that are infected with E. coli O157:H7 become acutely ill, but mature cattle are asymptomatic carriers.^1,9

Ground beef has been a common source of E. coli O157:H7 infection, but fruits and vegetables have also caused outbreaks and illness.¹ Fruits and vegetables are often fertilized with cattle manure, and beef can be contaminated with fresh manure at slaughter. Cattle have been suspected as being a primary source of E. coli O157:H7.9 However, it should be noted that other animals and wildlife can carry this bacterium.¹ Early work indicated that only small numbers of cattle carried E. coli O157:H7,9 but sensitive detection methods that used immunomagnetic beads indicate that as 30% to 50% of the cattle may be infected.^15,16

Hancock and his colleagues examined the effect of diet on E. coli O 15 7 :H7, but detection methods were insensitive and dietary correlations were either weak or inconsistent.^9,11 Recent work indicated that grain feeding, a practice common in the cattle industry, can alter the physiology and survival of E.coli, but a brief period of hay feeding was able to counteract this potentially dangerous effect.