Under a magnification of 6836x, this colorized scanning electron micrograph (SEM) depicted a number of Gram-negative Escherichia coli bacteria of the strain O157:H7, which is one of hundreds of strains of this bacterium. Although most strains are harmless, and live in the intestines of healthy humans and animals, this strain produces a powerful toxin, which can cause severe illness.
E. coli O157:H7 was first recognized as a cause of illness in 1982 during an outbreak of severe bloody diarrhea; the outbreak was traced to contaminated hamburgers. Since then, most infections have come from eating undercooked ground beef.
The combination of letters and numbers in the name of the bacterium refers to the specific markers found on its surface, which distinguishes it from other types of E. coli.
Escherichia coli O157:H7 is an emerging cause of foodborne illness. An estimated 73,000 cases of infection, and 61 deaths occur in the United States each year. Infection often leads to bloody diarrhea, and occasionally to kidney failure. Most illness has been associated with eating undercooked, contaminated ground beef. Person-to-person contact in families and child care centers is also an important mode of transmission. Infection can also occur after drinking raw milk, and after swimming in, or drinking sewage-contaminated water.
Consumers can prevent E. coli O157:H7 infection by thoroughly cooking ground beef, avoiding unpasteurized milk, and washing hands carefully. Because the organism lives in the intestines of healthy cattle, preventive measures on cattle farms and during meat processing are being investigated.
Source: CDC/ National Escherichia, Shigella, Vibrio Reference Unit at CDC