Pump Handles and Cholera Genes

On the night of August 31 in 1854, a terrible epidemic of cholera broke out in the Soho neighborhood of London. Hundreds of residents were stricken with severe diarrhea and vomiting and, in the next three days, 127 people living on or near Broad Street died. By September 10, the number of fatalities had climbed to more than 500. It was the worst outbreak of cholera ever seen in England. Residents of Soho fled their homes in terror, leaving businesses closed, homes locked, and streets deserted.

The Soho epidemic was witnessed firsthand by Dr. John Snow, a physician who lived on Sackville Street and saw the devastating effects and rapid spread of the disease. He had conducted research on cholera and suspected that it was spread through the water supply, but most medical authorities dismissed his suspicion.

Snow conducted a thorough survey of the Soho neighborhood, identifying all those who were sick with cholera. He plotted the locations of the cases on a map and observed that they clustered around one particular water pump located on Broad Street. Cholera cases did not cluster around other nearby water pumps. Snow contacted the parish officials and convinced them to remove the handle to the Broad Street pump and, with this simple action, the spread of cholera stopped dramatically. Snow later conducted additional studies of cholera outbreaks in London and established that the disease was spread in water contaminated with sewage.

Cholera has existed in Asia for at least 1000 years but, at the time of the Soho epidemic, it was a relatively new disease in England. Today, cholera is recognized as a severe infection of the intestine caused by Vibrio cholerae ( FIGURE 8.1). This bacterium produces a potent endotoxin that induces

I 8.1 Vibrio cholerae is the bacterium that causes cholera. (CMRI/Science Photo Library/Photo Researchers.)

copious diarrhea and vomiting. If untreated, the condition can lead to serious dehydration and death. Although the number of cholera deaths has dropped dramatically since the advent of oral rehydration treatment and antibiotics, the disease continues to be a serious public-health problem, particularly in areas that lack modern water-supply systems. A recent epidemic in South Africa infected more than 25,000 people in a 6-month period.

Many of cholera's secrets have now been revealed through the sequencing of V. cholerae's genome. Most bacteria have a single circular chromosome, but V. cholerae has two. One of the most significant findings to emerge from the sequencing of the V cholerae genome is that many of the bacterium's genes for pathogenesis were acquired from other bacteria. Vibrio cholerae apparently has a long history

Table 8.1 Advantages of using bacteria and 1

viruses for genetic studies

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