Science and Technology

How the Sari Guards Against Cholera

A woman dries her sari in the wind near Kolkata
A woman dries her sari in the wind near Kolkata (Photo: Deshakalyan Chowdhury/AFP-Getty Images).

Contrary to received wisdom, the pathogens responsible for cholera are experts at survival. Related to zooplankton, they know how to multiply and disperse themselves over long distances in water. Now research in Bangladesh shows that water filtered through old sari cloth is largely free of these diarrhea-causing bacteria.

Every year when the monsoon dumps rain on the foothills of the Himalayas, the Brahmaputra floods over its banks in Bangladesh. Along its course, hundreds of villages are turned into islands in a sea of muddy water.

At the same time, the waters of the mighty river spill into sewers and drainage ditches and carry their untreated broth to wells used for drinking water. A few days later the second plague begins, one that must be as old as the monsoon floods: cholera. At first there is a case here and there, then suddenly this infectious intestinal disease assumes epidemic proportions.

Just how many people get sick in the next few weeks as a result of the epidemic is not known. Experts estimate there are two to four cases of cholera per 1,000 people. With a population of 125 million, that works out to 250,000 to 500,000 cases of illness each monsoon. But at least the death toll has fallen drastically since even the most remote clinics have gotten the simple means to fight cholera’s diarrhea with fluid-replacement treatments. Up to the mid-1980s, every third person who got cholera died from the uncontrolled diarrhea that characterizes the disease.

The signs now look good for the Bengali people. In the future, they should not have to see cholera as a fact of nature, as inevitable as the Brahmaputra’s floods. This is because a dozen top scientists from institutions as varied as the International Center for Diarrheal Disease Research in Dhaka, the University of Maryland Marine Biotechnology Institute, and the Bloomberg School of Public Health at Johns Hopkins, both in Baltimore, are working at decoding this plague’s final mysteries. The new discoveries are in the pathogen’s molecular biology fundamentals, the epidemiology of the disease, and preventive measures.

To begin with, the researchers abolished one of the basic assumptions dating from the time of Robert Koch. (This German bacteriologist discovered a comma-shaped organism in an Egyptian patient in 1883.) Vibrio cholerae, say the textbooks even today, is an exceptionally sensitive pathogen, one that can survive outside the human intestinal tract for only a few hours. But in fact, the vibrio is a master of survival. In water it binds to the outside of and to the inner organs of tiny crustaceans known as zooplankton. The tiny crustaceans not only guarantee the vibrio’s survival, but by providing an inexhaustible food supply, they ensure that the microorganisms can multiply and spread over long distances.

Depending on the species and size of the tiny crustaceans, each one can provide a home for up to 10,000 vibrios. When the zooplankton bloom in the tributaries of the Brahmaputra and in the Gulf of Bengal in September and October, the density of pathogens in the water quickly reaches levels between 10,000 and 100,000 vibrios per milliliter—the concentration needed to induce infection. Just a thimbleful of such water is sufficient to transform a healthy person, in hours, into a deathly ill cholera sufferer.

Current research in molecular biology is aimed at finding out why cholera vibrios are such potent pathogens. Once they get inside the human body, they begin producing an endotoxin. This poison causes the mucous membrane of the intestines to release huge quantities of water into the intestines, thus setting off the uncontrollable diarrhea characteristic of cholera. The enterotoxin consists of two subunits encoded by the CTX gene fragment. This poison molecule, however, was not invented by the cholera vibrio. New research has revealed that the CTX gene sequence comes from a bacteriophage—a virus that specializes in infecting bacteria.

All variants of the cholera vibrio capable of causing disease also possess another weapon for attacking the human intestine: the toxin coregulated pilus, TCP. TCP functions as a “grappling hook” that the bacterium uses to attach itself to the surface of an intestinal cell. Its genetic code is embedded in a complex DNA structure called the TCP pathogenicity island. Interestingly enough, this grappling hook is also the attachment point that the CTX bacteriophage uses to gain entry into the vibrio.

It has also been determined that a gene sequence in the TCP island, called toxT, codes for a transcription factor that accelerates both the expression of the TCP gene and the cholera toxin gene as soon as environmental stimuli signal the bacterium that it has arrived in the human intestine. Even experienced microbiologists were astonished by this example of evolutionary co-adaptation, in which a virus lends a bacterium a pathogenic trait, uses an existing pathogenic surface structure as an entry point, and finally is able to alter the genetic substance of its host so cleverly that both factors are synthesized precisely when the cholera vibrio needs to begin reproducing rapidly. The 15 to 20 episodes of watery diarrhea per day ensure that the newly produced pathogens are efficiently dispersed into the environment.

Another group of genes is responsible for making the vibrios released into the environment so infective that just a small dose is enough to make a person ill. These “hyperinfective” pathogens are found only in human excrement. If one takes these vibrios and cultivates them on nutritional media in the lab, or uses them to infect animal models, their infectiousness declines rapidly.

Cholera vibrios are identified by serotype, meaning molecules on their cell surface that bind to specific antibodies. The classic cholera pathogen is called 01. When in 1992 a cholera vibrio was discovered in Bangladesh that was not in the 01 family, this was more than a scientific sensation. Infectious disease experts worried that they were facing a new pathogen that would not respond to existing vaccines. The first research seemed to confirm this fear, but conclusive results are not in yet.

The new variant, called the 0139 serotype, one of the 206 serotypes found so far, is in fact a dangerous pathogen. As long-term studies in Bangladesh have discovered, the vibrios of the 01 and 0139 serotypes have been engaged in bitter competition for 10 years. Comparing the struggle to business, it is as if a newcomer tried to drive an established firm out of the market. In the beginning, the 0139 serotype did manage to supplant the classic cholera pathogen in large areas of Bangladesh—but the 01 strain soon regained the upper hand. Meanwhile, dozens of variants of each serotype have been discovered: It seems that each appears, competes with others, and then disappears. Although this is a terrible threat to the people of Bangladesh, for bacteriologists it has offered a rare opportunity to follow the evolution of competing pathogens in real time.

Recent research in Bangladesh proved that preventing cholera does not demand any complicated technology but can be done by everyone at no cost. Here toilet paper is either expensive or unknown. Ignorance of hygiene leads most people to use their left hand for the purpose. And soap, which could be used to wash away infectious bacteria, is not available, given standards of living here. So hands are typically the intermediary that germs use in transit from excrement to the kitchen.

Scientists from the International Center for Diarrheal Disease Research in Dhaka have now demonstrated that this situation can be changed. They carried out a careful study, employing 115 women from several villages. The study was as convincing as it was simple. After they visited “the quiet place,” the women participants were to wash their hands in soap and water, in ashes and water, or in clay and water. A fourth group followed their usual practice and did not wash their hands at all. Then all the women pressed their fingertips into a sterile growth medium so that the number of infectious pathogens could be determined. The test showed that ashes and clay reduced the number of germs by the same high degree that expensive soap did.

Nor must drinking surface water during the monsoon season necessarily lead to infection with cholera. Since the cholera vibrios are, for the most part, either in or on copepods, the idea arose of using filtration to remove these zooplankton and thus the pathogens as well. Recent trials found that nylon mesh with threads 150 microns apart would trap 99 percent of cholera vibrios in water.

But another fabric found everywhere in South Asia works even better: used sari cloth. If this fabric is folded over eight times, it creates a filter with gaps just 20 microns in diameter—fine enough to trap even tiny bits of plankton. And old sari cloth is even better than new, because long use—and especially repeated washing—causes the fibers to fray and become better traps for microorganisms. A study done last year in 2,212 households in rural Bangladesh revealed how effective this simple filter technology is. The frequency of cholera infection in families who regularly filtered their water with old sari cloth was reduced from an average of four cases per 1,000 people to 0.64 cases.