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An electron micrograph of organisms found in biofilm, magnified 5,300 times. COURTESY THE HARVARD SCHOOL OF PUBLIC HEALTH

If you could take a fantastic voyage down the pipes that carry your drinking water, the variety of microbial creatures you would encounter might prove a bit unnerving. Most of us assume that filtration and chlorination provide us with safe, if not good-tasting, tap water. Not so, says Tim Ford, associate professor of environmental microbiology and director of the recently established Program in Water and Health at the Harvard School of Public Health.

Public drinking-water pipes provide a viable home for a variety of "opportunistic pathogens," says Ford, several of which can be hazardous to human beings. These microorganisms, including bacteria, protozoa, and viruses, create a thick, self-protective slime, better known by Ford and his colleagues as "biofilm," on the inner walls of pipes. Most utility companies use filtration methods that cannot repel these invaders--and once inside a water system, they can be immune to the highly praised effects of chlorine.

Ford, who has been studying biofilm for several years, recently chaired an American Academy of Microbiology colloquium in Guayaquil, Ecuador, entitled "A Global Decline in the Safety of Water: A Call for Action," where scientists from 12 countries discussed the growing threat of water-borne diseases and epidemics traceable to water sources. Such problems are not confined to developing countries, as proved by a 1993 outbreak of cryptosporidiosis in Milwaukee that infected nearly 400,000 people and left 100 dead. The culprit, Cryptosporidium, is a protozoan that enters water systems with insufficient filtration.

"Frankly, I'm surprised that more such outbreaks haven't occurred," says Ford. "Although it is relatively easy to destroy most loose, floating bacteria with chlorine, the chemical cannot penetrate the deeper layers of biofilm itself." The problem is that chlorine reacts with the biofilm's outer surface of organic matter, and this initial chemical bonding effectively prevents the disinfectant from reaching the microbes below. Ensconced in their nutrient-rich home, multitudes of microorganisms breed freely, and sometimes even swap genetic material. Not all are harmful, but at least a dozen or so--including Pseudomonas, Campylobacter, Clostridium, Flavobacterium, and Legionella--pose a growing threat.

In addition, Ford explains, certain pathogens like Cryptosporidium and Giardia possess survival mechanisms that help them resist disinfection even when they slough off from the protective colony of the biofilm. These microorganisms can form impenetrable cysts or go into "resting" stages, much like a dormant virus. Once free of the inhospitable environment produced by chlorine--i.e., inside our bodies--they can again assume their active, potentially deadly states.

Perhaps the greatest risk to water quality lies in the pipes themselves. Here again, microorganisms in the biofilm and their metabolic wastes may play a role by reacting with the metal pipes and speeding their inevitable decay. "Biocorrosion presents a large threat when you look at the possibility of waste water and public drinking water coming in contact with one another," Ford says. Most U.S. cities are already contending with insufficient or degraded infrastructures that may someday pose a health threat like that recently seen in Russia, where a compromised water system led to a recurrence of cholera in several cities.

Part of the problem lies in the limitations of current water-testing technology. Until recently, most testing has focused on the coliform group of bacteria (e.g., E. coli) as an indicator of relative water safety or fecal contamination. But as Ford points out in his textbook Aquatic Microbiology--An Ecological Approach, these particular bacteria do not adequately represent the complex and often pathogenic ecosystems that may exist within an urban water-pipe structure; they can hide and proliferate within the biofilm's walls, but are not central to its formation.

To prevent a repeat of the Milwaukee disaster, scientists like Ford have helped the Environmental Protection Agency identify new measures to safeguard public water supplies. "Not all cities are in compliance," Ford points out. Some, he explains, are still unwilling to bear the financial costs associated with newer surface treatment techniques, which involve better testing and filtration methods. New York, for example, whose main drinking-water source is the cattle-inhabited Catskills, has not fully implemented the new procedures. Since cattle are common hosts for Cryptosporidium, the potential for another outbreak is present.

Meanwhile, don't give up drinking your tap water just yet. Not all the microorganisms living there are bad news, and a number of them even have some merit. As Ford points out, "Our immune systems need a constant challenge to stay strong." This is especially true for anyone who frequently travels abroad, as he does. There, ironically, a lack of prior exposure to some of these micromonsters of the biofilm might prove even more dangerous.

~ Dan Boyne

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