WATER QUALITY

Water is the universal solvent. Many compounds that can dissolve in water are used as food sources by a variety of microbiological life forms. These microorganisms are themselves water-based and their constituent molecules are designed to function in aqueous environments. Thus, water can widely support the growth of microorganisms.

Some of this growth is advantageous. For example, the strains of yeast whose fermentative abilities make possible the brewing of beer, the production of wine, and the baking of bread. In addition, the growth of bacteria in polluted water is used as a means of decontaminating the water. The bacteria are able to use the pollutant compound as a food source. In contrast, some forms of microbial growth can detrimental to products being produced or dangerous to the health of people consuming the water. Ensuring the quality of water from a microbiological standpoint is thus of extreme importance.

The main concern surrounding water quality is the freedom of the water from microorganisms that can cause disease. Typically, these agents are associated with the intestinal tract of warm-blooded animals including humans. Examples of disease causing bacteria are those in the genera of Salmonella, Shigella, and Vibrio. As well certain types of the intestinal bacterium Escherichia coli can cause infections. Escherichia coli O157:H7 has become prominent in the past decade. Contamination of drinking water with O157:H7 can be devastating. An infamous example of this is the contamination of the municipal water supply of Walkerton, Ontario, Canada in the summer of 2000. Several thousand people became ill, and seven people died as a direct result of the O157:H7 infection.

The contamination of the well water in Walkerton occurred because of run-off from adjacent cattle farms. This route of water contamination is common. For this reason, the surveillance of wells for the presence of bacteria is often done more frequently following a heavy rain, or at times of the year when precipitation is marked.

The intestinal tract also harbors viruses that can contaminate water and cause disease. Some examples of these viruses are rotavirus, enteroviruses, and coxsackievirus.

A number of protozoan microorganisms are also problematic with respect to water quality. The two most prominent protozoans are in the genera Giardia and Cryptosporidium. These microorganisms are resident in the intestinal tract of animals such as beaver and deer. Their increasing prevalence in North America is a consequence of the increasing encroachment of civilized areas on natural areas.

Municipal drinking water is usually treated in order to minimize the risk of the contamination of the water with the above microbes. Similarly, the protection of water quality by the boiling of the water has long been known. Even today, socalled "boil water orders" are issued in municipalities when the water quality is suspect. The addition of disinfectant compounds, particularly chlorine or derivatives of chlorine, is a common means utilized to kill bacteria in water. Other treatments that kill bacteria include the use of a gaseous ozone, and irradiation of water with ultraviolet light to disrupt bacterial genetic material. In more recent decades, the filtering of water has been improved so that now filters exist that can exclude even particles as tiny as viruses from the treated (or "finished") water. The killing of the protozoan microorganisms has proved to be challenging, as both Giardia and Cryptosporidium form dormant and chemically resistant structures called cysts during their life cycles. The cyst forms are resistant to the killing action of chlorine and can pass through the filters typically used in water treatment plants. Contamination of the water supply of Milwaukee, Wisconsin with Cryptosporidium in 1993 sickened over 400,000 people and the deaths of at least 47 people were subsequently attributed to the contamination.

Water quality testing often involves the use of a test that measures the turbidity of the water. Turbidity gives an indication of the amount of particulate material in the water. If the water is contaminated with particles as small as bacteria and viruses, the turbidity of the water will increase. Thus, the turbidity test can be a quick means of assessing if water quality is deteriorating and whether further action should be taken to enhance the quality of the water supply.

Water quality is also addressed in many countries by regulations that require the sampling and testing of drinking water for microorganisms. Testing is typically for an "indicator" of fecal pollution of the water. Escherichia coli is often the most suitable indicator organism. The bacterium is present in the intestinal tract in greater numbers than the disease-causing bacteria and viruses. Thus, the chances of detecting the indicator organism is better than detecting the actual pathogen. Additionally, the indicator does not usually multiply in the water (except in tropical countries), so its presence is indicative of recent fecal pollution. Finally, Escherichia coli can be detected using tests that are inexpensive and easy to perform.

Because the prevention of water borne disease rests on the adequate treatment of the water, underdeveloped regions of the world continue to experience the majority of water borne diseases. For example, in India the prevalence of cholera is so great that the disease is considered to be epidemic. But, as exemplified by communities like Walkerton and Milwaukee, even developed countries having an extensive water treatment infrastructure can experience problems if the treatment barriers are breached by the microorganisms.