Characterization of the Staphylococcal enterotoxin A: Vß receptor interaction using human receptor fragments engineered for high affinity.

Staphylococcal food poisoning is a gastrointestinal disorder caused by the consumption of food containing Staphylococcal enterotoxins. Staphylococcal enterotoxin A (SEA) is the most common enterotoxin recovered from food poisoning outbreaks in the USA. In addition to its enteric activity, SEA also acts as a potent superantigen through stimulation of T cells, although less is known about its interactions than the superantigens SEB, SEC and toxic shock syndrome toxin-1. To understand more about SEA:receptor interactions, and to develop toxin-detection systems for use in food testing, we engineered various SEA-binding receptor mutants. The extracellular domain of the receptor, a variable region of the beta chain (Vß22) of the T-cell receptor, was engineered for stability as a soluble protein and for high affinity, using yeast-display technology. The highest affinity mutant was shown to bind SEA with a Kd value of 4 nM. This was a 25 000-fold improvement in affinity compared with the wild-type receptor, which bound to SEA with low affinity (Kd value of 100 µM), similar to other superantigen:Vß interactions. The SEA:Vß interface was centered around residues within the complementarity determining region 2 loop. The engineered receptor was specific for SEA, in that it did not bind to two other closely related enterotoxins SEE or SED, providing information on the SEA residues possibly involved in the interaction. The specificity and affinity of these high-affinity Vß proteins also provide useful agents for the design of more sensitive and specific systems for SEA detection.

Rivers of life: the challenge of restoring health to freshwater ecosystems.

The world is moving into a period of widespread water stress and competition, with enormous implications for food security, the health of the aquatic environment, and social and political stability. In the second half of the 20th Century water demand more than tripled, the major factor being irrigation for food production. Over the same period, the number of large dams worldwide climbed from 5,000 to 45,000-bringing about a major alteration of river hydrology and ecosystem function. Rivers have been disconnected from portions of their channels, their floodplains, their deltas, and from the seas into which they empty. The resulting loss of aquatic habitat has put freshwater life in grave jeopardy. In the 21st Century it will be possible to satisfy the needs of 8-9 billion people while protecting the health of aquatic ecosystems, but only with a fundamental shift in the way society uses, manages and values freshwater. It will likely require a doubling of water productivity over the next 25 years--including more efficient irrigation practices and greater recycling of wastewater. To guide policy, nations need to systematically determine their freshwater ecosystem flow requirements in a way that perhaps only South Africa is attempting so far. Underpinning all these measures is the need for a guiding water ethic that states that enough water should be provided for all living things before some get more than enough.

When the world's wells run dry.

PIP: Over the past 3 decades, the depletion of underground water reserves (known as aquifers) has spread from isolated pockets of the agricultural landscape to large portions of the world's irrigated land. Groundwater drafting and groundwater exploiting are now rampant in the crop-producing regions all over the world, as is exemplified by the cases of India, the western US, and parts of Pakistan. Globally, it is in agriculture that the greatest social risks lie. Irrigated land is disproportionately important to world food production. Along with groundwater depletion, there is the problem of the build-up of salts in the soil, the silting up of reservoirs and canals, the mounting competition for water between cities and farms and between countries sharing rivers, the rapid population growth in regions that are already water-stressed, and the uncertainties of climate change. Few governments are taking adequate steps to address any of these threats. It is only in recent times that groundwater issues have begun to appear in some countries' national agendas. Although workable plans will vary from place to place, clear policies should be made for sustainable groundwater use because the future of agriculture and of humanity itself will depend on how well water is managed.^ieng

Sharing the rivers. Overview.

PIP: Globally, water use has more than tripled since 1950, and the answer to this rising demand generally has been to build more and bigger water supply projects, particularly dams and river diversions. As population and consumption levels grow, more and more rivers are being dammed, diverted, or overtapped to supply increasing volumes of water to cities, industries, and farms. Among these rivers are the Nile in northeast Africa, the Ganges in south Asia, the Amu Dar'ya and Syr Dar'ya in the Aral Sea basin, the Huang He (Yellow River) in China, and the Colorado. Subsequently, such massive change in the global aquatic environment generated deterioration, decline, and in some cases, collapse in aquatic systems. In addition, competition for water is increasing not only between the human economy and the natural environment, but also between and within countries. Water scarcity is a potential source of conflict. Forces such as the depletion of resources; population growth; and unequal distribution or access can create political conflicts. Achieving more sustainable patterns of water use, restoring and maintaining the integrity of river systems, and cooperation within and between countries will not only protect the aquatic environment, but also avert conflict.^ieng

The politics of water.

PIP: Water scarcity in some regions is a leading source of economic and political instability. Upstream countries have a clear advantage over downstream countries. Almost 40% of the world's population relies on river systems used by at least 2 countries. Water conflicts are most evident in the Middle East where population growth rates are among the world's highest and agricultural productivity depends almost exclusively on irrigation. Water scarcity is most critical in the Jordan River basin which Israel, Jordan, the occupied West Bank, and part of Syria share. Israel exceeds its renewable water supply by 15%. Even though Jordanians use less than 50% of the water/capita Israel uses, its population grows 3.4%/year of Israel's water supply is the Yarqon-Taninim aquifer whose recharge area is on the West Bank. Israel draws water from this aquifer for its own use, but does not let West Bank Arabs draw from it. Another water supply lies in the Golan Heights with Israel seized from Syria. Its other source is an overpumped coastal aquifer. 9 nations claim the Nile with Egypt being the last country to receive its waters. Egypt has very few of its own water sources plus is has rapid population growth. Turkey plans on constructing 22 dams, 19 hydropower stations, and 25 irrigation systems on the Euphrates river, resulting in a 35% reduction in water flow to Syria in normal years and even more in dry years. This project would also pollute the river with irrigation runoff. International cooperation is needed to address wait crisis. Israel could share its drip irrigation technology with others, such as it has done with the Islamic Central Asian republics. Ethiopia could store Nile water in its highlands which have a lower evaporation rate than that at Egypt's Aswan Dam, resulting in more available water. Perhaps the mutual gains possible from cooperation will unite long standing enemies toward peace.^ieng

Towards a water ethic. Viewpoint.

PIP: We continue to expand a water supply that has ecological and economical limits. Drip irrigation techniques, rainwater harvesting, and use of water=saving plumbing fixtures can help solve our water shortage problem. The core of the predicament is that society is no longer connected to water's life=giving qualities. Modern society does not respect the natural river, the complexity of a wetland, and the intricate web of life. It considers water to be a resource only to control for human consumption. Humans do not realize that they should preserve and protect water. We need guidelines to force us to act appropriately when we must make complex decisions about natural ecosystems whose workings evade us. The ultimate goal of this water ethic should be protection of water ecosystems. Adoption of this integrated, holistic ethic would call for the use of less water when possible and to share what we have. This ethic would be part of a sustainable development code which blends economic goals with ecological criteria. The water ethic would have indicators monitoring the breakdown of ecosystems, therefore allowing us to make corrections to restore ecosystems to health. We see some of this now as Florida tries to restore the Everglades damaged by unsustainable development. We should watch to see whether Botswana will continue to keep economic development from the Okavango Delta. Governments, the World Bank, and other lending institutions should make investment decisions based on ecological sustainability. The water ethic must include a social and political commitment to meet the basic needs of the poor. International relations must also consider equity and fairness when it comes to developing water-sharing terms and treaties. Individuals need to reduce their water consumption and consumption of goods whose manufacture requires water use resulting in water pollution. Population growth needs to slow down considerably to secure out water future.^ieng

Water tight.

PIP: Many cities worldwide have gone beyond the limits of their water supply. Growing urban populations increase their demand for water, thereby straining local water supplies and requiring engineers to seek our even more distant water sources. It is costly to build and maintain reservoirs, canals, pumping stations, pipes, sewers, and treatment plants. Water supply activities require much energy and chemicals, thereby contributing to environmental pollution. Many cities are beginning to manage the water supply rather than trying to keep up with demand. Pumping ground water for Mexico City's 18 million residents (500,000 people added/year) surpasses natural replenishment by 50% to 80%, resulting in falling water tables and compressed aquifers. Mexico City now ambitiously promotes replacement of conventional toilets with 1.6 gallon toilets (by late 1991, this had saved almost 7.4 billion gallons of water/year). Continued high rural-urban migration and high birth rates could negate any savings, however. Waterloo, Ontario, has also used conservation efforts to manage water demand. These efforts include retrofit kits to make plumbing fixtures more efficient, efficiency standards for plumbing fixtures, and reduction of water use outdoors. San Jose, California, has distributed water savings devices to about 220,000 households with a 90% cooperation rate. Boston, Massachusetts, not only promoted water saving devices but also repaired leaks and had an information campaign. Increasing water rates to actually reflect true costs also leads to water conservation, but not all cities in developing countries use water meters. All households in Edmonton, Alberta, are metered and its water use is 1/2 of that of Calgary, where only some households are metered. Tucson, Arizona, reduced per capita water use 16% by raising water rates and curbing water use on hot days. Bogor, Indonesia, reduced water use almost 30% by increasing water rates. In the US, more and more states are mandating use of water-efficient plumbing fixtures. Multilateral development agencies have identified some developing country cities as demonstrated sites for urban water conservation.^ieng

Trouble on tap.

PIP: Water is central to food production, industrial expansion, and urban growth. The creative use of water has resulted in both the advance of the standard of living and the destruction of the environment. Exhausted rivers, falling water tables, and shrinking lakes indicate the abuse that the world water system has suffered. The Middle East, the region with the biggest problem, has a high population growth rate, which increases the load on the food production system and is the most heavily dependent upon irrigation. Water sources do not know political boundaries, as such, conflict may result in water-sharing agreements are not reached. Egypt is worst off because it relies entirely upon the Nile for its water, yet none of its tributaries are in Egypt. The water cycle has been most adversely affected in India where over US$12 billion has been spent on 1554 large dams and other measure designed to control water. However, over 10,000 villages across the subcontinent live with shortages. Heavy pumping has salinated much of the ground water, and New Delhi's water requirement is so high that it is available for only a few hours a day. China is also in serious trouble with 21% of the world population and only 8% of its renewable fresh water. The Aral sea in the Soviet Union is going to become several small brine lakes if drastic measures are not taken quickly. However, current plans will cost the Soviet economy US$78billion and a proposed Siberian river diversion project could cost US$150 billion. The most common solution to water shortages is the marketing of water; however, this system is still very inefficient. Government subsidies for water still greatly hinder the adaptation of water use policies that value this scarce resource equitably. There are no quick fixes to this problem as was speculated in the 1970s. People and governments must increase their awareness of the magnitude of the problem and begin to value water accordingly. Conservation, recycling, and other measures must be adopted.^ieng