Sulfur-Eating Bacteria Sense Water Pollutants

When factories accidentally release dangerous chemicals into waterways, scientists need an early warning system so that they can mitigate environmental damage. Now researchers have demonstrated that a bacterium commonly found in wastewater sludge can sound the alarm when toxic water pollutant levels rise (Environ. Sci. Technol., DOI: 10.1021/es1036892).

Oxidized contaminants such as nitrates, perchlorate, and chromate enter waterways from fertilizers, explosives, and manufacturing processes. At high concentrations, these chemicals harm fish, animals, and people. To detect such chemicals, scientists usually monitor changes in the physiology or bioluminescence of certain microorganisms, but such assays often require specialized equipment and are labor intensive and slow. 

What's more, explains Steven Van Ginkel, a research scientist at the Swette Center for Environmental Biotechnology at Arizona State University, Tempe, "Most sensors are specific to just one chemical, but the toxicity of individual chemicals is additive." So Van Ginkel, Sang-Eun Oh, an environmental biologist at Kangwon National University, in South Korea, and their colleagues wanted to develop a bacterial biosensor that detects the overall toxicity of oxidized pollutants.

They wondered if Acidithiobacillus, a genus of bacteria found in wastewater sludge, could provide a simple, comprehensive readout. This bacterium eats elemental sulfur and, in the presence of oxygen and water, catalyzes its conversion to sulfate and protons. The investigators reasoned that oxidized contaminants would inhibit this reaction by crippling bacterial enzymes. The bacteria then would generate less SO₄^2- and H⁺. So the researchers could detect the contaminants by the resulting drop in the electrical conductivity or rise in pH of the solution surrounding the bacteria.

To test their hypothesis, the researchers added sulfur particles and simulated stream water to sludge from a Korean wastewater treatment plant and bubbled oxygen through the mix. Under these conditions, Acidithiobacillus bacteria thrived, whereas most other bacteria in the sludge died. When the investigators introduced water containing oxidized contaminants to the biosensor, the pH and conductivity of the liquid changed substantially within minutes to hours, depending on the type and concentration of contaminant. Measurable change occurred with as little as 5 parts per billion of chromate.

The new method is faster and more sensitive than existing biosensors, the researchers say. Moreover, detection requires only a simple conductivity or pH meter, which is standard equipment for most environmental labs. According to Van Ginkel, scientists at riverside detection stations could use the bacteria to monitor contaminants in real time: All they would need is a pump to introduce river water into the bacterial reactor. "Any change in pH or electrical conductivity would let us know that some type of toxic event occurred, which could help us protect fish and people downstream," he says.

Kim Rogers, a research chemist at the U.S. Environmental Protection Agency's National Exposure Research Laboratory, calls Van Ginkel and Oh's technique "novel" and "potentially useful." However, Rogers says that "commercial successes for environmental bioanalytical methods are not particularly common." Yet this track record hasn't stopped Oh from patenting the technology, which he hopes to commercialize soon.