Cover illustration:

The cover picture shows nitrite oxidizers and ammonium oxidizers. These types of bacteria serve to reduce nitrogen compounds in wastewater. The vital bacterial cells were marked with specific gene probes and then photographed using fluorescence microscopy.

(Image: Helga Gehrke, Kooperationslabor von Ruhrverband, Emschergenossenschaft und Lippeverband (laboratory run by the Ruhrverband in cooperation with the Emscher Cooperative and the Lippe Association), Essen, Germany)