Natural microbial habitats include various interfaces--liquid-liquid, gas-liquid, solid-liquid, and solid-gas. An interface, the boundary between two phases, has physical and chemical properties that differ from those of either phase. Bacteria, yeasts, and algae often concentrate at interfaces, and the ability of microorganisms to exploit resources in their environment may be markedly affected by the nature of the available interfaces. Included within the realm of microbial activity at interfaces are such wide-ranging topics as predator-prey relations, tooth decay, gastrointestinal tract...