Responses of microorganisms to physical and chemical gradients.

Most microbial ecosystems are spatially organized heterogeneous structures where microbes proliferate in gradients of biologically active solute molecules as well as in physical gradients of temperature, pressure, light, ionic strength, redox potential, pH and so on. Some of these ecosystems are discussed in this paper; however, the importance of investigating them in the laboratory is stressed. My group has developed a number of model systems. Seven of these are discussed and include four experimental, two numerical and one conceptual models. These are briefly described. (1) The gradostat consists of a number of bidirectionally linked fermenter vessels fed with solutes from each end of the array. Steady-state solute counter-gradients are established. A number of results in which different microbes are grown in different gradient systems are described. (2) The gel-stabilized system: organisms are grown in a solute gradient diffusing from a source agar layer beneath a semisolid layer containing agar and cells. (3) A constant dimension thin film fermenter. (4) The bacterial colony. (5, 6) The two numerical models, devised to simulate growth in the gradostat and in gel-stabilized systems respectively. (7) A conceptual model in which cells are regarded as compartments surrounded by activity domains; the importance of vectorial solute transfer in natural ecosystems is stressed.