Atomic force microscopy of bacterial communities.

This chapter discusses atomic force microscopy (AFM) for the benefit of microbiologists who are interested in using this technique to examine the structures and dynamics of bacteria. AFM is a powerful technique for imaging biological samples at the nanometer to micrometer scale under nondestructive conditions. In order to be imaged with AFM, bacteria must be supported by a surface, which presents challenges because many laboratory strains of bacteria are planktonic. Still, in nature many bacteria live at surfaces and interfaces. This chapter discusses the benefits and difficulties of different methods that have been used to support bacteria on surfaces for AFM imaging and presents two methods in detail used to successfully grow and image bacteria at solid-liquid and solid-air interfaces. Using these methods it is possible to study bacterial morphology and interactions in a native state. These explorations by AFM have important applications to the study of different kinds of bacteria, interfacial bacterial communities, and biofilms.

Investigations into the life cycle of the bacterial predator Bdellovibrio bacteriovorus 109J at an interface by atomic force microscopy.

Atomic force microscopy was used to image Bdellovibrio bacteriovorus 109J, a gram-negative bacterial predator that consumes a variety of other gram-negative bacteria. In predator-prey communities grown on filters at hydrated air-solid interfaces, repeated cycles of hunting, invasion, growth, and lysis occurred readily even though the cells were limited to near two-dimensional movement. This system allowed us to image the bacteria directly without extensive preparation or modification, and many of the cells remained alive during imaging. Presented are images of the life cycle in two species of prey organisms, both Escherichia coli (a small prey bacterium that grows two-dimensionally on a surface) and Aquaspirillum serpens (a large prey bacterium that grows three-dimensionally on a surface), including high-resolution images of invaded prey cells called bdelloplasts. We obtained evidence for multiple invasions per prey cell, as well as significant heterogeneity in morphology of bdellovibrios. Mutant host-independent bdellovibrios were observed to have flagella and to excrete a coating that causes the predators to clump together on a surface. Most interestingly, changes in the texture of the cell surface membranes were measured during the course of the invasion cycle. Thus, coupled with our preparation method, atomic force microscopy allowed new observations to be made about Bdellovibrio at an interface. These studies raise important questions about the ways in which bacterial predation at interfaces (air-solid or liquid-solid) may be similar to or different from predation in solution.