Bacterial Biofilms in Nature and Disease

The growth of bacteria in pure cultures has been the mainstay of microbiologicaltechnique from the time of Pasteur to the present. Solid media techniques have allowed the isolation of individual species from complex natural populations. These pure isolates are intensively studied as they grow in batch cultures in nutrient-rich media. This experimental approach has served well in providing an increasingly accurate understanding of prokaryotic genetics and metabolism and in facilitating the isolation and identification of pathogens in a wide variety of diseases. Further, vaccines and antibiotics developed on the basis of in vitro data and tested on test-tube bacteria have provided a large measure of control of these pathogenic organisms.During the last two decades microbial ecologists have developed a series of exciting new techniques for the examination of bacteria growing in vivo, and often in situ, in natural environments and in pathogenic relationships with tissues. The data suggest that these organisms differ profoundly from cells of the same species grown in vitro. Brown & Williams (12) have shown that bacteria growing in infected tissues produce cell surface components not found on cells grown in vitro and that a whole spectrum of cell wall structures may be produced in cells of the same species in response to variations in nutrient status, surface growth, and other environmental factors (67). We and others (28) have used direct ecological methods to examine bacterial cells growing in natural and pathogenic ecosystems, and we find that many important populations grow in adherent biofilms and structured consortia that are not seen in pure cultures growing in nutrient-rich media. In fact, it is difficult to imagine actual natural or pathogenic ecosystems in which the bacteria would be as well nourished and as well protected as they are in single-species batch cultures.In this review we summarize and synthesize the data generated by the new direct methods of studying mixed natural bacterial populations in situ. Generally, morphological data give us a basic concept of community structure, direct bioch emical techniques monitor metabolic processes at the whole community level, and specific probes define cell surface structures in situ. Any in vitr o techniques used in these ecological studies are sel ected to mimic the natural ecosystem as closely as possible. In our estimation, data from studies of bacter ia growing in single-species batch cultures continue to be very valuable. However, these data represent a single, and perhaps unrepresentative, point in the broad spectrum of bacterial characteristics expressed in response to altered environmental factors. In retrospect, it may become apparent that the phenotypic plasticity of bacteri a (12, 107) and their ability to form structured and cooperative consortia will prove to be their most remarkable characteristics.