Vol. 182, No. 10
Bioﬁlm, City of Microbes
Infectious Disease Unit, Massachusetts General Hospital, Boston, Massachusetts 02114,1 and Department of Microbiology and Molecular Genetics, HarvardMedical School, Boston, Massachusetts 021152 In most natural environments, association with a surface in a structure known as a bioﬁlm is the prevailing microbial lifestyle. Surface association is an efﬁcient means of lingering in a favorable microenvironment rather than being swept away by the current. Taken to the extreme, we may view the planktonic or free-swimming microbial phase primarily as amechanism for translocation from one surface to another. Genetic studies of single-species bioﬁlms have shown that they form in multiple steps (46), require intercellular signalling (7), and demonstrate a proﬁle of gene transcription that is distinct from that of planktonic cells (35). From this perspective, bioﬁlm formation may be viewed as a developmental process that shares some of thefeatures of other bacterial developmental processes such as sporulation of gram-positive bacteria (9), fruiting body formation in Myxococcus xanthus (33, 40, 44), and stalked-cell formation by Caulobacter crescentus (13, 19, 24, 37, 48). In natural environments, however, the bioﬁlm is almost invariably a multispecies microbial community harboring bacteria that stay and leave with purpose, share theirgenetic material at high rates, and ﬁll distinct niches within the bioﬁlm. Thus, the natural bioﬁlm is less like a highly developed organism and more like a complex, highly differentiated, multicultural community much like our own city. There are several steps that we must take to optimize our lives in a city. The ﬁrst is to choose the city in which we will live, then we must select the neighborhoodin the city that best suits our needs, and ﬁnally we must make our home amongst the homes of many others. Occasionally, when life in the city sours, we leave. The same steps occur in the formation of a bacterial bioﬁlm (Fig. 1). First, the bacterium approaches the surface so closely that motility is slowed. The bacterium may then form a transient association with the surface and/or other microbespreviously attached to the surface. This transient association allows it to search for a place to settle down. When the bacterium forms a stable association as a member of a microcolony, it has chosen the neighborhood in which to live. Finally, the buildings go up as a three-dimensional bioﬁlm is erected. Occasionally, the bioﬁlm-associated bacteria detach from the bioﬁlm matrix. Micrographs ofthese steps in bioﬁlm formation by a single bacterial species are shown in Fig. 2. Although these micrographs are static views of the steps in bioﬁlm formation, a bioﬁlm is not a motionless heap of cells. Figure 3 shows the ﬁrst frame of a real time movie, accessible at http//gasp.med.harvard.edu/bioﬁlms/jbmini/movie.html, that documents the activity in a mature bioﬁlm. In this frame, the pillars ofa mature bioﬁlm are visible, distributed on top of a monolayer of surface-associated cells. The associated movie shows that, in addition to ﬁxed cells, there are motile cells that maintain their association with the bioﬁlm for long periods of time, swimming between pillars of bioﬁlm-associated bacteria. The bioﬁlm, therefore, demonstrates a level of activity similar to that of a bustling city.The genetic basis of the steps in bioﬁlm formation has been investigated for a number of bacterial species, including Escherichia coli (34), Pseudomonas aeruginosa (31) and Vibrio cholerae (46). For these studies, a simple genetic screen was utilized in which random transposon mutants are grown in 96-well plates (5, 16, 32). After removal of the planktonic cells, the remaining bioﬁlm-associated...