A new understanding of these microbial communities is driving a revolution that may transform the science of microbiology
When we think about bacteria, most of us imagine a watery milieu, with single-celled organisms swimming about. We might envision these solitary entities getting together with some of their brethren now and then to cause some disease or spoil some food, but once the job is done they return to their isolated existence. This image of bacterial existence, it turns out, is not only oversimplified but perhaps misleading as well. In nature, the majority of microorganisms live together in large numbers, attached to a surface. Rather than living as lonely hermits in the so-called planktonic form, most bacteria spend much of their lives in the microbial equivalent of a gated community—a biofilm.
A mature biofilm is a fascinating construction: It can form layers, clumps and ridges, or even more complex microcolonies that are arranged into stalks or mushroom-like formations. The residents of the biofilm may be a single species or a diverse group of microorganisms distributed in various neighborhoods. Their common bond is a matrix made of polysaccharides, DNA and proteins, which together form an extracellular polymeric substance—what many microbiologists just call slime.
It's becoming increasingly clear that the communal life offers a microorganism considerable advantages. The physical proximity of other cells favors synergistic interactions, even between members of different species. These include the horizontal transfer of genetic material between microbes, the sharing of metabolic by-products, an increased tolerance to antimicrobials, shelter from changes in the environment and protection from the immune system of an infected host or from grazing predators. The formation of a biofilm has even been likened to the program by which cells within a multicellular organism differentiate.
An appreciation of the significance of biofilms is a relatively recent phenomenon. Only within the past 15 to 20 years have biologists begun to examine the physiology of these microbial communities. This is an extraordinary state of affairs, given that the Dutch microscopist Antonie van Leeuwenhoek first described biofilms in the late 1600s. Using acetic acid, he had tried to kill a biofilm—the dental plaque on his dentures—but noted that only the free-swimming cells could be destroyed. Despite the early discovery of microbial communities, microbiology departed from these observations to focus primarily on planktonic bacteria.
To be sure, not everyone agrees that biofilms are the predominant form of bacteria in nature. The vast majority of laboratory methods used today examine cultured microorganisms in their planktonic mode. But we believe that microbiology is experiencing a shift in how bacteria are conceptualized. We predict that this new perspective of how microorganisms live will have fundamental consequences for medicine, industry, ecology and agriculture.