Copyright 2001 Scientific American, Inc.SCIENTIFIC AMERICAN 75BiofilmsTHEWARISAGAINSTBACTERIAL COLONIES THATCAUSESOMEOFTHEMOSTTENACIOUS INFECTIONS KNOWN. THE WEAPON IS KNOWLEDGE OFTHE ENEMY’S COMMUNICATION SYSTEMBY J. W. COSTERTON AND PHILIP S. STEWART Photographs by Sam OgdenCONTACT LENSES (left) are among the familiarsurfaces that may be colonized by biofilms—slime-enclosed communities of microorganisms.The film shown above, from a contact lens case, presumably caused a corneal infection diagnosed in the lens wearer.BattlingMICROGRAPH: LOUISE MCLAUGHLIN-BORLACE Institute of Ophthalmology, Department of Pathology, London;LICENSED FOR USE, ASM MICROBELIBRARY(www.microbelibrary.org)Copyright 2001 Scientific American, Inc.a great deal nowadays with information warfare.Why? Because interfering with a foe’s ability tocommunicate can be far more effective than de-stroying its bunkers or factories. In the battleagainst harmful bacteria, some investigators areconsidering the same strategy. The microbes that cause many stubborn infec-tions organize themselves into complex and tena-cious films—biofilms—that can be nearly impossi-ble to eradicate with conventional antibiotics. Inthe past few years, medical researchers have dis-covered that the microorganisms in biofilms de-pend critically on their ability to signal one an-other. Drugs able to interfere with this transmis-sion might then bar the microbes from establishinginfections or undermine their well-fortified posi-tions; such drugs might thus combat maladiesranging from the pneumonia that repeatedly afflictspeople with cystic fibrosis to the slow-burning in-fections that often form around medical implants. Signal-dampening compounds are currentlybeing evaluated in animal studies, but why is itthat such elegant weapons are only now beingreadied to enter the medical arsenal? The answer,in short, is that microbiologists took a very longtime to size up the enemy. Ever since the late 19thcentury, when Robert Koch’s laboratory studiesin Germany validated the germ theory of disease,most people, scientists included, have envisionedbacteria as single cells that float or swim throughsome kind of watery habitat, perhaps part of thehuman body. This picture emerged from the wayinvestigators usually examine such organisms: bytraining their microscopes on cultured cells sus-pended in a fluid droplet. That procedure is con-venient but not entirely appropriate, because theseexperimental conditions do not reflect actual mi-crobial environments. As a result, the bacteria intypical laboratory cultures act nothing like theones encountered in nature.In recent years, we and other bacteriologistshave gained important insights into how commondisease-causing microbes actually live. Our workshows that many of these organisms do not, infact, spend much time wafting about as isolatedcells. Rather they adhere to various wetted sur-faces in organized colonies that form amazingly di-verse communities.In retrospect, it is astonishing that investigatorscould overlook this microbial lifestyle for so long.After all, bacterial biofilms are ubiquitous—den-tal plaque (which most of us confront daily), theslippery coating on a rock in a stream, and theslime that inevitably materializes inside a flowervase after two or three days are but a few commonexamples. And bacteria, the focus of our studies,are not alone in the ability to create biofilms. In-deed, the genetic diversity of the microorganismsthat can arrange themselves into living veneers andthe breadth of environments they invade convinceus that this ability must truly be an ancient strate-gy for microbial growth. Scientific appreciationand understanding of that strategy is, however, amodern phenomenon.Germs in FlatlandSOME BIOLOGISTShad, in fact, attempted longago to examine the bacteria living in biofilms usingordinary microscopes; a handful even employedelectron microscopes. They always saw some bac-teria, but being unable to obtain clear images fromdeep within living layers, they concluded that thecells inside were mostly dead and jumbled in ran-dom clumps. This view changed little until abouta decade ago, when bacteriologists began em-ploying a technique called laser scanning confocalmicroscopy. That technology enables investigatorsto view slices at different depths within a livingbiofilm and to stack these planes together to cre-ate a three-dimensional representation.Applying this approach in a concerted effortto study the structure of biofilms, John R. Law-rence of the Canadian National Water ResearchInstitute, Douglas E. Caldwell of the University of76 SCIENTIFIC AMERICAN JULY 2001TROUBLE IN TUBESBiofilms that form in urinarycatheters are a common sourceof infection. When the tubes stay in only briefly, they poselittle risk, but the dangerincreases with prolonged use. A 1996 study found, for example, that after a week,infections strike 10 to 50 percent of catheterizedpatients; after a month, virtually all such patientsare affected.entagon planners concern themselvesPCopyright 2001 Scientific American, Inc.www.sciam.com SCIENTIFIC AMERICAN 77MICROGRAPH: R. BOS, H. J. BUSSCHER, W. L. JONGEBLOED AND H. C. VAN DER MEI Laboratory for Materia Technica, University of Groningen, The Netherlands; LICENSED FOR USE, ASM MICROBELIBRARY(www.microbelibrary.org)Saskatchewan and one of us (Costerton) demon-strated for the first time in 1991 that the bacteriagrow in tiny enclaves, which we called micro-colonies. Bacteria themselves generally constituteless than a third of what is there. The rest is agooey substance the cells secrete, which invariablyabsorbs water and traps small particles.The goo—or, more formally, the extracellularmatrix—holds each microcolony together. A bio-film is built of countless such groupings, separat-ed by a network of open water channels. The flu-id coursing through these tiny conduits batheseach congregation of microbes, providing dis-solved nutrients and removing waste products.The cells situated on the outside of a microcolonyare well served by this plumbing system, but thosein the interior are largely cut off. The dense aggre-gation of cells surrounding them and the organicmatrix that cements things together act as barriersto water flow. So the cells inside the colony mustmake do with the nutrients than can diffuse in-ward to them. Actually, the supply is not all thatmeager: because the glue is mostly just water,small molecules can move through it freely—albeitwith certain