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E. coli stick to the right E.
coli are picky little things. George Whitesides and colleagues at Harvard University and the Rowland Institute at Harvard have discovered that when the bacteria are confined to narrow channels made out of agar and PDMS, they swim along the right-hand side of the channel. The investigators suggest that the penchant for the right could be due to a bacterial partiality for agar over PDMS (Nature, 2005, 435, 1271–1274). The idea that E. coli ’s motion is influenced by the surrounding material can have implications for the development of biosensors and cell-based assays. Willow DiLuzio, the first author on the paper, says that the researchers achieved preferential movement of the bacteria by controlling the materials that made up the channel walls. “In the future, we can envision enclosed microdevices where we wouldn’t need fluid flow,” she says. “The cells could swim through the device, and they could be directed by the materials of the channel.” To create microchannels of two materials, Whitesides and colleagues fabricated grooves in films of PDMS. A grooved PDMS film was oxidized to make the surface hydrophilic and then sealed to an agar surface to produce the channels (1.3–1.5 µm tall and 7–10 µm high). A solution of nutrients, which seeped out of the agar, completely filled the microchannels so that no net fluid flow occurred. When they are near surfaces, bacteria are propelled in a clockwise, circular trajectory by flagella that jut out from their bodies. Most E. coli strains in Whitesides and colleagues’ experiments moved preferentially along the wall to their right. The movement of the bacteria to the right meant that the cells were following
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When presented with a three-way junction, bacteria head to the right, following the agar surface on the bottom of the microchannel. Scale bar = 20 µm. (Adapted with permission. Copyright 2005 Nature Publishing Group.)
a trajectory closer to the bottom agar surface than to the top PDMS surface. “It’s a very clever demonstration of how bacteria can behave in a confined environment,” says Stephen Quake at Stanford University. “There are many cases occurring in nature where bacteria swim in confined environments. For example, in urinary infections the bacteria grow in confined channels, not unlike the microfluidic ones.” The investigators quantified the directional preference of the bacteria by presenting them with a three-way junction in a microchannel. In channels where agar was on the bottom and PDMS on the top, 75–88% of the bacteria entered the channel to the right. When the positions of agar and PDMS were switched, only 16% of the bacteria
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entered the right-hand channel. This result demonstrated that the bacteria were moving along the agar surface. Daniel Chiu of the University of Washington notes that the investigators’ approach was a good example of applying engineering methods to biological problems. The group “found out something completely new about how bacteria behave that otherwise would be very difficult to [observe],” he says. “You wouldn’t be able to find this kind of [behavior] just in a petri dish. Through the techniques of microfabrication, microfluidics, and different materials chemistry, you actually see this phenomenon.” How do the bacteria distinguish between agar and PDMS? “It’s not well understood at this point,” says Whitesides. “[The bacteria] swim by pushing against the fluid using helical flagella. For whatever reason, it seems the properties of the fluid are such that the method works better when [the bacteria] are closer to the porous medium than to a hard, elastomeric medium.” The ability of bacteria to make a choice of material could shed light on how bacteria are recruited to biofilms and cause infection. “In growing biofilms, a lot of [bacterial] cells secrete polysaccharides that surround the cells. This means that as cells approach the growing biofilm, they may move along it for a much longer period of time than [they would along] a solid surface. It [could] enhance the recruitment of cells to the biofilm,” suggests DiLuzio. “Most of our cells are coated with a thin polysaccharide layer, the glycocalyx, [in which the] surface chemistry is similar to agar. That also means when bacteria infect our bodies, they may move along our cells for a longer period of time.” a — Rajendrani Mukhopadhyay © 2005 AMERICAN CHEMICAL SOCIETY
