Urinary catheters are sterile upon insertion, but they generally become colonized by bacteria shortly thereafter. This can lead to urinary tract infections and further serious complications in catheter users, who are often already weak from surgery or chronic health problems. Armed with a better understanding of how bacteria move under flow conditions, however, scientists could design catheters with interiors that slow or stop pathogen movement and subsequent infection.Led by Hür Köser, researchers from Yale University in New Haven, Conn., and Bogazici University in Istanbul, Turkey, have utilized microscopy and digital imaging to help explain the movement pattern of E. coli near surfaces in flow contexts. Researchers found that a number of forces and torques interact to move bacteria toward the left when they are exposed to flowing liquid. Under quiet conditions, bacteria show a clockwise circular trajectory.They began the study by running nutrient broth at varying rates past E. coli K12 cells in a flow channel. Broth was chosen instead of saline because it significantly reduced nonhydrodynamic interactions — such as adhesion — between the bacteria and the channel surfaces. Simultaneously, they viewed the bacterial movement patterns with a Zeiss inverted microscope and imaged this movement for 20-s periods using a digital camera operating at 30 fps from Silicon Imaging Inc. of Costa Mesa, Calif. A frame grabber from Epix Inc. of Buffalo Grove, Ill., acquired the data. Using computer programs, the investigators analyzed the photos to determine bacterial orientation and net direction of movement. They found that bacteria generally became more aligned with the flow direction farther from the channel’s center, and that the bacteria’s front ends most often pointed upstream. On both the glass lower surface and polymer upper wall of the flow channel, the bacteria also tended to migrate in the negative X-direction (toward the left) in the relative coordinate plane of the surface. In a simple microfluidic channel, bacteria tend to move in the negative X-direction and affix themselves along this side of the surface.The researchers suggest that these phenomena result from the interaction of opposing torques created by swimming and by shear flow, the latter of which decreases as side walls are approached. Most importantly, the behavior tends to result in upstream movement of the organisms along the channel’s left trench — a potential evolutionary advantage that may contribute to bacterial pathogenicity in analogous systems. According to the researchers, Salmonella spp. and other bacteria with a similar flagellation pattern to that of E. coli should demonstrate this tendency as well. “We intend to see if our findings can be applied to help reduce infections caused by motile bacteria in catheters,” Köser said. These secondary infections, he noted, cost hospitals an estimated $3 billion per year. Researchers also could utilize the findings to sort cells based on differences in motility or to develop more sanitary channels for irrigation and other purposes.Physical Review Letters, Feb. 9, 2007, 068101.