Berg and Block describe the design and fabrication of a miniature flow cell for studying bacteria under high-power microscope objectives. The cell is constructed from stainless steel and features glass windows, with internal dimensions of approximately 0.04 cm depth and 0.05 ml volume. It enables rapid, controlled exchange of media around tethered bacterial cells—bacteria anchored by their flagella to glass slides. The cell uses a transverse channel that acts as a constant pressure source to distribute fluid uniformly across a longitudinal channel, creating nearly uniform laminar flow. Detailed fabrication procedures using specialized machining techniques are provided. Testing with blue dyes showed that media exchange is 95% complete within 2-7 seconds, faster than predicted by parabolic flow theory. However, diffusion limitations near the cover slips restrict the ability to expose cells to concentration changes faster than approximately one second. The authors also note complications when using peristaltic pumps instead of vacuum-driven flow. This device proved valuable for studying bacterial flagellar motor dynamics and chemotactic responses by allowing rapid chemical stimulation of tethered cells during microscopic observation.

Key findings

• A miniature stainless steel flow cell was designed with 0.04 cm internal depth and 0.05 ml volume for rapid media exchange around tethered bacteria under high-power microscope objectives.
• The cell uses a transverse channel as a constant pressure source to achieve nearly uniform laminar flow across the entire longitudinal channel width.
• Media exchange reaches 95% completion in 2-7 seconds, exceeding theoretical predictions for parabolic flow, benefiting rapid stimulus exposure of tethered cells.
• Diffusion limitations near cover slips restrict temporal resolution to approximately one second or longer for concentration changes, despite the small cell dimensions.
• Detailed machining procedures were provided to enable fabrication of multiple units with consistent performance.