The OptoPACE Bioreactor

Continuous culture systems are essential for synthetic biology techniques like Directed Evolution (DE) and Phage-Assisted Continuous Evolution (PACE), which require metabolically active bacterial cultures for extended periods. Turbidostats, which maintain cell density using real-time optical density (OD) feedback, offer ideal conditions for inducing gene expression during mid-log phase, which is the optimal growth stage for synthetic circuit activity. However, current PACE-compatible bioreactors are either cost-prohibitive or require significant technical expertise. Here, we present the OptoPACE Bioreactor (OPB), a low-cost, user-friendly turbidostat system optimized for PACE and optogenetic control. The OPB uses a 650 nm laser and photoresistor to measure turbidity, an Arduino microcontroller for feedback control, and NeoPixel LEDs to enable multispectral optogenetic stimulation. Designed with 3D-printed components and off-the-shelf electronics, the system operates with a single peristaltic pump and uses the siphon effect to simplify fluidics. A custom motor control shield and an LCD-rotary encoder interface enable code-free operation. The OPB supports dual bioreactors connected unidirectionally, ensuring continuous culture while maintaining precise control over optical inputs and nutrient flow. The entire system can be built for under $200, significantly lowering the barrier to entry for labs interested in continuous evolution. By combining affordability, modularity, and ease of use, the OptoPACE Bioreactor provides a robust platform for protein engineering and synthetic biology, enabling broader adoption of advanced continuous evolution methods like PACE.