Watson describes a novel method for maintaining microbial continuous cultures at maximum specific growth rate (μmax) by controlling carbon dioxide production rather than turbidity or dilution rate. The system uses an infrared CO2 analyzer connected to an electronic controller that regulates medium supply via a peristaltic pump, maintaining constant CO2 production rates. Unlike turbidostats, which suffer from wall growth and optical fouling, this CO2-controlled apparatus operated reliably for over a month with only minimal daily calibration. The method was validated using Saccharomyces cerevisiae and a respiratory-deficient mutant, determining μmax at temperatures between 14.5°C and 30°C and generating Arrhenius plots. The approach maintains constant CO2 production by adjusting organism concentration inversely with changes in growth rate, and offers practical advantages for long-term continuous culture operation using commercially available equipment.

Key findings

• A new continuous culture method controls μmax by maintaining constant CO2 production rate through an automated feedback system linking a CO2 analyzer to a medium pump.
• The system operated reliably for extended periods (>1 month) with minimal maintenance, overcoming the wall-growth and optical-fouling problems that plague turbidostat operation.
• Maximum specific growth rates were successfully determined for wild-type and respiratory-deficient S. cerevisiae strains across multiple temperatures, demonstrating the method's precision and versatility.
• Unlike turbidostats, organism concentration varies with growth rate to compensate for changes in yield factors and maintain constant CO2 production, as predicted by mathematical theory.