Fluid dynamics impact on bacterial physiology: Biochemical oxygen demand

The interaction between fluid dynamics and microbiological organisms is an integral aspect of aquatic ecosystems that has received only sporadic attention over the years. The effects of fluid motion on bacterial growth kinetics and on the traditional biochemical oxygen demand (BOD) test were investigated. Laboratory experiments were performed to study the physiology of E. coli in a well-defined flow system with an emphasis on BOD measurements. Several methods were applied to quantify bacterial abundance and viability including viable plate counts; membrane integrity Live/Dead BacLight; respiratory 5-cyano-2,3-ditolyl tetrazolium chloride; and direct bacterial count 4′6-diamidino-2-phenylindole staining methods. A turbulent fluid flow was generated using an oscillating grid reactor. Microscale fluid flow in the experimental setup was analyzed using a laser-Doppler velocimeter. Our results demonstrate that bacterial physiology and the corresponding BOD measurements depend on hydrodynamic mixing conditions. Microbial growth and BOD were facilitated by small-scale fluid motion. BOD was minimal in a stagnant fluid that is the condition that replicates the traditional BOD test.