Rheology of Escherichia coli suspensions with various bacterial morphologies and motion characteristics

Motile bacteria can interact with surrounding fluids, creating complex rheological behavior of suspensions. However, studies involving paralyzed flagella or de-flagellated bacteria remain limited, leaving the separate roles of motility, flagella, and cell morphology poorly resolved. This study experimentally investigates the rheology of bacterial suspensions using three strains of Escherichia coli (E. coli), ATCC9637 motile with rotating flagella, HCB136 non-motile mutant with paralyzed flagella, and HCB137 non-motile mutant without flagella, to understand the role of bacterial morphology and motility in suspension rheological behaviors. The results show that the ATCC9637 suspension exhibits a notable decrease in viscosity, particularly pronounced in the low shear rate regime, whereas the HCB136 suspension shows an increase in viscosity, especially in concentrated suspensions. This contrast underscores the influence of active swimmers on modifying the flow field and subsequently fluid viscosity. Deflagellated bacteria reduce fluid viscosity, despite the absence of the organelles necessary for propulsion, driven by flow-induced collective behavior arising from their elongated body shape. Two dimensionless numbers Pe_f1 and Pe_f2 are introduced to delineate the bacterial stress dominant and flow stress dominant regimes along with the normalized shear rate. Finally, a prediction model is formulated to correlate the viscosity of bacterial suspensions with the shear rate, cell concentration, bacterial morphology, and bacterial motility.