Experimental study of swimming flagellated bacteria and their collective behaviour in concentrated suspensions
This thesis investigates bacterial motility from the mechanism permitting individual selfpropulsion to the complex collective flocking motility in Escherichia coli and Bacillus subtilis cells. Understanding bacterial swimming has intrigued scientists for decades and recently there has been a growing interest in collective swimming behaviour. The first part of this thesis reviews the characteristics of E. coli and B. subtilis cells subsequently describing the governing physics and constraints of self-propulsion in the low Reynolds regime. The second part of this thesis presents three self-contained experimental sections, examining individual swimming in non-conventional body shaped cells and subsequently focusing on concentrated bacterial swimming in normal cells. We first investigated motility in mutant spherical E. coli cells KJB24 motivated by simulations, which often model bacteria as self-propelled spheres. Somewhat unexpectedly these spherical cells do not exhibit runs and tumbles but diffuse slower than expected. As an introduction to working with microbiology and to familiarise with microbiology techniques we investigated why these spherical cells do not swim. Secondly we investigated how cellular motility varies as a function of body length by inhibiting cell division in wild-type E. coli with cephalexin; which remained motile despite body elongation. Fluorescent flagella visualization provided evidence of multiple bundle formations along the lateral walls as a mechanism to sustain motility. The average swimming velocity, body and flagella rotation rates, the number of flagella and number of flagella bundles were extracted experimentally as a function of length. The extracted experimental parameters for normal sized cells were consistent with Purcell’s model. We explored simple adaptations and scaling of this model to describe motility for filamentous cells, which agrees with experimental values. The main focus is on collective behaviour of B. subtilis by examining the onset from individual swimming to collective motility using time-lapse microscopy. Results demonstrated a smooth transition where cells self-organize into domains expanding rapidly by recruiting cells. We present advancements in B. subtilis fluorescent flagella staining which revealed unexpected multiple flagella bundle arrangements during runs, contradictory to general conjectures. Novel visualisation of flagella filaments during reversal events is presented in both E. coli and B. subtilis cells, providing experimental evidence for complex flagella ‘flipping’. Cellular reversal is hypothesized as a mechanism for quorum polarity facilitating collective swimming. We present novel flagella imaging in the setting of collective behaviour showing evidence to support quorum polarity. Subsequently we extracted the run length distributions of cells as a function of concentration, yielding a decreasing trend with increasing concentration. Using particle tracking we quantitatively extracted the mean squared displacement of swimming cells versus passive tracers at different concentrations during collective swimming, these novel results are discussed in respect to recent simulations. These presented experiments provide new insights into collective behaviour improving current understanding of this phenomenon.