|dc.contributor.author||Harshitha, S K||
|dc.description.abstract||Swarming is a unique example of social behaviour in bacteria. They represent the collective
effort of bacteria to translocate on moist substrates. The bacteria extract fluid from the
substrate through osmosis and produces surfactants to ease the spreading of the extracted
fluid, thereby colonising the substrate. The spreading colony forms distinct patterns specific to
the swarming species. We are interested in understanding the local interaction that leads to
colony-level patterns. In specific we study the self-organisation of Pseudomonas aeruginosa
into long sparse branches as they swarm.
We probe the swarm behaviour in complex environments such as physical obstacles, multiple
colonies in close proximity, antibiotics and foreign bacteria. We propose a complete model for
bacterial pattern formation by coupling active bacterial motility to passive fluid dynamics. We
have verified the qualitative similarity of the patterns obtained in the simulations with that of
experiments implying the model captures most of the dominant forces that determine the
swarm behaviour. We have studied the variation of patterns in different nutrient and substrate
conditions and have verified that the model can account for such changes.
We also study the behaviour of swarm on inclined surfaces. The swarm behaves like a fluid
drop that quickly de-pins to slide down the surface, exhibiting how bacteria can use gravity to
transport themselves on inclined surfaces quickly. We experimentally show that the osmosis
dynamics plays a major role here and agrees well with the simulation using our model.
The swarms' response to the antibiotic in preliminary experiments showed that the bacteria in
the swarm could steer the direction of the branch, possibly away from the antibiotic. In addition
to the change in direction, we observe that the bacterial tips merge to form aggregates of high
cell density. The aggregation, instead of escaping from the region of high concentration of
antibiotic, is non-intuitive. We study the response for different classes of antibiotics and see
that the response may be unique to the aminoglycosides. We find that there are live bacteria
in the aggregate, and the individual bacteria are not resistant to the antibiotic, which tells us
that the aggregation is offering the group a survival advantage and questions the efficacy of
antibiotic in the presence of such social behaviour of the swarm. We propose a plausible
mechanism to explain the enhanced survivability of the high-density bacterial aggregations.
We inspect the swarm at the single-cell level to understand the hydrodynamic forces at play.
The high density of bacteria in a swarm makes it difficult to track the behaviour of cells at this
level for a long duration and using a laser beam for fluorescence needed for tagging the
bacteria also affects the swarm adversely to the extent of inhibiting this behaviour. The singlecell
studies have thus only been used to measure a few experimental parameters and verify
the assumptions used in our model.
We have therefore studied the swarm in different environmental conditions and have verified
that our model can predict its behaviour in most of the conditions. We also show scenarios
where our model fails and suggest ways to improve the model for future work.||en_US
|dc.rights||I grant Indian Institute of Science the right to archive and to make available my thesis or dissertation in whole or in part in all forms of media, now hereafter known. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part
of this thesis or dissertation||en_US
|dc.subject.classification||Research Subject Categories::NATURAL SCIENCES::Biology::Organism biology||en_US
|dc.title||Self-organisation of bacteria through swarming||en_US
|dc.degree.grantor||Indian Institute of Science||en_US