  Trajectories and conformations of uni- and multiflagellar bacteria are
studied with a coarse-grained model of a cell comprised of elastic flagella
connected to a cell body. The elasticities of both the hook protein (connecting
cell body and flagellum) and flagella are varied. Flexibility plays contrasting
roles for uni- and multiflagellar swimmers. For a uniflagellar swimmer, hook
and/or flagellar buckling occurs above a critical flexibility relative to the
torque exerted by the flagellar motor. Addition of a second flagellum greatly
expands the parameter regime of stable locomotion, because flexible hooks that
would lead to buckling instability in the uniflagellar case provide the
flexibility required for flagellar bundling in the biflagellar case. Similar
observations hold for tri- and quadriflagellar swimmers. Indeed the stability
regimes for uni- and quadriflagellar swimming are virtually inverted -- to a
first approximation what is stable in one case is unstable in the other.
Swimming speed is also examined: it increases very weakly with number of
flagella and a simple theory is developed that explains this observation.
