  Peritrichous bacteria synchronize and bundle their flagella to actively swim
while disruption of the bundle leads to tumbling. It is still not known whether
the number of flagella represents an evolutionary adaptation towards optimizing
bacterial navigation. Here, we study the swimming dynamics of differentially
flagellated Bacillus subtilis strains in a quasi-two-dimensional system. We
find that decreasing the number of flagella N reduces the average turning angle
between two successive run phases and enhances the duration and directional
persistence of the run phase. As a result, having less flagella is beneficial
for long-distance transport and fast spreading, while having a lot of flagella
is advantageous for the processes which require localization and slow dynamics,
such as biofilm formation. We develop a two-state random walk model that
incorporates spontaneous switchings between the states and yields exact
analytical expressions for transport properties, in remarkable agreement with
experiments. The results of numerical simulations based on our two-state model
suggest that the efficiency of searching and exploring the environment is
optimized at intermediate values of N. The optimal choice of N, for which the
search time is minimized, decreases with increasing the size of the environment
in which the bacteria swim.
