  We studied the swimming of \textit{Escherichia coli} bacteria in the vicinity
of the critical point in a solution of the non-ionic surfactant
\chem{C_{12}E_{5}} in buffer solution. In phase contrast microscopy, each
swimming cell produces a transient trail behind itself lasting several seconds.
Comparing quantitative image analysis with simulations show that these trails
are due to local phase re-organisation triggered by differential adsorption.
This contrasts with similar trails seen in bacteria swimming in liquid
crystals, which are due to shear effects. We show how our trails are
controlled, and use them to probe the structure and dynamics of critical
fluctuations in the fluid medium.
