How will swimming microorganisms affect the mixing of a diffusive contaminant in chaotic flows? Answers to this question can lead to improved understanding of the spread of pollutants (e.g. algal blooms) in oceans and lakes, as well as potentially useful applications in biofuel and vaccine productions. In this talk, I present recent experiments on the chaotic mixing of a passive pollutant in dilute suspensions of swimming Escherichia coli. I will first discuss the results in a two-dimensional (2D) time-periodic flow that preserves spatial symmetry. Second, I will extend these results to a more realistic case of spatially disordered 2D vortical flows that resemble geological flows in nature. Overall, these results show that the interaction between active swimming particles and dynamical structures of the flow (i.e. Lagrangian coherent structures) can lead to profound changes in the transport and mixing of passive scalar quantities.