The entrapment of bacteria near boundary surfaces is of biological and practical importance, yet the underlying physics is still not well understood. We prove that it is crucial to include a commonly neglected entropic effect arising from the spatial variation of hydrodynamic interactions, through a model that provides analytic explanation of bacterial entrapment in two dimensionless parameters: a_1 the ratio of thermal energy to self-propulsion, and a_2 an intrinsic shape factor. For a_1 and a_2 that match an Escherichia coli at room temperature, our model quantitatively reproduces existing experimental observations, including two key features that have not been previously resolved: The bacterial “nose-down” configuration, and the anticorrelation between the pitch angle and the wobbling angle. Furthermore, our model analytically predicts the existence of an entrapment zone in the parameter space defined by {a_1, a_2}.

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