['Chemical-in-plug assays are commonly used to study bacterial chemotaxis, sometimes in the absence of stringent controls.', 'We report that non-chemotactic and non-motile mutants in two distinct bacterial species (Shewanella oneidensis and Helicobacter pylori) show apparent zones of accumulation or clearing around test plugs containing potential attractants or repellents, respectively.'] Is the chemical-in-plug bacterial chemotaxis assay prone to false positive responses?