Large increases in attractant concentration disrupt the polar localization of bacterial chemoreceptors. 
In bacterial chemotaxis, the chemoreceptors [methyl-accepting chemotaxis proteins (MCPs)] transduce chemotactic signals through the two-component histidine kinase CheA. At low but not high attractant concentrations, chemotactic signals must be amplified.  The MCPs are organized into a polar lattice, and this organization has been proposed to be critical for signal amplification.  Although evidence in support of this model has emerged, an understanding of how signals are amplified and modulated is lacking.  We probed the role of MCP localization under conditions wherein signal amplification must be inhibited.  We tested whether a large increase in attractant concentration (a change that should alter receptor occupancy from c. 0% to > 95%) would elicit changes in the chemoreceptor localization.  We treated Escherichia coli or Bacillus subtilis with a high level of attractant, exposed cells to the cross-linking agent paraformaldehyde and visualized chemoreceptor location with an anti-MCP antibody.  A marked increase in the percentage of cells displaying a diffuse staining pattern was obtained.  In contrast, no increase in diffuse MCP staining is observed when cells are treated with a repellent or a low concentration of attractant.  For B. subtilis mutants that do not undergo chemotaxis, the addition of a high concentration of attractant has no effect on MCP localization.  Our data suggest that interactions between chemoreceptors are decreased when signal amplification is unnecessary.