Nanometer-sized composite particles, which consisted of gold nanoparticles encapsulated by an N-isopropylacrylamide copolymer, were successfully synthesized using a one-step process. Shape complementary cavities of the O157-antigen were formed on the composite utilizing temperature-dependent affinity changes of the copolymer. The composite bound to enterohemorrhagic Escherichia coli (E. coli) O157 at 298 K and enhanced light-scattering intensity of the cell due to the optical properties of the gold nanoparticles. Moreover, the composite showed excellent selectivity (>15) against other types of E. coli such as O26 and O Rough. Recognition of the O157-antigen ceased upon heating to 313 K but was restored upon cooling to 298 K. During repeated temperature cycling around the phase transition temperature of the copolymer (305 K), the composite reproducibly showed recognition behavior at 298 K. The binding ability of the composite could be switched reversibly. Therefore, it was concluded that the molecular structure of the O157-antigen was memorized by the composite, rather than being molded into it. This technique is applicable not only for the detection of a target bacterium but also for an identification of new bacterial threats by the simple formation of the specific antigen-imprinted composite.