Described herein are compositions and methods for detecting the presence or absence of a microorganism in a sample comprising contacting the sample with an aptamer capable of binding to a cell-surface protein of the microorganism to form a complex, contacting the mixture with a second aptamer capable of binding to the first cell-surface protein or a second cell-surface protein of the microorganism; and performing an assay to detect the second aptamer, wherein detecting the second aptamer indicates that the microorganism is present in the sample, and wherein not detecting the second aptamer indicates that the microorganism is absent from the sample.