For the computer mouse study, we recruited nine healthy adults (four female and five male, all 20–35 years of age) who worked in the same building on the University of Colorado campus. Using the swabbing technique described above, the entire exposed surface of each computer mouse and the palm surface of the individual's dominant hand (the hand typically used to operate the mouse) was swabbed. Care was taken to ensure that the mouse had last been touched by the owner 12 h before the swabbing (the mice remained at room temperature during this period). Palm surfaces were sampled midday and the volunteers were told to follow their typical hand hygiene practices before the sampling. All swabs were stored at −80 °C before DNA extraction. We estimated the accuracy of matching the mouse to the owner of the mouse by measuring the degree of similarity between bacterial communities on each computer mouse to the hands of the mouse's owner and to the hands that had never touched the mouse. We compiled a database of bacterial communities from 270 other hands sampled for other projects (8, 9). The 270 hands bacterial communities included in this database came from both left and right palm surfaces belonging to male and female volunteers in equal proportions that were healthy and between the ages of 18 and 40 years. The palms were sampled and the bacterial communities analyzed using procedures identical to those described here.