Assessment of background for ATP and stamp methods. (A) The background of the ATP method was estimated by a spiked experiment with a diluted bacterial solution of three bacteria (E. coli, S. aureus, and B. subtilis) (100–106 CFU). The background value was 74.3 + 1 standard deviation (SD) (52.6) RUL, equivalent to 10,000 CFU. The experiments were independently performed at least three times. The data show the mean ± SD. *p < 0.05 vs. the value of the sample spiked with 100 CFU (Student’s t test). (B) The background of the stamp agar method was randomly examined on student tables (n = 19) in a lecture room of our department (Faculty of Health Sciences, Hokkaido University) during winter vacation, and the total CFU detected before and after treatment with a neutral detergent was determined as the detection limit of the stamp method (See Methods). The detection limit was estimated as 3.3 + 1 SD (2.1) CFU.