Plasma sterilization efficiency of the spore-contaminated surfaces was characterized with use of
the standard B. subtilis spore-immobilized test strips (about 106 spores per each strip). The strips to be
treated were stood edgewise toward the jet. After the treatment, each strip was aseptically transferred
to an individual tube containing the test medium with added tryptone and phenol red dye. The tubes
were incubated at 37 °C in an incubator. The effect of plasma sterilization was judged from the changes
in the medium color in 7 days of the incubation. The absence of changes in color (still red) meant that
the sample was sterile, while a change in color from red to yellow indicated the presence of the living
microorganisms in the sample.