An engineering timing criterion for microbial biofilm inactivation by UV-C irradiation and cold atmospheric plasma

Microbial surface colonization is difficult to control because a treatment applied at one growth stage can become insufficient after the colony has crossed a critical coverage. This work develops a phenomenological engineering rule for selecting the activation time of an external inactivation source (EIS). A logistic balance with natural growth rate r , effective inactivation rate I , and carrying capacity K gives the critical intensity I c = r K / 4 and, in the subcritical regime, an explicit activation boundary t 0 = f ( θ 0 ; [ r , I ] ) . The criterion classifies treatment as being on the extinction side of the boundary when θ ( t 0 ) < θ 1 = K ( 1 − D ) / 2 , where D = 1 − I / I c , and on the survival side when this threshold has already been exceeded. Three previously obtained Aspergillus brasiliensis trajectories are used as illustrative cases: UV-C at 24 h and cold atmospheric plasma (CAP) at 117 h lie on the survival side of the coverage threshold, whereas CAP at 72 h lies on the extinction side. The signed coverage margins are + 0.58 , + 19.4 , and − 44.4 percentage points, respectively. The model is intended as a timing-oriented screening rule, not as a statistical efficacy model or a plasma-chemistry model. Replicate-level uncertainty, absorbed biological dose, and plasma-species diagnostics were not available for these archived trajectories. No species-resolved mechanism or head-to-head predictive comparison with conventional kinetic models can therefore be inferred from the present data; claims are restricted to phenomenological proof-of-concept for the tested fungal system and reactor configuration.