Replication capacity as a basis for assessing the sensitivity of micro-organisms to disinfectant agents

BACKGROUND: Interpretation of microbial tolerance and resistance to disinfectants has long been inconsistent, with heterogeneous definitions and no clinically meaningful threshold. We propose the concept of replication capacity after use (RCAU) as a practical endpoint to assess whether microbial survival after disinfectant exposure constitutes a clinically relevant phenomenon under recommended use conditions. RCAU is defined as the ability of micro-organisms to replicate after exposure at recommended application concentration and exposure time. A critical RCAU corresponds to failure of a standardized quantitative suspension test. METHODS: We reassessed published evidence across the most common disinfectant substances listed by the German Association for Applied Hygiene. Reported findings on survival, tolerance and resistance were re-evaluated against the RCAU definition, with particular attention to whether testing was performed using quantitative suspension methods at application concentration. RESULTS: No disinfectant group has demonstrated a critical RCAU under application conditions in standardized suspension testing. Reports of reduced susceptibility or microbial survival exist, but many were not based on suspension tests at use concentrations, making interpretation with respect to RCAU uncertain. Transient or reversible adaptations have been described, yet without evidence of a critical RCAU. Only triclosan and silver compounds show established resistance mechanisms, though even here no critical RCAU has been confirmed under standardized testing. CONCLUSION: RCAU provides a transparent, use-condition-anchored framework to differentiate non-critical survival from clinically relevant resistance development. Applied across disinfectant classes, it shows that no critical failures have occurred at use concentrations, although many reported findings were not assessed by standardized suspension tests.