Quaternary ammonium compounds (QACs) are among the most used disinfection agents in consumer products and in industrial applications. Because of their poor biodegradabil ity, after use, they will persist in technical and natural aquatic systems at sub - inhibitory concentrations. Implications of QACs on the development of antimicrobial resistance was detected in several studies, however, the effect of a constant sub - inhibito ry concentration on the development of bacterial resistance remains unknown. The aim of this study was to determine if constant sub - inhibitory concentrations of benzalkonium chloride (BAC) and cetyltrimethylammonium chloride (CTMA) are able to promote a st able adaptation of populations of P. aeruginosa . Additionally, the study investigated cross - resistances between QACs and with antibiotics and nanomechanical properties of the membrane using atomic force microscopy (AFM) were explored. The exposure to sub - i nhibitory concentrations of QACs led to a maximal but reversible increase of the minimum inhibitory concentration (MIC) by a factor 1.45 for CTMA and a maximal irreversible increase of the MIC by a factor 1.9 for BAC. Cross - resistance was observed with the QACs but not with antibiotics: bacteria exposed to CTMA had an increased MIC for BAC of a factor 1.8 and bacteria exposed to BAC had an increased MIC for CTMA of a factor 1.75. Analyses of the membrane by AFM showed increase in the cell's roughness and st iffness after treatment with QACs. In populations of P. aeruginosa, constant sub - inhibitory concentrations of QACs were able to promote adaptations mechanisms, which were irreversible in the case of BAC. Furthermore, cross - resistance between the selected Q ACs were observed. Since low sub - inhibitory concentrations of QACs can be detected in natural and technical aquatic systems, this may lead to a dissemination of bacteria with higher QAC resistance or with QAC multiresistance, which can be a threat to the u rban water management.