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Colistin (polymyxin E) is a membrane-destabilizing antibiotic used against Gram-negative bacteria. We have recently reported that the outer membrane prevents the uptake of antibacterial chlorophyllin into Gram-negative cells. In this study, we used sub-toxic concentrations of colistin to weaken this barrier for a combination treatment of <i>Escherichia coli</i> and <i>Salmonella enterica</i> serovar Typhimurium with chlorophyllin. In the presence of 0.25 &#181;g/mL colistin, chlorophyllin was able to inactivate both bacteria strains at concentrations of 5&#8722;10 mg/L for <i>E. coli</i> and 0.5&#8722;1 mg/L for <i>S.</i> Typhimurium, which showed a higher overall susceptibility to chlorophyllin treatment. In accordance with a previous study, chlorophyllin has proven antibacterial activity both as a photosensitizer, illuminated with 12 mW/cm², and in darkness. Our data clearly confirmed the relevance of the outer membrane in protection against xenobiotics. Combination treatment with colistin broadens chlorophyllin&#8217;s application spectrum against Gram-negatives and gives rise to the assumption that chlorophyllin together with cell membrane-destabilizing substances may become a promising approach in bacteria control. Furthermore, we demonstrated that colistin acts as a door opener even for the photodynamic inactivation of colistin-resistant (<i>mcr-1</i>-positive) <i>E. coli</i> cells by chlorophyllin, which could help us to overcome this antimicrobial resistance.