Despite the advances made over the last decade, infections caused by multi-drug resistant bacterial strains are increasingly important societal issues that need to be addressed. New approaches have already been developed in order to overcome this problem. Photodynamic antimicrobial chemotherapy (PACT) could provide an alternative to fight infectious bacteria. This approach has already inspired the development of innovative materials. Interesting results have been obtained against Gram-positive bacteria, but it also appeared that Gram-negative strains, especially Pseudomonas aeruginosa, were less sensitive to PACT. Enhanced efficacy against Gram-negative bacteria had been previously obtained with photosensitizers bound to antimicrobial peptides. In this work, we designed a photobactericidal organic material, CNCsc6-PMB, consisting of cellulose nanocrystals to which the photosensitizer chlorin-e6 and the antimicrobial polypeptide polymyxin B (PMB) were covalently attached. These modified nanocrystals were characterized by IR spectroscopy, zeta potential measurements and elemental analyses, after which antibacterial assays were carried out. Following light irradiation, CNCsc6-PMB demonstrated efficiency against Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus and Staphylococcus epidermidis) by inhibition of bacterial growth. An amplifying effect of chlorin-e6 has been highlighted against these Gram-negative strains, based on membrane weakening and a potential docking effect from the polymyxin moiety. Such results confirmed the importance of using an antimicrobial peptide in order to broaden the spectrum of PACT.