During the synthesis of peptidoglycans in the bacterial cell wall, the nascent peptide chains are specifically capped with D-alanines, which can be selectively exchanged by exogenously provided unnatural D-amino acids (Figure ). To monitor the effect of the amino acids in bacterial cells, we grew Gram-negative Escherichia coli in the presence of compounds 12 or 13 and imaged the cells shortly after by fluorescence microscopy. We observed that E. coli cells incubated with the D-amino acid 13 -but not the L-amino acid 12-showed uniform labeling of the bacterial septa, indicating that the benzoselenadiazole scaffold retained the selectivity of D-alanine (Figure ). We confirmed by mass spectrometry that the D-amino acid 13 was covalently incorporated into the peptidoglycan cell wall (Figure ) and that the incorporation was independent of the bacterial strain, as shown in experiments with Gram-positive bacteria, such as the model species Bacillus subtillis and the pathogen Staphylococcus aureus (Figure ). We also examined whether compound 13 could kill E. coli after light irradiation. E. coli bacterial colonies were treated with compounds 12 (as a negative control) or 13 and kept in the dark or irradiated with light, followed by proliferation measurements under both conditions. As shown in Figure , high levels of cell death were observed in 13-treated and lightirradiated E. coli cells (<10% cell viability) whereas minimal toxicity was detected in non-irradiated cells or cells that had been incubated with the control compound 12. Finally, we performed time-lapse imaging of E. coli co-cultures that had been separately pre-labeled with compound 13 or NADA (Figure ). NADA is a reported nitrobenzoxadiazole D-alanine analogue lacking the selenium atom and able to fluorescently stain bacterial cell walls. The amino acids NADA and 13 fluorescently labeled E. coli cells (λem (NADA): 530 nm, λem (13): 590 nm), and their growth was monitored for 30 minutes while irradiating the cells at regular intervals. Remarkably, most NADA-labeled cells showed clear signs of continued growth and proliferation over time, whereas cells that had been pre-treated with compound 13 appeared as lysed or showed arrested cell growth (Figure ). These results support that the local production of singlet oxygen after the irradiation of peptidoglycan-incorporated benzoselenadiazoles can selectively target and kill bacterial cells with high spatial resolution and without affecting neighboring cells in the close vicinity. Given that compound 13 can be effectively incorporated into Gram-negative and Gram-positive bacteria and cause cell death upon light irradiation, these molecules could open avenues to develop new antibiotic regimes overcoming complications associated to antimicrobial resistance in pathogenic bacteria. Glioblastomas are the most common and one of the most aggressive types of brain cancer, where surgical treatments require highly precise and safe strategies to remove malignant cells without affecting healthy tissues. Brain tumor cells are very glycolytic and show increased levels of glucose consumption when compared to healthy tissue, hence we decided to assess whether glucosederivatized benzoselenadiazoles could kill human glioblastoma cells after exposure to non-toxic light.