A novel homoarginine-containing cyclic peptide pioamide with selective antipseudomonal activity isolated from the nematode symbiont <i>Photorhabdus khanii</i>

The rise of multidrug-resistant Gram-negative bacteria is a growing threat to global public health. Narrow-spectrum antibiotics minimize disruption of the host microbiota and reduce the risk of resistance development in off-target bacteria. In the field of antibacterial discovery, developing compounds effective against <i>Pseudomonas aeruginosa</i> remains particularly challenging. Although <i>Photorhabdus</i> species are known to produce various antibiotics, their potential remains largely underexplored. In this study, we applied differential screening to a highly concentrated culture extract of <i>Photorhabdus khanii</i> HGB1456 and discovered pioamide, a novel cyclic peptide with unusual selective activity against <i>P. aeruginosa</i>. Mutations in pmrB confer pioamide resistance to <i>P. aeruginosa</i>. However, the mechanism of action is distinct from that of colistin, which also involves resistance conferred by <i>pmrB</i> mutations. These findings underscore the untapped potential of <i>Photorhabdus</i> species as an attractive source of species-specific antibiotics and highlight the utility of differential screening for discovering compounds with targeted antibacterial activity.