L-type pyocins kill Pseudomonas aeruginosa by inhibiting the β-barrel assembly machinery (BAM) complex externally.

Pseudomonas aeruginosa is a formidable Gram-negative pathogen, notorious for its intrinsic defense mechanisms, including an impermeable outer membrane, rendering many conventional antibiotics ineffective. The escalating challenge of antimicrobial resistance necessitates novel therapeutic strategies targeting unique bacterial vulnerabilities. The β-barrel assembly machinery (BAM) complex, crucial for outer-membrane protein biogenesis, represents a promising, underexplored target, particularly for agents that can act extracellularly to bypass resistance mechanisms.

Researchers investigated the mechanism of action of L-type pyocins against P. aeruginosa. They employed single-particle cryo-electron microscopy to visualize pyocin binding to the BAM complex. Further studies utilized genetics, multi-omics (transcriptomics, proteomics), and cryo-electron tomography to characterize the cellular response to BAM complex inhibition. The effects of L-type pyocins were compared to the cyclic-peptide antibiotic darobactin, another known BAM complex inhibitor, to confirm the pathway.

L-type pyocins effectively kill P. aeruginosa by directly targeting the β-barrel assembly machinery (BAM) complex at the cell surface. Cryo-electron microscopy revealed that L-type pyocins bind to a surface-exposed region of BamA, a key component of the BAM complex.

The pyocins deploy a C-terminal peptide that competitively inhibits the BAM complex, crucially demonstrating that cell entry is not required for their potent antibiotic activity. This inhibition halts the assembly of outer-membrane proteins, leading to a profound cellular response. Multi-omics analyses showed that BAM complex inhibition by L-type pyocins, or by darobactin, triggers a multifaceted transcriptomic, proteomic, and morphological cascade. Ultimately, this disruption culminates in a catastrophic loss of membrane integrity and subsequent cell death.