Impact of FiuA outer-membrane receptor polymorphism on the resistance of towards peptidoglycan lipid II-targeting PaeM pyocins.

Certain Pseudomonas aeruginosa strains produce a homolog of colicin M, namely, PaeM, that specifically inhibits peptidoglycan biosynthesis of susceptible P. aeruginosa strains by hydrolyzing the lipid II intermediate precursor. Two variants of this pyocin were identified whose sequences mainly differed in the N-terminal protein moiety, i.e., the region involved in the binding to the FiuA outer membrane receptor and translocation into the periplasm. The antibacterial activity of these two variants, PaeM1 and PaeM2, was tested against various P. aeruginosa strains comprising reference strains PAO1 and PA14, PaeM-producing strains, and 60 clinical isolates. Seven of these strains, including PAO1, were susceptible to only one variant (2 to PaeM1 and 5 to PaeM2), and 11 were affected by both. The remaining strains, including PA14 and four PaeM1 producers, were resistant to both variants. The differences in the antibacterial spectra of the two PaeM homologs prompted us to investigate the molecular determinants allowing their internalization into P. aeruginosa cells, taking the PAO1 strain that is susceptible to PaeM2 but resistant to PaeM1 as the indicator strain. Heterologous expression of fiuA gene orthologs from different strains into PAO1, site-directed mutagenesis experiments, and construction of PaeM chimeric proteins provided evidence that the cell susceptibility and discrimination differences between the PaeM variants resulted from a polymorphism of both the pyocin and the outer membrane receptor FiuA. Moreover, we found that a third component, TonB1, a protein involved in iron transport in P. aeruginosa, working together with FiuA and the ExbB/ExbD complex, was directly implicated in this discrimination. IMPORTANCE Bacterial antibiotic resistance constitutes a threat to human health, imposing the need for identification of new targets and development of new strategies to fight multiresistant pathogens. Bacteriocins and other weapons that bacteria have themselves developed to kill competitors are therefore of great interest and a valuable source of inspiration for us. Attention was paid here to two variants of a colicin M homolog (PaeM) produced by certain strains of P. aeruginosa that inhibit the growth of their congeners by blocking cell wall peptidoglycan synthesis. Molecular determinants allowing recognition of these pyocins by the outer membrane receptor FiuA were identified, and a receptor polymorphism affecting the susceptibility of P. aeruginosa clinical strains was highlighted, providing new insights into the potential use of these pyocins as an alternative to antibiotics.