Production of Norspermidine Contributes to Aminoglycoside Resistance in Mutants of Pseudomonas Aeruginosa

Emergence of resistance to polymyxins in is mainly due to mutations in two-component systems that promote the addition of 4-amino-4-deoxy-l-arabinose to the lipopolysaccharide (LPS) through upregulation of operon () expression. Here, we demonstrate that mutations occurring in different domains of histidine kinase PmrB or in response regulator PmrA result in coresistance to aminoglycosides and colistin. All seventeen clinical strains tested exhibiting such a cross-resistance phenotype were found to be mutants. As shown by gene deletion experiments, the decreased susceptibility of the mutants to aminoglycosides was independent from operon but required the efflux system MexXY-OprM and the products of three genes, PA4773-PA4774-PA4775, that are cotranscribed and activated with genes Gene PA4773 (annotated as in the PAO1 genome) and PA4774 () are predicted to encode enzymes involved in biosynthesis of polyamines. Comparative analysis of cell surface extracts of an selected mutant, called AB16.2, and derivatives lacking PA4773, PA4774, and PA4775 revealed that these genes were needed for norspermidine production via a pathway that likely uses 1,3-diaminopropane, a precursor of polyamines. Altogether, our results suggest that norspermidine decreases the self-promoted uptake pathway of aminoglycosides across the outer membrane and, thereby, potentiates the activity of efflux pump MexXY-OprM.