Dual uptake mode of the antibiotic phazolicin in Gram-negative bacteria prevents resistance acquisition

Phazolicin (PHZ) is a peptide antibiotic exhibiting narrow-spectrum activity against rhizobia closely related to its producer Rhizobium sp. Pop5. Here, we show that PHZ enters sensitive Sinorhizobium meliloti Sm1021 cells through BacA and YejABEF, which belong to SLiPT and ABC transporter families, respectively. The simultaneous and independent uptake by transporters from two distinct families dramatically decreases the naturally occurring rate of PHZ resistance. Both BacA and YejABEF are essential for the development of functional symbiosis of rhizobia with leguminous plants. Therefore the acquisition of PHZ resistance via inactivation of these transporters is further disfavoured. A whole-genome transposon-sequencing screen revealed that the capsular polysaccharide KPS, an additional putative novel envelope polysaccharide, as well as the peptidoglycan layer also contribute to the sensitivity level of S. meliloti to PHZ. Most likely, these macromolecular surface structures constitute barriers that reduce the amount of PHZ transported inside the cell.