During symbiosis with the legume Medicago truncatula, the bacterium Sinorhizobium meliloti differentiates into nitrogen-fixing bacteroides in nodules formed on the plant roots. This occurs through a process that requires the bacterial protein BacA and the plant NCR AMPs (nodule-specific Cys-rich antimicrobial peptides), although the precise role of each in establishing this symbiotic relationship was unclear. In this study, the authors observed that BacA was crucial for protecting S. meliloti from NCR AMPs. In vitro, BacA-deficient bacteria were more sensitive to the activity of NCR AMPs, which induce loss of membrane integrity. Furthermore, BacA-deficient bacteria showed reduced viability in vivo compared with wild-type S. meliloti. Interestingly, the mutant bacteria could survive in the M. truncatula dnf1 mutant, which lacks the signal peptidase required for NCR AMP trafficking. Together, these findings suggest that S. meliloti BacA protects the bacterium from membrane damage caused by the host NRC AMPs.