# The Fsr transporter of Sinorhizobium meliloti contributes to antimicrobial resistance and symbiosis with alfalfa

## TL;DR

This study shows that the Fsr transporter in Sinorhizobium meliloti helps the bacteria resist certain antibiotics and improves its ability to form symbiotic relationships with alfalfa.

## Contribution

The study reveals the role of the Fsr transporter in antibiotic resistance and symbiosis, despite the fsr gene not functioning as a fosmidomycin efflux pump as previously thought.

## Key findings

- The fsr gene does not confer resistance to fosmidomycin but is crucial for resistance to fosfomycin, H2O2, and DOC.
- Fsr is a proton-dependent efflux pump that influences biofilm formation, motility, and symbiotic efficiency with alfalfa.
- Alfalfa seed exudate increases fsr transcription, suggesting a role in host interaction.

## Abstract

Major facilitator superfamily (MFS) transporters in bacteria participate in both the uptake and export of ions, metabolites or toxic compounds. In rhizobia, specific MFS transporters increase resistance to plant-produced compounds and may also affect other phenotypic traits, including symbiosis with legume host plants. Here, we describe the importance of the Sinorhizobium meliloti 1021 Fsr efflux pump in resistance to selected antimicrobial compounds and in modulating biofilm formation, motility and symbiotic efficiency with alfalfa. The fsr gene (smc00990) is annotated as encoding an MFS family fosmidomycin efflux pump. Unexpectedly, both the 1021 wild type and an fsr null mutant were highly resistant to fosmidomycin. Our assays indicate that this is due to an inability to transport the antibiotic. Unlike the wild type, the fsr mutant was highly sensitive to the fosmidomycin structural analogue fosfomycin, and moderately more sensitive to hydrogen peroxide (H2O2) and deoxycholate (DOC). Root and seed exudates from alfalfa did not inhibit the growth of the wild type or fsr mutant. fsr transcription significantly increased proportionally to the concentration of fosfomycin added to cultures but was unaffected by the addition of other antibiotics, H2O2, DOC or SDS. Alfalfa seed exudate moderately increased fsr transcriptional expression. Fluorometric assays using ethidium bromide as a substrate and carbonyl cyanide m-chlorophenyl hydrazone as an energy decoupler showed that Fsr was a proton-dependent efflux pump. Biofilm formation and swimming motility were decreased and increased, respectively, in the fsr mutant, and its symbiotic efficiency with alfalfa was decreased in terms of nodule numbers per plant and plant dry weights.