Taxonomic distribution of SbmA/BacA and BacA-like antimicrobial peptide transporters suggests independent recruitment and convergent evolution in host–microbe interactions

Antimicrobial peptides (AMPs) are often produced by eukaryotes to control bacterial populations in both pathogenic and mutualistic symbioses. Several pathogens and nitrogen-fixing legume symbionts depend on transporters called SbmA (or BacA) or BclA (BacA-like) to survive exposure to AMPs. However, how broadly these transporters are distributed amongst bacteria, and their evolutionary history, is poorly understood. We used computational approaches, including phylogenetic and sequence similarity analyses, to examine the distribution of SbmA/BacA and BclA proteins across 1,255 species spanning the domain <i>Bacteria</i>, leading to the identification of 71 and 177 SbmA/BacA and BclA proteins, respectively. <i>In vitro</i> sensitivity assays using legume AMPs and several BclA proteins confirmed that AMP transport is a common feature of BclA homologues. Our analyses indicated that SbmA/BacA homologues are encoded only by species in the phylum <i>Pseudomonadota</i> and are primarily found in just two orders: <i>Hyphomicrobiales</i> and <i>Enterobacterales</i>. BclA homologues are somewhat more broadly distributed and were found in clusters across four phyla. These included several orders of the phyla <i>Pseudomonadota</i> and <i>Cyanobacteriota</i>, the order <i>Mycobacteriales</i> (phylum <i>Actinomycetota</i>) and the class <i>Negativicutes</i> (phylum <i>Bacillota</i>). Many of the clades enriched for species encoding SbmA/BacA or BclA homologues are rich in species that interact with eukaryotic hosts in mutualistic or pathogenic interactions. These observations suggest that SbmA/BacA and BclA proteins have been repeatedly co-opted to facilitate associations with eukaryotic hosts by allowing bacteria to cope with host-encoded AMPs.