Novel mechanism of plasmid-DNA transfer mediated by heterologous cell fusion in syntrophic coculture of Clostridium organisms

The evolution of bacteria is driven by random genetic mutations and horizontal gene transfer (HGT) of genetic material from other bacteria. HGT can occur via transformation, transduction, and conjugation. Here, we present a potential new mechanism of HGT which occurs in a syntrophic Clostridium coculture. We have previously shown that in syntrophic cocultures of Clostridium acetobutylicum and Clostridium ljungdahlii, the two organisms undergo heterologous cell fusion, which includes fusion of the peptidoglycan cell walls and membranes. Heterologous cell fusion facilitated a large-scale exchange of cytoplasmic protein and RNA between the two organisms, leading to the formation of hybrid bacterial cells containing cytoplasmic material of the two parent organisms. Here we present new evidence that cell fusion events also facilitate the exchange of plasmid DNA between the two organisms of the syntrophic coculture. Through the use of a selective subculturing process, we successfully isolated wild-type C. acetobutylicum clones which have acquired a portion of the plasmid DNA - containing the antibiotic resistance marker - from a recombinant strain of C. ljungdahlii. Fusion events led to formation of persistent aberrant hybrid cells with distinct morphogenetic characteristics. Furthermore, our data support the concept of a novel, interspecies, mechanism of acquiring antibiotic resistance. Since neither organism contains any known conjugation machinery or mechanism, these findings expand our understanding of multi-species microbiomes, their survival strategies, and evolution. IMPORTANCEInvestigations of natural multispecies microbiomes and the field of synthetic and syntrophic microbial cocultures are attracting renewed interest based on their potential application in biotechnology, ecology, and medical fields. A variety of synthetic and natural cocultures have been examined in terms of their metabolic output, but relatively few systems have been interrogated at the cellular and molecular level. Previously, we have shown the syntrophic coculture of C. acetobutylicum and C. ljungdahlii undergoes heterologous cell-to-cell fusion, which facilitates the exchange of cytoplasmic protein and RNA between the two organisms, and leads to the formation of hybrid bacterial cells. Continuing this line of investigation, we now show that heterologous cell fusion between the two Clostridium organisms can also facilitate the exchange of DNA between the two organisms. By applying selective pressures to this coculture system, we isolated clones of wild-type C. acetobutylicum which acquired the erythromycin resistance (erm) gene from the C. ljungdahlii strain carrying a plasmid with the erm gene. Fusion led to persistent hybrid cells containing DNA from both parents but with distinct properties and morphologies. Moreover, we provide evidence for a novel mechanism of acquiring antibiotic resistance mediated by the syntrophic interactions of this system. This is a major finding that may shed light on a new mechanism of bacterias ability to acquire antibiotic resistance.