Plasmids have a crucial role in the spread of antimicrobial resistance (AMR) genes in bacterial populations through horizontal gene transfer. For instance, incompatibility group HI (IncHI) plasmids, which are prevalent in Enterobacteriaceae species such as Salmonella enterica and Escherichia coli, carry several AMR genes, including those that confer resistance to colistin and carbapenems, thus posing a major challenge to the treatment of bacterial infections in the clinic. A novel feature of IncHI plasmids is that they encode large molecular mass proteins containing bacterial immunoglobulin-like (Big) domains (Big proteins), which are found on the surface of bacteria such as in flagella and conjugative pili. Given their presence on bacterial surfaces, these plasmid-encoded Big proteins are potential targets for vaccines aimed at limiting the spread of AMR. In a recent study, Prieto et al. demonstrate that a plasmid-encoded Big protein is antigenic and can protect mice from antibiotic-resistant infections, and that a nanobody targeting this protein interferes with the conjugative transfer of IncHI plasmids.

Next, the authors selected and isolated a nanobody that specifically binds and neutralizes the RSP protein. Analyses showed that this nanobody has a high affinity for RSP and that it agglutinates S. Typhimurium SL1344 carrying the R27 plasmid in the presence of an E. coli strain expressing the nanobody on its cell surface. The authors then investigated whether an E. coli strain displaying the nanobody on its surface could interfere with the conjugation of the R27 plasmid from S. Typhimurium SL1344 to a recipient strain. Their results showed that the presence of the nanobody significantly reduced the plasmid conjugation frequency, with up to a ~1,000-fold decrease in conjugation at 24 hours and a ~100-fold decrease at 36 and 48 hours.