Antibiotics have several ways to kill or prevent growth in bacteria. Bacteria try to figure out ways to circumvent this. And they are good at it: multi-resistant bacteria exist that managed to get resistant to most known antibiotics. If an infection with these occurs, antibiotics cannot longer help, making these infections very difficult to treat. In human healthcare, an increasing number of diseases is known to become harder to heal due to this problem. And in the food chain we also see more multi-resistant bacteria emerging, especially in farm animals. Pathogens can spread via animals or food to human healthcare.

Exchanging resistance via mobile genetic packages

Bacteria can acquire specific genes that help them to become resistant. These genes are called resistance genes. Often these genes are present on mobile genetic elements, allowing bacteria to share them with others. Plasmids are an example of such elements. They are small circles of DNA that bacteria can share between each other. If a plasmid carrying a resistance gene is transferred from one bacteria to another, the bacteria instantly becomes resistant as well.

The transfer of plasmids has been known in microbiology for a long time already, as well as the fact that resistance genes are shared this way. For this study, the researchers used different E. coli strains isolated from meat. Isolates were found to be carrying one to five known plasmids.

A method to study resistance transfer

As part of her PhD research, Tania Darphorn set up a method to incubate the isolates together with a recipient strain carrying no plasmids, and analyze them at several time-intervals. This allowed the team to follow the dynamics and transfer rates of resistance plasmids. They found that transfer rates were relatively low, only about three percent of bacteria received plasmids in the tested populations. But it does seem to be initiated quickly, within an hour of co-incubation of bacteria. Moreover, it seems that the transfer process is quicker when bacteria have several instead of just one plasmid, meaning that multi-resistant bacteria might have more potential of transferring their plasmids. To visualize what is happening, the researchers collaborated with Anita Grootemaat and Nicole van der Wel from the Electron Microscopy Centre Amsterdam at the AUMC to visualize the structures that the bacteria use to transfer. Bacteria with faster transfer rates showed to have more of these structures. Why this is the case is something to further investigate. 

With this study, the researchers developed a method to reliably study transfer of resistance plasmids in bacteria. By doing so we can learn more about the exact details of how multi-resistant bacteria spread among strains, hopefully leading us to potential weak spots that can help us slow down or even prevent spread of multi-resistance genes.  

Publication

Darphorn TS, Koenders-van Sintanneland BB, Grootemaat AE, van der Wel NN, Brul S, ter Kuile BH (2022) Transfer dynamics of multiresistance plasmids in Escherichia coli isolated from meat. PLoS ONE 17(7): e0270205.