How resistance genes spread

Posted By News On June 16, 2017 - 5:02pm

Do resistance genes actually originate from the microorganisms producing the antibiotic?

It's been a microbiological hypothesis for 30 years but a new paper shows for the very first time shows that antibiotic resistance genes do originate from the same place as the antibiotic compounds, i.e. from a group of soil bacteria called Actinobacteria. Basically, disease-causing bacteria get their resistance genes in a complex process involving bacterial 'sex'. More than three fourths of all current antibiotics used to treat human infections are produced by Actinobacteria, which at the same time carry antibiotic resistance genes.  In these experiments, the researchers found that many resistance genes in disease-causing microbes (gram negative pathogens) were very similar to resistance genes found in Actinobacteria. Especially in one case, the genes were 100 percent identical.

Actinomycetes is in a plate. Credit: DTU/DTU Biosustain 

Gram negative pathogens constitute a big group of different species; amongst others pseudomonas, which can cause lung and urinary tract infections.

At first, it was difficult to imagine how pathogens can acquire genes from Actinobacteria, because they are so different and not at all related with each other. But by investigating the DNA sequence around the resistance genes, the team figured out how the resistance genes transfer occurred through a new mechanism named "carry back", where the pathogen basically has a primitive form of "sex" with the Actinobacterium and takes up its resistance genes after it dies.

This gene transfer by carry back could in principle happen where pathogens come into contact with Actinobacteria, like in an animal farm or in soil polluted with untreated hospital waste. In this way, the pathogen can become resistant and endanger human lives in the next round of infection.

Understanding the origin is hence key to counteracting the spread of antibiotic resistance, explains Senior Researcher Tilmann Weber from DTU Biosustain:

"We can't stop this gene transfer, but when you know, which resistance genes pathogens may harbor, you can personalize the antibiotic treatment. Also, with this knowledge you can try to develop new antibiotics with other properties that the pathogens don't have a defense against."

Bacterial sex act reveals the mechanism:

By following the DNA-transfer, scientists for the very first time showed an unknown mechanism called carry back in which pathogens were able to snatch genes from far-related bacteria via the carry back mechanism. Here is how, in short, the carry back process works:

1. The Gram negative pathogen injects its DNA into the Actinobacteria. Gram negative bacteria naturally have an ability called conjugation by which bacterial cells can inject their own DNA into other bacterial cells. It is called the bacterial equivalent of sex, because it is usually used to exchange gene information between Gram negative bacteria. But sometimes Gram negative bacteria can also use this mechanism to inject DNA into far-related Gram positive bacteria like Actinobacteria.

2. Inside the Actinobacteria, the injected DNA recombines with the host's DNA containing resistance genes. After the Actinobacterium dies, the recombinant DNA is released into the environment.

3. Lastly, the injected DNA can act as "gluing DNA" and mediate the uptake of resistance gene back to the pathogens through a phenomenon called natural transformation.