Same but different: how closely related bacteria differ from each other
- Joint press release by Kiel University and the Max-Planck-Institute for Evolutionary Biology in Plön -
Study lead by researchers from Kiel University and the MPI-EB in Plön shows how strains of the disease-causing bacteria Pseudomonas aeruginosa differ in their type VI secretion system
P. aeruginosa bacteria can cause life-threatening infections. However, different P. aeruginosa bacteria are not all the same,
the type VI secretion system, a molecular mechanism enables bacteria to fire effector proteins out of the cell and with that change their environment or take up nutrients,
the researchers show which genes of the type VI secretion system are the same and which differ between closely related P. aeruginosa bacteria. Those genes might enable competition between these bacteria or cause selective advantages in specific environments,
these findings help to understand how closely related bacteria evolve and could be used in the future to improve diagnosis and treatment of infections.

Wherever bacteria live, they constantly compete with other bacteria for space or nutrients – and not always do so peacefully. One molecular mechanism that bacteria make and use to gain an advantage over other bacteria is the type VI secretion system (T6SS), which functions similar to a crossbow. Dr Luke Allsopp, a co-author of the study, says: “Bacteria use this T6SS to fire proteins out of the bacterial cell similar to how a crossbow fires arrows.” These so-called effectors proteins can be thought of like a poison arrow. They have a range of functions like killing of neighbouring bacteria, manipulating of eukaryotic cells or helping with nutrient uptake. “The T6SS is a kind of superpower for bacteria, enabling it to fight and alter its environment” says Dr Daniel Unterweger, corresponding author of the study. How disease-causing bacteria differ in their T6SS is not fully understood.

Researchers from Kiel University, the Max-Planck-Institute for Evolutionary Biology (MPI-EB), and the National Heart and Lung Institute at Imperial College London have now discovered that the DNA of disease-causing bacteria P. aeruginosadiffers massively in their T6SS effector genes. In this study recently published in the scientific journal Nature Communications, the scientists compared the DNA of around 2000 different P. aeruginosa bacteria and analysed which effectors are found in each bacterium. Using this population genetics approach, they found that some T6SS effectors occur in all P. aeruginosa bacteria and others do not. “This difference in occurrence of the effectors was initially surprising to us,” says Dr Antonia Habich, lead author of the publication and scientist in the BMBF-funded research group Infection Biology at Kiel University and MPI-EB, led by Dr Daniel Unterweger, adding, “But ultimately, we were able to explain this. For example, effectors responsible for nutrient uptake are present in all P. aeruginosa bacteria, whereas effectors used for bacterial competition differ.”

The researchers also investigated how such differences arose. They performed further bioinformatic analyses and found that P. aeruginosa bacteria can exchange the T6SS effector genes between each other through a process known as horizontal gene transfer. Horizontal gene transfer is widespread in bacteria and best known for the spread of antibiotic resistance genes. For the researchers, the findings of horizontal gene transfer of effectors genes were particularly interesting from an evolutionary perspective, as it provides insights into the emergence of bacterial diversity within a species. “Normally, humans inherit their traits from their parents e.g. eye colour or height, which is called vertical gene transfer. Bacteria can also get a piece of DNA from other bacteria. That’s horizontal transfer of DNA, and can make bacteria more successful in an environment” says Luca Robinson, co-author on the study.

P. aeruginosa is an opportunistic bacterial pathogen that causes many infections such as those of open wounds, the urinary tract or the respiratory tract in humans. Each year, around 600,000 people die from P. aeruginosa infections. Antibiotic resistances make it increasingly difficult to treat these infections. The world health organization rates research on P. aeruginosa and the development of novel treatment approaches as high priority. This study contributes to this effort with its new findings on the occurrence and diversity of T6SS genes amongst global P. aeruginosa bacteria which enables us to better understand pathogens and the factors they use in their environments.