A widespread Salmonella strain appears to have gained ground after losing certain parts of its DNA.
Researchers focused on ST34, a Salmonella family now found in many countries. These bacteria can pass from animals to humans and cause intestinal infections. To trace their evolution, the team compared nearly 45,000 Salmonella genomes from different regions and time periods.
Bacterium with its flagella.
Pixabay illustrative image
The genome contains the instructions needed for an organism to function. These instructions are distributed across portions of DNA called genes. One might therefore think that a bacterium necessarily becomes less effective when it loses genes. In the case of ST34, the researchers observed the opposite.
The first change affects a gene called dinB. This gene normally plays a role when the bacterium has to copy damaged DNA. In ST34, a mutation disrupts its function, and this change subsequently affects the activity of other genes.
The researchers observed effects on the flagella in particular. These long filaments located on the surface of the bacterium allow it to move. After the change in dinB, ST34 lost another gene involved in producing these flagella.
This succession of changes appears to have given the bacterium an advantage in the intestine. The researchers verified this in experiments in which different Salmonella strains were placed in competition. ST34 was then able to grow better than some other lineages.
The order of the changes appears to be important. Analysis of ST34's evolution indicates that the mutation in the first gene appeared before the loss of the second. These successive changes accompanied the spread of this Salmonella family around the world.
This study provides a lead for monitoring the emergence of new lineages. Researchers may look for similar sequences of mutations in other disease-causing bacteria.