Defence systems shape evolution of major bacteria

Evolutionary defence systems help a common bacterial species to create and maintain distinct genetic groups, research has shown.

This influences how bacteria adapt to different hosts and acquire beneficial new traits, the study shows.

Staphylococcus aureus is a common cause of infections in people and animals, and is well known for developing resistance to antibiotics.

One reason S. aureus adapts so readily is its known ability to acquire DNA from distant relatives, gaining new genes that can help it survive in different environments or to resist drug treatment.

The bacteria’s capability to share genes is shaped not only by opportunity to cause infection, but also by built-in biological barriers and by the range of hosts the bacteria infect, Roslin scientists have discovered.

Their findings show that within a single bacterial species, different sub-groups exist that have their own distinct sets of genes. This insight could help inform future approaches to infection control and mitigating antimicrobial resistance.

Genetic groups

To understand why S. aureus forms clearly defined genetic subgroups, researchers analysed the DNA of many different strains representing the main groups within the species.

The team compared how often different groups of S. aureus exchange genes, and variation within the full set of genes found across the species.

Results showed striking differences. Groups that can infect a wide range of hosts, including humans, sheep, pigs, dogs and birds, tended to have a broader and more varied set of genes than those which do not.

In contrast, groups largely restricted to one host species, such as those almost exclusively infecting cattle, had more limited variety in their genes.

This may limit how easily they acquire new traits, such as antibiotic resistance, compared with strains that circulate across multiple hosts.

Bacterial security system
A key reason why these bacterial groups remain genetically distinct is the presence of molecular defence systems made up of specialised genes, known as restriction–modification systems.

These systems provide an important defense against molecular parasites that hijack bacteria to spread their own DNA. They recognise and cut up unfamiliar DNA that enters the bacterial cell, limiting which new genes the bacterium can acquire, preventing most foreign genes from being taken up.

"Each bacterial lineage has its own set of defence systems, and once they’re in place, the lineage becomes very cohesive," explains Dr Jamie Gorzynski. "These systems act as a critical barrier against incoming DNA from other lineages, helping maintain the identity of each group."

These defence systems can themselves be acquired from other bacteria, meaning that lineages evolve not only by gaining new traits but also by adopting new systems that control which incoming DNA can enter their genomes.

Gene mobility

It is well established that many key traits, including antibiotic resistance and toxin production, are often carried on mobile, transferrable pieces of DNA.

A better understanding of how these are blocked or admitted into genomes could eventually help inform strategies to slow the spread of harmful strains, the research team says.

This study was published in Cell Reports, in collaboration with colleagues from Imperial College London. 

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Research publication

Image credit CDC, Creative Commons