Genetic diversity helps to reduce the spread of diseases by limiting parasite evolution, according to new research from the University of Liverpool.

he idea that host diversity can limit disease outbreaks is not new. For example, crop monocultures in agriculture – which lack genetic diversity – can suffer severe disease outbreaks that sweep through the entire population. But why is this?

A new study, led by the University of Exeter in collaboration with academics in Liverpool, France and the USA, provides an answer.

To study the effects of host diversity on disease spread, the researchers used a virus that can infect and kill bacteria. The bacteria defend themselves using a sophisticated immune system, known as CRISPR-Cas, which captures random DNA fragments from the virus. This ‘genetic memory’ protects the bacteria against future infections.

Diversity matters

CRISPR-Cas generates lots of diversity because every bacterium captures a different piece of virus DNA. Hence, after virus exposure every bacterium with CRISPR-Cas immunity is unique and diversity in the population is high.

To test if and why host diversity limits the spread of disease, the researchers isolated individual bacteria, and grew them in monoculture or mixed them together in diverse populations before exposing them to the virus.

The team found that the virus could spread on monocultures because they were able to evolve rapidly to overcome CRISPR-Cas immunity. However, on mixed bacterial populations – which have much more genetic diversity in the CRISPR-Cas system – the virus was unable to evolve and therefore went extinct.

Sequencing technology

The experiments showed that the ability of viruses to evolve high infectivity directly depends on the level of host genetic diversity. Therefore, mixing monocultures together can increase the immunity level of the population as a whole, a feature known as herd immunity.

Professor Steve Paterson, Co-Director of the University’s Centre for Genomic Research, said: “We were able to use our DNA sequencing technology at Liverpool to show how just a single mutation in the virus genome allowed them to sweep through the monocultures.”

The conceptual insights from this fundamental research on bacteria and their viruses could have future application in areas such as agriculture and conservation biology.

The paper, The diversity-generating benefits of a prokaryotic adaptive immune system, is published in the journal Nature.