Microevolution key to Salmonella’s success

Research led by Dr Rob Kingsley from the Institute of Food Research has shown how Salmonella rapidly “microevolves” during an epidemic. It highlights how whole genome sequencing is important in tracking pathogenic bacteria.      Rob Kingsley

Salmonella are one of the most common causes of bacterial disease in humans and animals. Part of their success comes from their ability to adapt to environmental changes. To better understand how this happens, the researchers looked at how the genomes of Salmonella associated with an epidemic that emerged in 2005 to find clues as to what made this epidemic successful, and also understand the drivers of this “microevolution.”

They found that the Salmonella related to the epidemic had acquired genes increasing its resistance to heavy metals such as zinc and copper. Heavy metals are used as a livestock feed supplement to promote growth and as an antimicrobial, as an alternative to antibiotics, whose use for this purpose was banned in 2005.

The acquisition of genes involved in resistance mechanisms may help this form of Salmonella outcompete other bacteria in environments containing heavy metals – Dr Rob Kingsley

But this wasn’t the only trick Salmonella had up its sleeve. Between 2005 and 2012, the epidemic-associated clones further evolved other adaptations to help them maintain a foothold in their animal host population – amassing a surprising amount of variation in a short time. Many strains lost genes needed for the flagella, the hair-like appendages used for movement.

The strains also picked up a significant weapon – a gene called sopE that allows the bacteria to make a toxin it can inject into host cells to help it invade. This also triggers diarrhoea, which helps Salmonella transmit from host to host.

This remarkable ability to rapidly adapt, or microevolve, is important to consider for surveillance of these bacteria in the food chain and in the environment. The adaptations, which are likely to have contributed to the success of the epidemic, are only picked up when the whole genome is sequenced. Relying on the tracking of a single characteristic, such as the flagella, isn’t a reliable to way to keep tabs on these resourceful pathogens. And by understanding this microevolution, we can be better placed to combat the specific threats and help keep these dangerous bacteria out of the food chain.

Read more on the Gut Health and Food Safety blog

Reference: Microevolution of Monophasic Salmonella Typhimurium during Epidemic, United Kingdom, 2005–2010 Emerging Infectious Diseases. doi: 10.3201/eid2204.150531

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