Extreme heatwaves disrupt life in soil. Soil bacteria that produce and resist antibiotics switch into survival mode, temporarily reducing the number of resistance genes. At the same time, heat and drought are causing significant damage to soils and ecosystems, especially in cold, wet regions such as the Netherlands. This is evident from a large-scale European study by soil ecologist Franciska de Vries (University of Amsterdam) and colleagues.
According to De Vries, many people are unaware that soils form an important reservoir of both antibiotics - which we can extract - and resistance genes. 'In the soil, there is a kind of "war" going on: bacteria both produce antibiotics and arm themselves against them with resistance genes,' she explains.
At warm temperatures and in wetter conditions, the number of resistance genes increases. Bacteria then become insensitive to antibiotics and can pass this trait on to other bacteria. It becomes problematic when those genes end up in pathogenic bacteria that infect humans or animals.
It has often been assumed that climate change accelerates the spread of resistance genes. But what happens during extreme weather conditions such as heat, drought and floods that occur not slowly, but suddenly?
Soils from Across Europe
''Before we got to that specific question, we first wanted to understand in general how soil organisms respond to extreme weather conditions," says De Vries. "Soils, for example, are very important for sequestering CO2. What happens to this function during extreme weather?"
With her colleagues, she collected soils from all over Europe:"From Iceland to Greece and from Sweden to Spain and even the Russian steppe." The researchers exposed these soils to extreme conditions in climate chambers: severe drought, flooding, alternating frost and thaw, and a short but intense heatwave.
Hot Soils can Handle More Than Wet, Cold Soils
The researchers turned out to be very good at predicting how certain soils react to extremes. "Quite logically, hot soils can withstand heat better than cold and wet soils like those we have in Sweden, Iceland and the Netherlands," says De Vries.
"Soils from warm, dry countries are already accustomed to heat and drought," she adds. "The soil organisms there remain relatively stable when the weather becomes extreme. In cold, wet soils, bacteria and fungi are disrupted much more quickly by heat and drought. Those soils then lose their functions, such as CO2 storage, more easily. And it is primarily these areas that are warming up significantly."
What do Climate Extremes do to Resistance Genes?
In a new collaboration and using new DNA techniques, the question subsequently arose as to what effect these extreme conditions have on the type and quantity of resistance genes in soils. And whether this is also accurately predictable.
Droughts, floods and freeze-thaw periods were shown to cause only mild changes in the resistance repertoire. Moreover, these responses were reasonably predictable based on climate and soil type.
Heatwaves, however, stood out. The reactions that arose from them were the least predictable, and bacteria really struggled. "They exchange fewer resistance genes and produce fewer antibiotics," says De Vries. "That was very surprising: just like plants, they also go into survival mode. And after such an extreme, those genes don't grow back quickly either."
Location Proves Decisive
Geography proved to be the most important indicator. "Where the soil comes from is more important than what extreme weather is unleashed upon it," says De Vries. "Wet soils find it much harder to cope with drought and heat, whereas we saw much larger quantities of resistance genes in countries like Greece and Spain."
Not Necessarily Good News
At first glance, it could appear to be good news that heatwaves temporarily reduce the amount of resistance genes in the soil. However, De Vries warns, the heat and droughts are also causing significant damage to soils and ecosystems. The temporary dip in resistance therefore does not mean that the health risks from antibiotic resistance disappear.
"We now know that climate extremes do not always affect antibiotic resistance in the same way," De Vries concludes. "That knowledge helps us more easily predict where the real risks lie and to tailor policy and monitoring specifically to particular soils and regions."