Bacteria are omnipresent on Earth, found in the land, air, and sea, as well as in the guts of almost every animal. While some of these bacteria are beneficial, others are simply neutral, and some are harmful. It is well-known that bacteria have a tendency to evolve resistance to antibiotics used to kill them. Recently, scientists have uncovered something even more disconcerting: bacteria may be able to spread this resistance by hitchhiking around the world via clouds.
To understand how this happens, it is important to know how antibiotic resistance generally spreads. For example, when exposed to quinolones (a class of antibiotics which interrupts bacteria’s ability to copy its own DNA), some bacteria may survive due to genetic mutations which prevent the quinolone molecule from sticking where it needs to, or stop the bacteria from taking up the antibiotic in the first place. This superpower is written into the bacterium’s DNA in the form of an antibiotic resistance gene, and after surviving its encounter with the antibiotic, the bacterium can replicate and create a population of antibiotic-resistant bacteria.
Additionally, bacteria can share their resistance genes with other bacteria through conjugation (extending a tube between two bacteria and transferring the plasmid DNA), transformation (picking up plasmids in the environment left behind by dead bacteria), and transduction (transferring genes indirectly from one bacteria to another via a viral infection).
When antibiotics are present in the environment, bacteria can build up resistance without us even knowing it, which is very concerning according to the World Health Organization. It is thus essential to understand how antibiotic resistance can travel around the world, including via clouds.
Bacteria can be lifted into the atmosphere by a breeze or human activity, and when they get high enough, they can get incorporated into clouds. This allows them to hitchhike for long periods of time, as evidenced by the Puy de Dôme, a dormant volcano in central France. Between 2019 and 2021, a group of researchers went to the 1,500-meter-high summit to collect samples of clouds and the bacteria swimming inside. First, the team counted up individual bacterial cells in their cloud samples, which varied widely in how many microbes they contained, with one milliliter of cloud water holding anywhere from 330 to over 30,000 bacteria. Less than half of those were probably alive and kicking, but since DNA can spread through the environment, it was important to count them all. Then, they analyzed the bacterial DNA, hunting for 33 known antibiotic resistance genes. After sampling just 12 clouds, they found 29 of them. One of the most common genes they found provides resistance to quinolones and also makes bacteria better at surviving stressful environments, such as the cold, wet, wasteland inside a cloud.
This means that clouds could be a legitimate transportation system for even more antibiotic resistance genes, which could be spread internationally. The researchers estimated that globally, roughly 2.2 septillion copies of antibiotic resistance genes get rained, snowed, or otherwise precipitated back to Earth’s surface each year. This doesn’t sound great, but now that we know just how legitimate clouds are as a way for antibiotic resistant bacteria to spread their genetic knowledge, we can develop better ways to stop antibiotic resistance before it takes to the skies.
If you’d like to take a closer look at what’s inside clouds, you can order a microscope from our sibling channel, Journey to the Microcosmos. Head over to microcosmos.store to get your own microscope and have a blast looking at all the tiny stuff living right under our noses.