The ability of human pathogens to develop resistance to antibiotics poses a serious threat to public health, but this is probably not news to you.

Even people without recent antibiotic use have higher antibiotic-resistant strains of pathogens in countries with high antibiotic use than healthy people in countries with comparatively lower antibiotic use.

Recent research from the Earlham Institute and Quadram Institute in the UK in conjunction with an international team of scientists suggests that any and all of the bacteria and other microorganisms that set up home in and on human bodies may be a source of antibiotic resistance.

And you might be surprised to learn that antibiotic use among the people around you appears to affect you directly.

"Even a healthy individual, who hasn't taken antibiotics recently, is constantly bombarded by microbes from people or pets they interact with that can lead to resistance genes becoming embedded in their own microbiota," says Earlham Institute and Quadram Institute microbial ecologist Chris Quince.

"If they exist in a population with a heavy burden of antibiotic consumption, that leads to more resistance genes in their microbiome."

When antibiotics are used to fight infection, they have the unfortunate side effect of damaging the host microbiome and also increasing the prevalence of resistance genes being passed among different microbiome strains.

Studies on the topic of antimicrobial resistance (AMR) have concentrated mostly on the degree of resistance seen in the microbes that cause infections in humans. But microbes are ubiquitous in the human body, and the vast majority are harmless, and many are beneficial.

Due to the ease with which antibiotic-resistant genes are passed on, the use of antibiotics can have an effect on the entire microbiome, not just the pathogens that are supposed to be killed. The resistome – the collection of resistance genes found in commensal microbiota – is an important topic for research as a result.

"Our bodies are continually importing and exporting microbes and pathogen strains," Quince explains.

"These strains are themselves passing genes back and forth, which means the challenge of AMR has to be tackled at both the micro and macro level."

While the entire microbiome plays a part in health and disease, the gut microbiome is especially important with functions in digestion and the formation of the immune system. The digestive tract of a human is a veritable paradise for millions of different kinds of microbes, housing more of them than anywhere else in the body.

So the study authors speculated that microbes living in the gut could be a significant contributor to antimicrobial resistance.

The group analyzed 3,096 gut microbiome samples from people in 14 different countries to learn more about the effects of antimicrobials on the gut microbiome, with a focus on samples from people who weren't likely to be taking antibiotics so as to get a 'clean' gene profile of the gut microbiome.

In order to learn how AMR genes are transferred from microbe to pathogen, researchers made careful comparisons between the data from the samples and the Comprehensive Antibiotic Resistance Database, a public health resource where resistance genes are documented.

Prior research already established that there were notable differences in antimicrobial resistance gene (ARG) profiles across different countries with enough data available at the time.

And this time, by studying people who were not taking antibiotics, the team uncovered two distinct trends.

They found two clusters, or resistome profiles, of ARG types in the gut microbiome, and they found that median total ARG abundance and diversity varied based on the level of antibiotic consumption in that country.

"We found that, in countries where antibiotics are taken more regularly, their populations also have higher numbers of resistance genes in their gut microbiome," says Quince.

The connections were remarkably strong. Thus, widespread antibiotic use may cause ARGs to become prevalent throughout the population, not just among those directly exposed, and these ARGs may spread throughout the microbiota.

The researchers suggest further studies could focus on investigating this in more countries and on public health strategies.

According to research, antimicrobial resistance kills thousands of Europeans every year and is only going to get worse in the future. Alarmingly, the third leading cause of death worldwide in 2019 was AMR, which was responsible for 4.95 million deaths.

"This study is so important because it can, for the first time, quantify the impact national antibiotic usage has on our commensal bacteria, as well as giving us insights into the common types of resistance we can expect to evolve," says Earlham Institute and Quadram Institute microbial bioinformatician Falk Hildebrand.

The research has been published in Nature Communications.