In recent years, concern has grown over the widespread use of antibiotics. Despite what you may think, most of antibiotics are not used to treat diseases in people but are used in livestock, aquaculture and agriculture.

Due to their widespread and continued use, antibiotics end up reaching streams, rivers, lakes … One of the main entry routes for antibiotics to freshwater are treatment plants since they are unable to fully degrade them.

Once they reach ecosystems, antibiotics can have negative effects on living beings and can promote resistance to antibiotics. These are a priority issue of concern due to the consequences they have for human health.

It has been estimated that in 2050 there will be more deaths related to bacteria that carry antibiotic resistance genes than from cancer. The European Union (EU) has already started to take action. In 2018, it established that the presence of certain antibiotics such as amoxicillin, ciprofloxacin, erythromycin, clarithromycin, and azithromycin in freshwater should be monitored and controlled.

Invisible pollutants in water

This recommendation, together with the growing evidence of the presence of microplastics (plastics with a size smaller than 5 millimeters) in most of the planet’s ecosystems, has led researchers from the University of Alcalá and the Autonomous University of Madrid to study the effects that the joint presence of both pollutants: antibiotics and microplastics could be causing in the environment.

Microplastics are found in products that we use in our daily lives, such as toothpaste or exfoliating creams. We pour them down the drain and think they disappear, but they don’t.

Once they reach the treatment plants, a large percentage is eliminated, but the amount we discard is so high that many end up reaching freshwater.

In our study, we have evaluated whether two antibiotics closely monitored by the EU, azithromycin and clarithromycin, adhered to four types of microplastics frequently used in our day-to-day lives, and subsequently became free of them.

There are entry points for pollutants into ecosystems, such as wastewater treatment plants, where microplastics and antibiotics are in high concentrations. In general, the concentration of antibiotics decreases as we move away from the wastewater treatment plant due to the dilution effect. However, if antibiotics stick to microplastics, thanks to their small size and buoyancy, they could be transported to places in ecosystems that they could not reach before.

Microplastics, vectors of antibiotics

The results we have obtained, published in the journal Chemosphere, indicate that the antibiotics azithromycin and clarithromycin can adhere to different types of microplastics, especially polystyrene microplastics, and subsequently be partially released.

There were already previous studies showing that other antibiotics and antimicrobials could adhere to and subsequently free themselves from microplastics. But this is the first research to show that azithromycin and clarithromycin can do it too. This is important as they are two of the most widely used antibiotics today for which there is a European standard. They belong to one of the groups of antibiotics (macrolides) that take the longest to disappear from the environment.

Furthermore, we have studied whether the antibiotics released from microplastics, to which they had previously adhered, were still capable of generating negative effects on living beings. The results show that both antibiotics, after being released from the microplastics, caused damage to a primary producer, microalgae.

Primary producers of freshwater such as microalgae play a key role in the functioning of these ecosystems. They are the base of the food web and, if they are affected, consequences can be generated in the rest of the living beings that are in the higher links of the food chain.

Rational use of plastics and antibiotics

Taken together, the data from this research indicate that microplastics can act as vectors for antibiotics, transporting them to places they previously did not reach. It is the first step; the door is now open to future research on the role of microplastics as transporters of antibiotics between different ecosystems.

More studies are needed to know if this process is occurring beyond the laboratory, in the environment.

We must make rational use of plastics, which are not bad but are made to last, so let’s make them last, and avoid, as far as possible, single-use plastics. That would be a key first step in reducing its presence in the environment. Another is to strictly follow the medical indications for the use of antibiotics prescribed by specialists to avoid negative consequences for us and nature.