The waters that evacuate the cities carry new pollutants whose impact is still unknown. One of these emerging contaminants are bacteria that are resistant to antibiotics. A comparative study of a dozen wastewater treatment plants in several European countries now shows that the wastewater reaching the plants in southern Europe carries a greater abundance of genes involved in the development of drug resistance. The result confirms the greatest antibiotic consumption in the south and converts the sewage treatment plants into an effective antibacterial surveillance system.
A consortium of European scientists has sampled three campaigns in 12 wastewater treatment plants in several countries, including Spain. They analyzed the waters both when entering the treatment plant and when leaving. They looked for pathogenic bacteria like Escherichia coli, the Klebsiella pneumonia or the Staphylococcus aureus. They stopped in particular to detect the presence of 229 genes known to intervene in the development of resistance and other 25 mobile genetic elements that could facilitate the spread of that resistance to other bacteria of the same or another species. It is necessary to clarify that the outgoing water was completely apt for the consumption, with levels of normal microorganisms. What interested them was the presence of resistance to antibiotics.
The work, published in the magazine Science Advances, shows that all wastewater reaches the treatment plants with resistant bacterial material. The plants function as true collectors of the urban microbiome and testify to the arms race unleashed between antibiotics and pathogenic bacteria. But the study configures two groups of countries. On the one hand, Germany, Norway and Finland with a relative abundance of genes of bacterial resistance significantly lower than that of the other group, the one formed by Portugal, Cyprus and Spain.
Wastewater contains bacterial resistance genes to several families of antibiotics
All the samples analyzed contained genes of bacterial resistance to various types of antibiotics, such as aminoglycosides (streptomycin), beta-lactams (penicillins or cephalosporins), sulphonamides or tetracyclines. In the majority they also detected multiresistant material, capable of surviving the action of more than one group of antibiotics. Except in the case of tetracyclines, resistance to the other drug families was more abundant in the waters to be treated in the southern countries.
"The European Center for Disease Prevention and Control [ECDC por sus siglas en inglés] already noted the difference between the north and the south in clinical infections. We have investigated if the same thing happened in the urban waste treatment plants. And indeed, that's the way it is, "says the researcher at the Portuguese Catholic University of Porto and senior author of the study, Célia Manaia. ECDC reports are based on data obtained from hospital samples, this work shows the ecosystem of bacterial resistance of the inhabitants of a whole city, what would be its urban resistoma.
The differences between the north and the south also reflect the highest consumption of antibiotics which has been detected in most of the countries of the south (and also of the east) of Europe. The emergence of resistance is an adaptive response of bacteria: to more antibiotics, greater development of bacterial resistance. However, the abuse of antibiotics is not the only explanation for the greater bacterial resistance in the south. To this we should add the difference in temperature: human pathogens do good human body temperature, so they proliferate above 30 degrees, an extreme that does not occur very often in northern Europe.
The researcher of the Catalan Institute of Water Research and co-author of the study (ICRA, in its acronym in Catalan), Sara Rodríguez-Mozaz., Comments that "the treatment plants are a reflection of our state of health". And as such, they can serve as sentinels. Focused on the study of emerging pollutants (such as drugs and endocrine disruptors), he worked with samples of the Spanish plant included in the study, which can not reveal its location, although it is a Catalan coastal city. For Rodríguez-Mozaz, the genes of resistant bacteria are an emerging pollutant of which "very little is known about their impacts on the environment and human health," he says.
Among the possible risks are the propagation of a resistance determined either by genetic recombination or by horizontal transfer to other bacteria of the same or another species. It is known, for example, that microscopic algae that green rivers and lakes (cyanobacteria) are very sensitive to antibiotics. But it is unknown what will happen to them if they develop resistances. It is also not clear until which points of the trophic chain resistant bacteria can reach: soils, vegetation, irrigation water, animals or humans.
There are not yet established maximum levels of bacterial resistance in drinking water
Fortunately, once treated, the water leaves the plants with a tiny part of its bacterial load. The purifiers here do their job well, but the problem of bacterial resistance is almost qualitative. The work shows that, although in smaller quantity, the salient waters still contained bacterial material resistant to a good part of the antibiotics.
"You can eliminate pathogenic bacteria but not necessarily all the genes of bacterial resistance and understand which is the most effective system to eliminate these genes can be important", highlights the researcher of the National Center of Biotechnology (CNB-CSIC) and co-author of the study, José Luis Martínez. In fact, it has been argued that on occasions, the plants themselves could favor the appearance of resistances. "The processes used, such as ozonation or ultraviolet radiation could cause the SOS response in bacteria[[to repair damaged DNA], favoring recombination and transfer of resistance. However, this is an issue that is still under study and what is clear from our work is that the treatment plants, as a whole, are very effective in eliminating the bacterial pathogens and the resistance genes they carry, "recalls Martínez.
For both the CNB microbiologist and the ICRA researcher, the work shows how sewage treatment plants can become the basis of a global surveillance system for bacterial resistance. But they also emphasize that it is urgent to establish maximum levels of their presence in water as they exist for other pollutants.