Antibiotic-resistant microorganisms in wastewater treatment plants

In September 2024, the prestigious journal Lancet [1] published a comprehensive analysis of drug resistance from 1990 to 2021, with an outlook to 2050. Antibiotic-resistant microorganisms and their development are currently one of the biggest public health challenges.

Drug resistance in numbers

Researchers estimated that in 2021 alone, more than 4.71 million deaths worldwide were linked to bacterial antimicrobial resistance. And in 2050, the annual number of deaths from such infections will exceed 8 million. However, these figures are likely to be severely underestimated due to the gaps that still exist in recording drug-resistant infections. In addition, many parts of the world lack monitoring systems, such as through wastewater, to assess the scale of the future threat. In the previously mentioned publication [1], researchers analyzed mortality data from 204 countries, focusing on 22 pathogens, 84 drug combinations used to treat infections, and 11 diseases, including bloodstream infections and bacterial meningitis.

Researchers have noted the highest increase in antibiotic resistance in the case of methicillin-resistant Staphylococcus aureus bacteria (a narrow-spectrum β-lactam antibiotic). Commonly found in the nasopharyngeal cavity and on the skin, the gram-positive bacterium can cause serious health problems once it enters the bloodstream. In 1990. Drug-resistant Staphylococcus aureus caused more than 261 thousand associated deaths and more than 57 thousand attributed deaths. However, by 2021, these numbers had already risen to 550,000 and more than 130,000, respectively. [1].

Gram-negative bacterial strains are also gaining resistance, most rapidly to carbapenems (broad-spectrum β-lactam antibiotics), widely used for their effectiveness [1]. From 1990 to 2021, the number of deaths associated with infections with gram-negative bacteria that are also resistant to this class of antibiotics rose from 619,000 to more than 1 million per year. And the number of confirmed direct infections increased from 127,000 to 216,000 per year [1].

The threat of drug-resistant infections in a social and geographic context

Although the number of deaths from drug-resistant infections in children is gradually decreasing (between 1990 and 2021, it fell by more than 50 percent in the under-5 age group), at the same time, the number of deaths in those over 70 years of age increased by an average of 80 percent during the analyzed period [1]. The decline in mortality in children is primarily the result of fewer infections with pneumococci and bacteria spread by so-called “dirty hands.

This phenomenon follows the wider availability of pneumococcal vaccines and measures to increase hygiene levels and access to water and sanitation services [2]. However, success in the fight against infections in children has been half-hearted, as at the same time the number of deaths from septicemia has increased in the under-5 group over the period analyzed (by 5 percent between 1990 and 2021), suggesting that over time bacterial infections are also becoming more difficult to treat in the youngest.

Researchers predict that the regions likely to be hardest hit by an antibiotic resistance pandemic in the future are South Asia, Latin America and the Caribbean [1]. But an increase in drug-resistant microbes will occur in all areas of the world, including Western and Central Europe, where such infections have so far been strictly controlled and periodically even managed to reduce the number of fatal cases.

According to the European Center for Disease Prevention and Control, 35,000 people die each year in the European Union from antibiotic-resistant infections, that’s up to 100 people a day [3]. In addition to antibiotic-resistant bacteria, drug-resistant fungi such as Candida auris are of growing concern. In this case, the number of reports doubled between 2020 and 2021. Researchers estimate that without urgent remedial action, 169 million people infected with drug-resistant bacteria will die between 2025 and 2050. The projected burden trends are largely driven by changes in population size (growth, especially in cities) and age structure (aging population) [1, 3].

Antibiotic-resistant microorganisms – where does this problem come from?

The decades between the 1930s and 1960s. In the 1970s. are considered the golden age of antibiotic development due to the number of compounds discovered during this period. The use of antimicrobials in clinical practice is one of the greatest achievements of medicine, which has significantly extended the lives of patients. However, progress has been slowed by the natural flexibility of microorganisms to develop mechanisms that build resistance to environmental stressors [2]. Bacterial evolution is not limited to random mutations in genes, inheritance through offspring and natural selection of appropriate phenotypes.

Bacteria modulate their own rate of DNA mutation after exposure to environmental stressors and are able to transfer genetic material among themselves. Bacterial genomes contain different types of mobile elements that allow DNA to move between chromosomal locations in a single cell and between different bacterial cells. Bacteria can associate with each other, or “conjugate,” allowing genetic elements to be transferred between each other. So we can say that bacterial DNA is extremely mobile, alive and flexible.

Horizontal (horizontal) gene transfer is one of the most well-known and widespread mechanisms among bacterial populations for gene transfer and enrichment of new traits [2, 4]. Microorganisms rapidly transfer resistance genes between strains, with the environment in which they are found playing a special role. Studies show that one of the more preferred environments for microbial evolution is human organisms [2].

Human microbiomes are complex ecosystems including bacteria, viruses, archaeons or eukaryotes that co-evolve in an environment subject to various selection pressures, such as antibiotic intake, contact with pollutants, disease and lifestyle. In the human body, commensal bacteria – non-pathogenic microorganisms found on the mucous membranes of the gastrointestinal, respiratory and urogenital tracts in mammals, including humans, among others – play an important role [4].

They provide essential nutrients to the host and help protect it from opportunistic pathogens, but they are also important factors in the antibiotic resistance gene pool. There is evidence of antibiotic resistance gene transfer between commensal bacteria and bacterial pathogens in the human gut. For example, Escherichia coli , commonly found in our intestines, has been well studied as an indicator of the surveillance and spread of acquired antibiotic resistance genes among pathogens in the environment [4-6].

The gut microbiome contains a wide range of resistance genes, but studies mostly distinguish resistance against tetracycline, β-lactams, aminoglycosides and glycopeptides [5]. Some antibiotic resistance genes present in healthy humans appear, already in infancy, because they can be passed on from the mother’s body. Among other things, ampicillin- and cotrimoxazole-resistant E. coli bacteria have been isolated from the fecal microbiota of newborns less than one month old who were not receiving antibiotic therapy [2].

There are various driving factors and trigger points contributing to the development and spread of drug resistance. Antibiotics are not only abused in human therapy, but also in food production. They are used in agriculture to combat bacteria that attack plants, aquaculture and livestock production. Scientific evidence supports a close link between the development of antibiotic resistance in human pathogens and the use of antibiotics in agricultural production.

Animal/plant foods and organic fertilizers can serve as important vectors for gene migration [7, 8]. Bacteria can colonize plants, animals, humans and the environment and can easily move between these elements to transfer genes. Many studies emphasize the importance of the aquatic environment in the transmission of antibiotic-resistant microorganisms, particularly surface water resources that are exposed to sewage inflows and surface runoff, such as from agricultural areas [9]. Also, wastewater treatment plants are considered “hot spots” for the transmission and spread of antibiotic resistance genes [10].

Importance of wastewater treatment plants in the development of antibiotic resistance

In addition to the fact that some of the microorganisms regularly excreted from the human body may possess as well as carry antibiotic resistance genes, wastewater also has unique characteristics conducive to initiating further gene migration. They are increasingly recognized as a potential source of new resistance genes due to the constant supply of low doses of antibiotics, the extreme genetic diversity of microbes and contact with the environment. Researchers examined DNA from thousands of samples taken from different environments to find those traits that link antibiotic-resistant microbes. Wastewater turned out to be a place where drug-resistant bacteria are more common than in any other environment [11].

Wastewater treatment includes mechanical, physical, biological and chemical processes that affect the fate of pharmaceuticals and microorganisms. While the treatment process itself is expected to reduce the number of bacteria in wastewater, especially pathogenic ones, it can simultaneously contribute to an increase in their resistance [12]. Studies show an increase, up to several percent, in the number of antibiotic-resistant bacteria of the genera Enterococcus, Acinetobacter, Bacillus, Mycobacterium, Nocardiopsis in treated wastewater [12]. Personal care products, disinfectants, and heavy metals are commonly present in wastewater, which promotes the building of bacterial resistance.

Antibiotics can undergo hydrolysis, degradation and adsorption processes on sludge – organic and inorganic particles [13]. However, they are not removed sufficiently in wastewater treatment plants. Most substances are non-volatile due to their high molecular weight. The efficiency of conventional processes (mechanical-biological treatment using activated sludge) ranges from 50 to 80 percent, depending on the technologies used, operational parameters and physicochemical properties of the drugs. Typically, treatment processes are focused on the efficient removal of nitrogen compounds, phosphorus, suspended solids and selected organic substances with high biodegradability.

Macrolides (e.g., erythromycin), sulfonamides, trimethoprim and quinolones are found in the highest concentrations in wastewater [12, 16]. Studies have shown that they are resistant to biodegradation and therefore also difficult to remove in biological wastewater treatment chambers. Both wastewater discharges and their use, such as for crop irrigation, are potential pathways for antibiotic-resistant genes and microorganisms, as well as antibiotics themselves, to enter the environment [12, 14]. Studies show that both antibiotic and antibiotic-resistant gene levels decrease with increasing distance from wastewater discharge [15, 16].

However, microorganisms can adhere to organic and inorganic sludge particles, so they persist in the environment and are subject to further transport. The presence of antibiotic-resistant genes in aerosols near wastewater treatment plants has also been confirmed, posing an additional threat to the operation of the facilities [15-17].

Antibiotic resistance – available medical support

Studies have confirmed a higher abundance of microorganisms and antibiotic resistance genes in wastewater in winter, which allows the problem to be partially linked to an increase in seasonal infections [17]. Therefore, one of the main recommendations for curbing the spread of the problem is balanced surveillance of antibiotic consumption in humans and ongoing education. Antibiotics cannot be used to treat viral infections. In addition, antibiotics are still sold over-the-counter in some countries, such as India, which encourages their overconsumption.

The search for new antimicrobial drugs is a growing challenge, so an urgent expansion of vaccination coverage is needed. According to WHO data, the introduction of vaccines against the seven most important resistant bacteria could save an estimated 1.2 million people from death each year. And antibiotic use would drop by 2.5 billion doses per year. Antibiotics don’t work against viruses, but viral infections can cause complications in the form of bacterial infections. The WHO estimates that better use of vaccines would reduce the incidence of infections because vaccinated people are more resistant to infections and secondary infections requiring antibiotic treatment [18].

During the COVID-19 pandemic, the use of antibiotics increased dramatically. Initially fighting an unknown adversary, they were used not only for bacterial and fungal infections that followed or were a complication of the virus, but also in many unwarranted cases. In addition, patients, in fear of worsening their condition, chose to use antibiotics without medical supervision. Both the viruses associated with Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) are accompanied by bacterial co-infections, for which the use of antibiotics is essential. However, their use should always be under medical supervision.

Most of the deaths during the 1918 influenza pandemic were caused by secondary bacterial infections, mainly due to Streptococcus pneumoniae (the pneumococcus we commonly know). During the 2009 H1N1 influenza pandemic, it is estimated that. 29-55 percent of global mortality was associated with secondary bacterial co-infections, in which antimicrobial resistance accounted for a clinically significant percentage. Patient mortality from penicillin-resistant Streptococcus pneumoniae accounted for about 1.8 percent, and erythromycin resistance contributed to the deaths of more than 2 percent of patients [19, 20].

This may be considered a small percentage, but we must take into account that reporting of drug-resistant infections has been conducted for a short time and data from this period is not fully available. Researchers predict a three- or even fourfold increase in deaths due to more frequent pneumococcal co-infections in future flu pandemics. A recent WHO report states that pneumococcal vaccination, covering 90 percent of the world’s child population, could reduce antibiotic use by 33 million doses by 2030. [18]. Pnemococci are responsible for the highest mortality rate among bacterial infections.

Children and people over 65 are most susceptible to them. Thus, it can be concluded that pneumococcal vaccination not only protects our health, but also reduces the use of antibiotics, which enter the water environment and promote the development of antibiotic resistance genes among microorganisms. Besides, studies have shown that the widespread use of non-steroidal anti-inflammatory drugs, such as , ibuprofen, naproxen, diclofenac, gemfibrozil and the β-blocker propranolol, increases natural transformation in bacterial populations, and thus increases the uptake of exogenous antibiotic resistance gene in bacteria. In an aging society, there will be increasing challenges to medical care. However, prevention can make a significant contribution to reducing the number of drug-resistant infections that end in death.

Antibiotic resistance – the importance of water and wastewater management

There is a strong link between antibiotic resistance and access to safe drinking-water, sanitation and hygiene (WASH) [21]. Studies have shown that better access to water and sanitation facilities and infrastructure was associated with a reduction in antibiotic resistance genes, particularly in urban areas. According to a recent UNICEF Report, to achieve universal access to safe drinking water, the pace of efforts to expand water and sanitation infrastructure, among other things, would have to increase sixfold [22].

No region of the world is currently on track to meet this goal by 2030. In addition, some 3.5 billion people still lack access to safely managed sanitation services, and more than 419 million must take care of their physiological needs outdoors on a daily basis. A fivefold increase in efforts to improve the availability and quality of sanitation services is needed to meet the goal of universal access to toilets by 2030, set by the UN Sustainable Development Goals [22].

Increasing access to water and improving sanitation can be an effective strategy to reduce the spread of antibiotic resistance in low- and middle-income countries. Moving from 0 to 100 percent in access to sanitation and water and wastewater infrastructure can help reduce the abundance of antibiotic resistance genes in the environment by up to 95 percent. [21].

Education, better management of antimicrobial use in humans and animals, and expanded monitoring of municipal wastewater are currently the main strategies for combating antibiotic resistance. But no less important is the inclusion of expansion and modernization of water and wastewater infrastructure. In middle- and high-income countries, investment and modernization are an important part of climate change adaptation, which will strongly affect the efficiency of water and wastewater management.

Floods, overflows, flooding and heavy rainfall promote the entry of microorganisms into the environment, including pathogenic and drug-resistant ones. Studies show that tertiary treatment techniques, such as ozonation, adsorption on activated carbon, coagulation, and nanofiltration and reverse osmosis, under optimal conditions, are effective in removing antibiotics and antibiotic resistance genes [22].

Disinfectants such as chlorine tend to act selectively on antibiotic resistance genes, reducing gene abundance (the number of gene copies per ml of sample), while the frequency of these genes (the number of gene copies per total number of bacteria) increases [23]. Sometimes disinfection processes destroy the DNA or cellular structure of bacteria, but drug resistance genes can persist for a long time in cellular debris and the environment. Sometimes they transfer and adapt to new bacteria, further generating the development of antibiotic resistance. Damaged bacteria have low metabolic activity, which becomes active under certain conditions.

They receive a large amount of DNA released from surrounding susceptible bacteria, making horizontal transfer occur more frequently. In addition, when E. coli, for example, is exposed frequently to low doses of chlorine, it induces a specific set of proteins, making it less susceptible to disinfection. Chemical disinfection methods also carry the risk of transforming some of the chemical compounds that have not been biodegraded in wastewater treatment processes, which can increase the toxicity of wastewater and cause further negative impacts in the environment. Therefore, it is necessary to increase the number of facilities with three-stage wastewater treatment systems [13, 21, 22].

Does the changing climate increase the risk of antibiotic resistance?

There are also a growing number of reports on the impact of climate change on the rise of drug resistance in microorganisms. Globally, antimicrobial resistance and climate change are recognized as two major threats to public health [23-25]. In addition, the two can be linked in a web of current environmental problems. Environmental degradation, deforestation, loss of biodiversity and climate change are increasingly causing human and animal pathogens to intermingle, leading to outbreaks of zoonotic diseases.

Climate change is a significant factor that will drive the increase in infectious disease incidence in the coming decades [24]. Although bacterial resistance develops largely under selective antibiotic pressure, there are other factors that may also play a role at the population level. Among other things, researchers have linked local temperature increases to increased antibiotic resistance in common pathogens such as Escherichia coli, Klebsiella pneumoniae and Staphylococcus aureus. These links apply to most classes of antibiotics and pathogens, and importantly may increase over time with climate pressure [25].

Dr.-Ing. Edyta Łaskawiec – water and wastewater technologist, science popularizer, author of educational profile on Instagram platform: wastewater_based.doctor and podcast About Wastewater. Winner of the POP SCIENCE Competition for Science Popularizers of the Silesian Science Festival Katowice 2024.

In the article, I used, among others. z:

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