Antibiotics are bactericidal and bacteriostatic agents used to treat bacterial infections, thereby providing a solution in the treatment of many diseases. Antimicrobial substances are also used for non-medical purposes, such as livestock, poultry, and fish growth stimulation. Remarkably, more antibiotics are used in the US for animal growth promotion than in human medicine
[1]. Approximately 24.6 million pounds of antibiotics are used each year in livestock farming
[2]. Animal husbandry utilizes more antibiotics than human therapeutic applications of these drugs. Importantly, the increased consumption of antibiotics has led to the exposure of aquatic ecosystems to contamination with these substances. This is because most antibiotics are only partially metabolized by the target organism; therefore, their residues (30–90% of ingested antibiotic doses) are excreted in urine and feces to reach wastewater treatment plants (WWTPs)
[3]. Wastewater contaminated with antibiotics undergoes treatment in treatment plants but complete removal of these compounds is impossible in conventional systems
[4]. WWTPs are designed to reduce the pollutant load in the majority of urban and rural wastewater, but they are not effective in reducing the loads of antibiotics and antibiotic resistance genes (ARGs)
[5]. Furthermore, WWTP effluents are discharged into surface water and the sludge can be used as manure fertilizer. Antibiotics are a problem around the world due to their frequent widespread use in large amounts, and they often become an inappropriate therapeutic pathway. Antibiotics are regarded as “pseudo-persistent” contaminants because of their continuous introduction into ecosystems—their entry rate into the environment is higher than their rate of elimination
[4].
2. Sources of Antibiotics in Mountain Rivers
The quality of water in mountain regions is shaped by many natural and anthropogenic factors. In the most pristine regions, including national parks and natural valuable protected areas, the natural factors include variable weather conditions, surface runoff, soil leaching, and snowmelt water [6][7]. Along the course of rivers, water pollution increases, which is strongly related to the influence of anthropopressure-related factors, including illegal discharge of sewage from households (human and animals feces), surface runoff carrying natural fertilizers from agricultural fields, and wastewater inflow from WWTPs increased by tourist traffic-related sewage inflow [6][8].
In many cases, anthropogenic pressure in mountain regions is related to tourist traffic contributing to increased wastewater inflow, thus contributing to the presence of antibiotics and bacterial contaminants in mountain rivers [9]. Constantly developing winter tourism can pollute the environment and affect the ecology of microorganisms in mountain aquatic ecosystems in various ways. Mountain hiking is one of the causes of the pollution of rivers with antimicrobial agents. Mountain areas attract tourists due to their unspoiled nature, hiking trails for mountain trekking, and areas for active recreation. One of the reasons for the growing number of tourists is the constant development of winter sports centers with snow-covered ski slopes, which also offer activities outside the winter season, such as downhill skiing, as well as facilities offering thermal pools with geothermal water (thermal spas).
The metabolism rate of these compounds varies for humans and animals depending on the antibiotic class. These pharmaceuticals may reach aquatic environments by effluents from WWTPs to rivers and groundwater, as well as leachate from unsealed sewage systems and manure and/or sewage storage tanks [10]. During the wastewater treatment process, bacteria are continuously mixed with sub-inhibitory concentrations of antibiotics, which creates suitable conditions for the development of drug resistance, and then released with antibiotic residues into water environments [11]. The removal efficiency of antibiotics and antibiotic resistance determinants from wastewater by conventional WWTPs is insufficient. WWTPs are not specifically designed to completely reduce levels of antibiotics and ARGs. Removal of different antibiotics occurs in different steps of the wastewater treatment process, and the effectiveness of their removal varies among antibiotics [5]. Removal efficiency depends on the physical and chemical properties of antibiotics and on the treatment process conditions.
Another significant source of antimicrobials in rivers, including mountain rivers, is their use in animal husbandry for therapeutic and preventive purposes. Mountain agriculture is dominated by livestock production based on grazing. Veterinary antibiotic usage is related to the treatment of infective diseases in animals. The use of antimicrobial agents in animal husbandry ensures the welfare and health of the animals. However, the use of antibiotics is extended to the whole livestock flock in order to limit pathogen spread, thus uninfected animals also take doses of antibiotics [12]. This is referred to as metaphylaxis—short-term antibiotic treatment of animal groups without disease symptoms that had contact with infected animals [13]. This action involves observation of a livestock flock and administration of high doses of antibiotics before clinical symptoms occur in order to counteract the effects of infection. In contrast, antibiotics can also be used for disease prevention (prophylaxis). This includes antibiotic administration in water and food for farm animals in low doses for longer periods of time. During this period, the risk of infection still exists [14]. Metaphylaxis and prophylaxis are common practices in livestock and poultry production to prevent whole livestock mortality and minimize losses, but they have boosted antibiotic consumption. From an epidemiological point of view, the preventive administration of antibiotics increases the risk of drug-resistant bacteria development in the livestock herd and significantly influences the contamination of the environment with antibiotics and drug resistance determinants. In addition, selection for antibiotic-resistant strains can be widespread in the environment via animal feces, thereby enhancing environmental drug resistance [15]. Residues of antibiotics and ARB are usually found in livestock and poultry manure and in waste from livestock companies, resulting in persistent environmental pollution [16]. Animal manure studies have proven the presence of various classes of antibiotics excreted in feces, for instance: enrofloxacin in broiler chicken feces (74% of orally applied enrofloxacin was excreted as the parent compound) [17], oxytetracyclines present in dairy cow feces (20% of injected oxytetracycline was detected in manure samples) [18], and sulfonamides in pig excreta (excretions of four sulfonamides reached 36–87%) [19]. Stored animal manure often reaches soil and surface water with runoff water after rain or due to leaks in manure tanks. Livestock manure is also used as a fertilizer to enrich the soil before growing crops. Mountain areas, in addition to their environmental and cultural functions, also have an agricultural function as they have abundant arable fields, meadows, and pastures. In sustainable and organic farming, the use of manure as a source of organic matter to improve soil quality is a common practice. However, manure is also a source of antibiotic residues, which can adsorb on soil particles, enter plant tissues, and end up in the food chain. There is a risk of enhanced antimicrobial resistance as a result of consumption of vegetables grown on manure [20][21]. Manure widely applied to agricultural lands as fertilizer has enriched the abundance of some ARGs (ermA, ermB, blaOXA-1, qnrS, and oqxA) in agricultural soil [22]. Antibiotic residues and drug resistance determinants in soil fertilized with manure enter rivers with surface runoff, thus polluting the aquatic environment. Active forms of antibiotics occurring in manure can act as a selective pressure and contribute to dissemination of antimicrobial resistance. Livestock animals are a constant link in the spread of ARGs and antibiotics in the aquatic environment because they are continuously exposed to large amounts of antibiotics. Livestock farming can be one of the main sources of antibiotics in rivers due to the excretion of incompletely metabolized antibiotics in animal feces and their further dissemination into the environment [16].
Mountain rivers provide water for the production of artificial snow to ensure snow cover on ski slopes in the winter season and for irrigation of green areas in the summer season. Mountain river water is also used by households to irrigate their crops. The use of water contaminated with antibiotics, drug resistance genes, or antibiotic-resistant bacteria results in further transmission of these micropollutants into the environment, thereby increasing the risk of spreading drug resistance and endangering public health (Figure 1).
Figure 1. Dissemination routes of antibiotics and drug resistance determinants in the mountain environment.
Another essential route of further transmission of antibiotics in the mountain environment is the irrigation of fields with antibiotic-contaminated water
[23]. Irrigating crops and green areas with antibiotic-contaminated water leads to crop contamination and dissemination of drug resistance genes
[24]. Antibiotics from irrigation water can accumulate in the edible parts of plants or grasses on which livestock feed. Plants irrigated with antibiotic-contaminated water increase the threat of adaptive resistance selection of the gut microbiome. The amounts of antibiotics found in the environment are considered as trace contaminants, nevertheless, they have a very significant impact on the environment
[9]. Although the concentrations of antibiotic residues in water environments range from ng/L to μg/L
[25], the continuous discharge and persistence of these contaminants at sub-inhibitory concentrations may cause changes in bacterial communities and stimulate the development of drug resistance. The transference of drug resistance genes from environmental bacterial strains to human pathogens is a major threat to public health. Water-polluting antibiotics cause the development of antimicrobial resistance among microorganisms, hence their presence in the environment is of critical importance to public health.
3. Stability of Antimicrobial Agents in Water Environments
Antibiotic degradation rates are important for predicting their environmental exposure and impact on bacterial populations. Antibiotics dissolved in water undergo physicochemical modifications caused by biotic and abiotic factors that affect their structural stability. The following processes affect the stability of antibiotics in surface water: hydrolysis, photolysis, sorption, and biological degradation
[26][27][28][29][30]. The occurrence of these processes depends on environmental conditions such as sunlight, water temperature, the abundance of microorganisms, water chemical composition, sediment properties, and organic matter content. Predicting the degradation pathways of antibiotics is essential for assessing their fate in the environment. The natural degradation pathways of antibiotics in water environments are presented in
Figure 2.
Figure 2. Natural degradation pathways of antibiotics in water environments.
Photocatalytic degradation and hydrolysis are two of the main abiotic pathways of antibiotic degradation in aquatic environments
[26][27][28]. The degradation of antibiotics in water depends on the pH value and temperature, which are the most important parameters affecting hydrolysis rates. These rates typically increase when the temperature increases. Additionally, aqueous compounds such as metals and organic matter can catalyze the hydrolysis reaction. Hydrolysis is the main degradation pathway in aquatic environments without abundant microbial populations, such as rivers and streams, but biodegradation pathways are predominant in wastewater where microbial populations are much more abundant than in surface waters
[26].
The second essential pathway for the degradation of antibiotics in water is photolysis. Degradation of antibiotics can occur through direct photolysis, which is caused by direct absorption of solar light, or indirect photolysis, which involves natural photosensitizers like nitrate and humic acid suspended in water. Under solar radiation, these constituents can generate excited compounds such as hydroxyl radicals and singlet oxygen
[31][32]. In addition, organic matter dissolved in water is characterized by high mobility and can promote the solubility of organic pollutants in surface water
[31]. Photolysis rates vary along with season, time of day, and water depth
[33].
Antibiotics can also be retained in the aquatic environment by sorption with organic matter such as humic substances and organic carbon. Hydrogen bonds stabilize antibiotics on the surface of organic molecules
[34]. Sorption of antimicrobial agents to the mineral components of the river sediment might protect these compounds against microbial degradation and thus prolong their half-lives in water
[29]. For instance, oxolinic acid (quinolone group antibiotic) is very stable in the aquatic environment (9 days in water and 48 to 300 days in sediment) and its long persistence in water is related to adsorption onto sediments
[35]. Antibiotics with high adsorption coefficients may undergo repeated adsorption and desorption in the aquatic environment. An even greater ecological threat to the environment is the deposition of antibiotics in river water adsorbed on solid particles, such as micropollutants, which include microplastics occurring in the aquatic environment
[36][37]. Antibiotics and microplastics are two classes of emerging pollutants with negative impact to the aquatic environment. Microplastics have different adsorption capacities for organic pollutants, including antibiotics, due to different surface characteristics, pore size distributions, and various degrees of crystallization
[36]. Adsorption of antibiotics on microplastics could result in their long-range transport and increase their exposure to aquatic environments. The main sources of both antibiotics and microplastics in the aquatic environment are wastewater treatment plants, the effluents of which are point sources of these micropollutants in rivers
[38].
4. Effect of Sub-Inhibitory Concentrations of Antibiotics on Bacterial Populations
Antibiotics entering aqueous environments as a result of anthropopressure could potentially affect the communities of microorganisms. They can be regarded as an ecological factor driving microbial evolution by changing the structures of microbial communities, inhibiting or promoting their ecological functions, and affecting drug resistance mechanisms
[39]. The impact of antibiotics on the aquatic ecosystem is related to their concentrations, bioavailability, exposure time, and the addition of substrates, e.g., metals
[39]. Antibiotic-induced changes in the ecological functions of the aquatic environment include the nitrogen transformation process, e.g., oxytetracycline inhibits the nitrification process in surface water
[40]; however, in some cases, increased nitrification activity has been observed when bacteria are exposed to antibiotics
[41].
Antibiotics polluting the natural environment do not reach the high therapeutic concentrations that inhibit the growth of bacteria (∼1
mg mL−1)
[42]. However, they are widely distributed at low concentrations (
ng−μg L−1)
[10][43] without reaching the minimum inhibitory concentration (MIC). The level of an antibiotic that is below the MIC concentration is referred to as sub-MIC (sub-minimum inhibitory concentration), or sub-inhibitory concentration in the literature. These levels are not considered as lethal concentrations, but they still affect individual cells of bacteria or their populations in various ways. The continuous increase in the prevalence of sub-inhibitory levels of antibiotics in the environment is a key aspect of the current problem of widespread drug resistance worldwide. Sub-inhibitory concentrations of antibiotics are increasingly found in many aquatic environments, such as sewage and sludge, rivers, lakes, and even drinking water and water in pristine environments
[9][44][45][46][47][48]. Aquatic environmental concentrations of antibiotics reaching from ng/L to μg/L are generally too low to inhibit bacterial activity, but environmentally relevant concentrations of antibiotics could enhance bacterial communication and transcriptional regulation. Sub-MIC concentrations of antibiotics found in the natural environment are essential to enriching and maintaining drug resistance among bacteria. At sub-inhibitory concentrations of antibiotics, bacteria do not die but their growth is slowed down. Resistance mutations caused by sub-MIC antibiotic concentrations require much less adaptation energy than mutations caused by MIC antibiotic concentrations. Therefore, mutations that incur less fitness cost could be more competitive and enriched in the microbial population
[49]. There is also the minimum selective concentration (MSC) of an antibiotic, meaning the lowest antibiotic concentration that is required to select for growth of the resistant mutant
[49].
The presence of sub-inhibitory concentrations of antimicrobial compounds in waters causes great concern about their harmful effects on microbial community composition
[50]. Direct effects of antibiotics on microbial populations might affect their abundance and species richness
[51]. Antibiotics can negatively impact microbial populations involved in key ecosystem functions. Thus, they reduce biodiversity, which is crucial for maintaining the correctness of biological processes in ecosystems
[50]. Importantly, sub-inhibitory levels of antibiotics can reduce bacterial community diversity by increasing the variance in fitness among taxa
[52].
5. Antibiotic Removal Processes from Water and Wastewater
Removal of antibiotics contained in wastewater (of human and animal origin) is a key aspect that could reduce the contamination of the aquatic environment (surface water and groundwater) with antibiotics. Antibiotic contamination of water creates direct and indirect routes through which antibiotics then enter human organisms. The direct route includes drinking contaminated water, while indirect routes include using contaminated water to irrigate crops that humans and livestock animals then eat or to water livestock animals that are then eaten by humans
[10]. From an environmental point of view, antibiotic residues can influence microbial populations by affecting their physiological functions or can lead to the disappearance of key environmental groups of microorganisms. The problem is that conventional WWTPs are not properly prepared to remove pharmaceuticals from raw wastewater using primary treatment methods
[53]. Additionally, undegraded antibiotics can adsorb onto sewage sludge in biological treatment plants.
Conventional WWTPs generally use primary (mechanical treatment: filtration and sedimentation) and secondary treatment processes (biological processes to remove organic matter using aerobic or anaerobic systems). The most commonly used biological method is conventional activated sludge. Membrane bioreactors (MBR) are less common
[54], probably because of their high operational costs related to maintaining sustainable filtration conditions and high energy consumption
[55]. The MBR process comprises aerobic and anaerobic methods, combining modern membrane filtration technology and biological degradation by active sludge. The main advantage of MBR is the high quality of the treated water suitable for its reuse
[56]. Membrane bioreactors contain micro- or ultra-filtration membranes ranging from 0.04 to 0.4 μm
[57], resulting in significant improvements in the microbial quality of the produced effluent by removal of a wide range of microorganisms by size exclusion
[55]. Research on a pilot-scale MBR
[58] showed the following percentages of antibiotic elimination from the influent from a Swiss hospital: 51% for ciprofloxacin, 47% for norfloxacin, <60% for erythromycin, 7% for sulfamethoxazole, and 96% for trimethoprim. Xiao et al.
[59] showed the reduction of sulfamethoxazole and trimethoprim from wastewater using an anaerobic membrane bioreactor (AnMBR) at levels of 67.8 ± 13.9% and 94.2 ± 5.5%, respectively. High removal of ampicillin (94.4%) was also achieved in MBR treatment
[60]. On the other hand, the activated sludge process removed 82% of ampicillin and the disinfection process eliminated 91% of ampicillin
[61] in a municipal wastewater treatment plant. MBR sewage treatment is distinguished by the increased quality of the effluent compared to conventional activated sludge systems. MBR is a compact process and its advantages are exploited in ski resorts, hotels, and trailer parks
[55]. MBR effluent could be reused for technical applications, such as irrigation, snow production on ski slopes, or other non-potable water industrial applications, and not only directly discharged into the environment. Reusing water is a huge advantage in the midst of the current worldwide problem of water scarcity and water management regulations. This enables reducing the consumption of water resources and eliminating or decreasing concentrations of emerging pollutants, such as antibiotics, introduced into the environment. More developed techniques combine MBR with advanced water treatment, such as activated carbon, UV-irradiation, post-ozonation, or reverse osmosis
[55][59][62].
Recently, metal-organic frameworks (MOFs), a multi-dimensional material held together by bonding between metal atoms and organic ligands, have been shown to be effective in treating wastewater with antibiotic residues
[63][64]. MOFs exhibit desirable characteristics, such as large surface area and pore volume, hierarchical structures, biocompatibility, non-toxicity, regeneration capabilities, and tunable pore size and functional groups, that are suitable for wastewater treatment processes
[64][65][66]. MOFs can maintain their structures in water conditions
[67]. Applying MOFs in WWTPs can significantly improve treatment efficiency. MOFs can be applied in wastewater treatment by conducting adsorption, filtration, and degradation
[68], including catalytic degradation of antibiotics by immobilized enzymes
[65]. Zhou et al.
[69] investigated the detection and removal of tetracycline solution (0.1 mM) in water with a luminescent MOF, resulting in 56% of this antibiotic being removed after 30 minutes. Dong et al.
[63] showed photocatalytic decomposition of oxytetracycline with a stable 8-connected Cd(II) MOF as a photocatalyst. Metal-organic frameworks are considered as relevant materials for the adsorption and removal of emerging pollutants, such as antibiotics, in wastewater.
Zeolites can also be used as useful materials in the treatment of antibiotics in sewage. Zeolites are sorption materials which—if appropriately developed and selectively functionalized—can retain antibiotic residues in wastewater treatment systems
[70]. The hydrophobicity of zeolites is a beneficial property that facilitates the adsorption of antibiotics in water solutions. High silica-zeolites almost completely (>90%) removed sulfonamide antibiotics from water
[71][72]. Natural and modified minerals are also employed in the processes of antibiotic elimination from water. They have unique properties, including high specific surface area, low cost, availability, and good removal efficiency
[73]. Natural colemanite mineral (mesoporous material) was used as an adsorbent material for the removal of four common fluoroquinolones from surface water and wastewater samples.
6. Conclusions
Antibiotics polluting the environment are recognized as emerging micropollutants affecting microbial populations. Water is the main dissemination pathway of antibiotics and drug resistance determinants between various environmental compartments. The rate of antibiotics entering the aquatic environment is higher than their rate of elimination. Long-term exposure to sub-inhibitory concentrations of antibiotics (ng/L-μg/L) in waters is the main driver of changes in the genomes of microorganisms, thus resulting in the emergence of drug resistance and exchange of drug resistance genes by HGT. Antibiotics (acting as signaling molecules) are the ecological factor driving the evolution of bacteria by interfering with their ecological functions and compositions of bacterial communities. This causes the reduction of bacterial biodiversity responsible for the proper occurrence of biological processes in ecosystems. Antibiotics at low concentrations and bioavailability are capable of modifying bacterial communities and affect transcriptional regulation, thereby causing drug resistant mutations. Microorganisms evolve in response to emerging factors, such as antibiotics, in their environment. This is particularly evident in sensitive environments such as pristine mountain ecosystems where rivers can be exposed to strong anthropogenic factors closely related to tourism, agriculture, and animal husbandry.
Contamination of mountain waters with antibiotics is already present in the upper river courses of high-mountain national parks under protection. Mountain shelters, which are not equipped with sewage systems, are also sources of antibiotic contamination. There is a conflict between maintaining the pristine mountain environment and the continuous development of mountain tourism. The main threat to public health is the development of drug resistance and possible transfer of ARGs from environmental strains to clinical strains. With the continuous supply of sub-inhibitory concentrations of antibiotics in the environment affecting changes in the genomes of microorganisms, there may be a risk of a link between environmental and clinical drug resistance. On the other hand, changes in the biodiversity and composition of microbial populations that are responsible for important ecological functions in the ecosystem pose a threat to the environment. For this reason, monitoring the contamination of surface waters with antimicrobial agents is an important aspect. Contamination of surface waters with antibiotics is particularly harmful in the mountain environment. This is due to the fact that mountain water supplies are a valuable natural resource found mostly in pristine and protected areas where they give rise to rivers and constitute a reservoir of drinking water in every country. Protected environments, which are a valuable source of biodiversity, should be taken care of. Antibiotics are a type of micropollutant that is not routinely tested. Therefore, monitoring concentrations of antibiotics in waters is crucial for maintaining the quality of water resources for human use and the microbiological biodiversity within water ecosystems.