Antibiotics have played an essential role in the treatment of numerous infectious diseases, contributing to a significant improvement in the health status of animals as well, and speeding up their growth processes through the control of food-related infections ^[1][2][3][21,22,23]. This improvement risks being undermined by the increasing spread of pathogens resistant to antibiotics ^[4][5][6][24,25,26]. The Antimicrobial Resistance Plan 2017–2020 reports that more than 50% of antibiotics used globally are consumed in veterinary medicine ^[7][8][27,28]. The incorrect use of antibiotics in veterinary medicine also increases selective pressure and the spread of resistant bacteria, which can be transferred from animals to humans ^[7][8][9][27,28,29] either by direct contact or through food of animal origin, or indirectly through more complex cycles of environmental contamination ^[10][30].

Indeed, the use of antibiotics in animal husbandry can disrupt the delicate balance of the intestinal microbiome, a microbial community that plays a crucial role in maintaining the health of both humans and animals ^[11][12][31,32]. The intricate connections between humans, animals, and the environment are evident in the impact that antibiotic use in animals can have on the microbiome, as it can potentially lead to imbalances with far-reaching consequences. Understanding and addressing this interference is important, not only for the well-being of animals, but also for the broader context of public health and environmental sustainability ^[13][14][15][33,34,35].

Also, as aspect not to be underestimated is an increase in the potential health risk for farmers, which may be responsible for a reduction in both farm efficiency and production safety.

The use of antibiotics in animal husbandry presents a multifaceted set of risks and concerns. On one hand, it is well-established that there are risks of environmental contamination due to the presence of antibiotic-resistant bacteria in the waste products of treated animals ^[16][36]. Additionally, there is a direct risk for veterinarians, breeders, and workers who may acquire antibiotic resistance through prolonged or repeated exposure to these drugs ^[17][18][19][37,38,39].

However, the impact of antimicrobial use in animal husbandry on the risk of transmitting antibiotic-resistant bacteria to humans, particularly through the consumption of animal-derived food products, remains an area that requires further investigation ^{[15][16][17][18][19]}[35,36,37,38,39]. This is a critical issue as it relates to food safety and public health, and ongoing research is needed to better understand the extent of this risk and how it can be effectively managed. The complex interplay of antibiotic use, resistance development, and transmission to humans, underscores the importance of a comprehensive and multidisciplinary approach to address this issue ^[20][21][40,41].

The prudent use of antibiotics, moreover, can only be closely linked to the application of high standards of farm welfare and biosecurity ^[16][17][22][36,37,42]. It follows that an integrated approach to the phenomenon of antibiotic resistance is a key element in combating its occurrence ^[18][19][21][38,39,41]. Key actions in veterinary medicine include a computerised system of traceability of veterinary medicines, including the Electronic Veterinary Recipe, compulsory from 16 April 2019; a drug classification system for risk categorisation within herds; good pharmaco-surveillance; appropriate training of veterinarians, pharmacists, and operators; guidelines for the prudent use of antibiotics; funding of research on antibiotic resistance in the veterinary field; participation in the European surveillance system on the sale of antimicrobials in the veterinary sector (ESVAC) project. The European Centre for Disease Prevention and Control (ECDC) states in one of its latest reports that the veterinary sector in Italy has significantly reduced the use of antibacterial drugs in the livestock sector since 2014, showing an even lower overall use of antibiotics per kg of biomass than in the human sector. The improvement described so far has also been achieved thanks to the electronic prescription system that started in Italy and Spain even two years before the entry into force of the aforementioned new European regulation, which was also adopted in response to antibiotic sales that were too high compared to those of other Member States ^[23][43]. The European Medicines Agency (EMA) has issued a scientific opinion on the designation of antimicrobials to be reserved for the treatment of certain human infections, to preserve their efficacy and reduce the risk of antibiotic resistance to public health ^[24][25][44,45].

The misuse of antibiotics in human and veterinary medicine has led to the development and proliferation of specific resistances in bacterial communities exposed to the effects of human activities all over the planet ^[6][26][6,26]. The abundance and diversity of resistance genes and resistant bacteria in the environment are closely related to the impact caused locally by human activities ^[23][27][19,43]. Antibiotic resistance resulting from resistance genes to synthetic and semi-synthetic antibiotics ^[3][19][28][13,23,39] spreads in the environment via multiple contamination pathways as a result of different anthropogenic activities ^[29][30][31][46,47,48], in which there is a high use of antibiotics ^[21][41]. Resistance genes can reach the environment both through diffuse sources of contamination ^[32][49], i.e., areas of intensive agriculture ^[33][34][50,51], industrial districts, and human activities distributed throughout the territory, and through point sources, such as intensive livestock facilities ^[29][46], aquaculture, urban ^[35][36][52,53] and hospital sewage discharges ^[30][33][47,50], and industrial activities for the production of antibiotic substances.

Antibiotic use in animals can impact human health in different ways, each presenting its own set of complex challenges and considerations. These multifaceted effects need to be thoroughly understood and addressed to develop effective policies and practices related to antibiotic use in agriculture. The impact of the massive use of antibiotics causes not only the release of resistant bacteria and resistance genes into the environment, but also significant quantities of the various antibiotics ^[37][54]. The parent substances not metabolised by the human body and their metabolites are where they are generally not completely removed. Antibiotics and metabolites are then released into streams, lakes, or the sea ^[38][55] through treated water or into the soil through the use of sewage sludge as fertiliser for fields ^[39][56]. This class of contaminants, despite its heterogeneity, is generally referred to as ‘semi-persistent’ because its use is continuous and massive: significant quantities are released into the environment on a daily basis as a result of use in human and veterinary medicine. In practice, although some substances degrade rapidly in the environment, they are always present due to continuous input. Prevention activities at the source of the discharge and disposal of antibiotic substances into the environment represent the priority strategy for action. It is important to strengthen environmental and urban wastewater monitoring networks to support knowledge-based intervention measures and best available techniques. In Italy, the Ministry of Ecological Transition, in collaboration with researchers from the Italian National Institute for Environmental Protection and Research (ISPRA), the National Research Council, universities, and scientific research institutes, has proposed a number of priority actions to facilitate the proper management of antibiotic resistance in the environment ^[40][41][42][57,58,59]; review the management of antibiotics and waste in intensive livestock farms ^[3][23]; convey, through the tools of information and education of the population, indications on the correct use and disposal of antibiotic drugs ^[41][58]; and promote research activities on the relationship between antibiotic resistance and the environment ^[43][60].

Furthermore, the role of degradable plastics should not be underestimated, which are not fully biodegradable and are increasing in use. What is relevant is the problem of microplastic pollution (MPs), one of the main environmental issues of the last decade. MPs are defined as particles of anthropogenic origin between 100 nm and 1 mm, if small, and between 1 and 5 mm, if medium-sized. MPs are purposely added in several products to change their consistency, stability or to impart functions, such as abrasive capacity; for these purposes, they are added, for example, in fertilisers and plant protection products, industrial and household detergents, paints, and products used in the oil and gas industry ^{[44][45][46][47][48][49][50]}[61,62,63,64,65,66,67]. MPs persist in the environment in large quantities, especially in marine and aquatic ecosystems, and it is estimated that in the oceans, more than 68% of MPs derive from undisposed or improperly disposed of and released into the environment.

All this contributes to the permanent pollution of ecosystems and their accumulation along the food chain, with both direct and indirect negative effects on human health. Human beings may, in fact, suffer a physical reaction, linked to the size of plastic fragments, a chemical reaction, due to the possible release of monomers, additives and chemical agents, and also due to the deterioration of plastic fragments, and finally, a biological reaction, due to the colonisation of MPs by pathogenic microorganisms ^[43][60]. The compounds that most commonly form MPs are compounds such as polyethylene and polyvinyl chloride, materials that due to their chemical–physical properties facilitate the binding and transport of chemical contaminants, such as antibiotics, and microbial agents ^{[51][52][53][54][55][56][57][58]}[68,69,70,71,72,73,74,75], such as bacteria, increasing their impact on the environment and human health ^{[59][60][61][62][63][64][65]}[76,77,78,79,80,81,82].

Some studies on plastisphere, examined the role of polystyrene take-away food containers in the formation of MPs and their potential contribution to the growing problem of antibiotic resistance ^[63][64][80,81]. Once polystyrene is transformed into MPs, it can serve as an ideal substrate for hosting microorganisms and chemical contaminants. Moreover, it can harbour genetic materials containing antibiotic resistance genes (ARGs) ^[65][66][82,83]. These studies highlighted how the aging of these MPs in the environment makes them particularly conducive to binding with ARGs ^[65][66][82,83].

The aging process of MPs can be triggered by factors such as mechanical abrasion, exposure to solar radiation, and biodegradation. These processes increase the surface area of MPs, break down their polymer structure, and alter their physical and chemical properties. This, in turn, promotes microbial adhesion and the formation of biofilms.

Additionally, the widespread and often inappropriate use of antibiotics in various human activities leads to their release into the environment, primarily through wastewater. In such environments, ARGs can transfer to pathogenic bacteria via horizontal gene transfer methods like conjugation, transformation, or transduction ^{[47][48][49][50]}[64,65,66,67].

The release of depolymerizing chemicals from MPs can change the membrane permeability of microorganisms, potentially facilitating the transfer of ARGs. Additives or pollutants accumulated on MPs can also influence the presence of ARGs. For example, the presence of copper and zinc on plastic surfaces can promote the binding of ARGs related to resistance against certain antibiotics. Organic pollutants like polycyclic aromatic hydrocarbons have been found to exert selective pressure on the transfer of ARGs through mechanisms like co-selection or cross-selection ^[65][67][82,84].

Specifically, in 2023, the study of Tuvo et al. ^[65][82] reveals that the presence of copper sulphate resistance genes can co-select resistance to various antibiotics that share the same genetic element. MPs have the capacity to transport or facilitate the exchange of ARGs and other pollutants across different environmental compartments, even over long distances.

In conclusion, the presence of ARGs on MPs dispersed in the environment presents an emerging concern that will need to be addressed in the near future through proper management of plastic waste disposal and effective water treatment methods. This is especially critical in the case of hospital wastewater, where the concentration of ARGs and the potential for antibiotic resistance spread is particularly high.

It is therefore essential to deepen our knowledge of the environmental behaviour of MPs and their role in the transmission of ARGs by studying the MPs resistome and how much of this is shared with the surrounding environment ^[3][4][23,24], to understand the potential risk of exposure for humans ^{[66][67][68][69][70]}[83,84,85,86,87]. The importance of the environment and food of both animal and plant origin in the emergence of emerging diseases, such as SARS and avian influenza, allowed the term ‘One Health’ to be coined in 2003 and laid the foundations for the importance of collaboration between different professions to respond with a strategic approach to prevention and surveillance of emerging diseases ^[53][71][72][3,8,70].

Bacterial infections plagued human societies throughout history until the discovery of antibiotics ^{[55][56][57][58][59]}[72,73,74,75,76], which led to adapt in different environments, the threat of antibiotics by means of different astute strategies ^{[26][28][56][73]}[4,6,13,73]. They can modify the quaternary structure of a target protein, substitute a metabolic pathway by synthesizing alternative biomolecules, expel antibiotics from their cell by efflux pumps, produce enzymes able to inactivate the antibiotic, hide their target structure, for example, behind an outer capsule ^{[59][70][74][75]}[76,87,88,89]. Antimicrobial resistance genes may be carried on the bacterial gene chromosome, plasmid, or transposons ^{[76][77][78][79][80][81]}[90,91,92,93,94,95].

Bacteria are also capable of forming biofilms that physically prevent host immune response cells and antibiotics from inhibiting the pathogen. In addition, biofilms protect the dormant cells called persistent cells that are an expression of antibiotic tolerance, in which case the microorganisms are resistant in vivo to high doses of antibiotics without manifesting resistance to the minimum inhibitory concentration (MICs), in vitro ^[43][45][60,62]. Antibiotic tolerance can arise when bacteria are exposed to environmental conditions of stress, temperature, reduced nutrient supply and treatment with antibiotics ^{[82][83][84][85][86][87]}[96,97,98,99,100,101]. A previous study in 2016 conducted by de Kraker et al. ^[83][97] on antimicrobial resistance estimated that worldwide, by 2050, bacterial infections will cause around 10 million deaths per year, far exceeding deaths from cancer (8.2 million), diabetes (1.5 million) or traffic accidents (1.2 million) with a projected cost for “Combatting Bacterial Resistance in Europe: COMBACTE-NET” exceeding euro 220,000,000.00.

The WHO List of Critically Important Antimicrobials for Human Medicine (WHO CIA List) was initially established based on the recommendations from two successive expert meetings jointly organized by the Food and Agriculture Organization of the United Nations (FAO), the World Organization for Animal Health (OIE), and the World Health Organization (WHO).

During the first expert workshop, it was concluded that there was compelling evidence of adverse impacts on human health due to the development of antimicrobial resistance in non-human usage of antimicrobials. This included an increased frequency of infections, higher rates of treatment failures (sometimes resulting in fatalities), and greater severity of infections. These consequences were well-documented, particularly in cases of fluoroquinolone-resistant human Salmonella infections ^[87][101].

In 2008, the American Society of Infectious Diseases coined the acronym ESKAPE. To date, ESKAPE pathogens are responsible for the majority of hospital infections and owe their name to their ability to escape (from the English ‘to escape’) the biocidal action of antimicrobial agents ^[88][89][102,103]. The acronym ESKAPE includes the six nosocomial pathogens that show multi-resistance and virulence: Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter Species. ESKAPE pathogens are responsible for the majority of nosocomial infections and are able to ‘escape’ the biocidal action of antimicrobial agents. They are also associated with the highest risk of mortality, leading to increased healthcare costs ^{[90][91][92][93]}[104,105,106,107]. The World Health Organisation also recently included ESKAPE pathogens in its list of 12 bacteria against which new antibiotics are urgently needed ^[91][105]. Three categories of pathogens are described: critical, high, and medium priority, depending on the urgency of the need for new antibiotics. Carbapenemase-resistant A. baumamannii and Pseudomonas aeruginosa ^[80][81][94,95] together with extended-spectrum β-lactamase (ESBL) or carbapenemase-resistant K. pneumoniae and Enterobacter spp. ^[94][95][108,109] are listed in the critical priority list of pathogens; whereas vancomycin-resistant E. faecium (VRE) and methicillin- and vancomycin-resistant S. aureus (MRSA AND VRSA) are in the high priority group list ^{[96][97][98][99][100][101]}[110,111,112,113,114,115].

According to the European Centre for Disease Prevention and Control (ECDC), the most common and clinically relevant bacterial species in European hospital isolates include Escherichia coli, Pseudomonas Aeruginosa, Klebsiella Pneumoniae, Staphylococcus Aureus, and Enterococcus Faecium ^{[102][103][104][105][106][107]}[116,117,118,119,120,121].

Among hospital-acquired (HA) infections in Europe, 41% of S. aureus infections are methicillin-resistant, and particularly in Northern Europe (i.e., the United Kingdom and France), a steady decrease in the prevalence of HA-MRSA was observed between 2015 and 2018 and was largely attributed to improved national infection control programs. In comparison, the rates of HA-MRSA in Southern Europe (i.e., Portugal, Spain, Italy, and Greece) remain high ^{[103][104][105][106][107][108][109][110][111][112]}[117,118,119,120,121,122,123,124,125,126].

In Europe 32% of P. aeruginosa infections are resistant to carbapenems (ESBL-carba) ^[105][119]. Furthermore, the bacteria remain a major contributor of hospital-acquired infection ^[106][107][120,121]. Widespread distribution of P. aeruginosa nosocomial isolates resistant to last-resort and polymyxin- and carbapenem-class antibiotics is well documented ^[101][113][115,127].

With regard to Klebsiella pneumoniae, the Italian 2019 statistics report that 30% of the strains isolated are multi-drug-resistant (MDR), and in particular, between 2005 and 2010, we showed an increase in isolates causing invasive infections ^{[101][114][115][116][117][118][119]}[115,128,129,130,131,132,133].

Twelve countries reported rates of 25% or more, six of which reported AMR rates of 50% or more (Belarus, Georgia, Greece, Republic of Moldova, Russian Federation, and Ukraine) ^{[102][120][121][122][123][124]}[116,134,135,136,137,138].

4. Antibiotic Resistance in Agroecosystems

Currently, communication across the One Health triad (humans, animals, environment) regarding agricultural AMR is hindered by ambiguous language, complicated by cultural and linguistic differences that can lead to the conclusion that the other participant is not aware of the facts or has ulterior motives ^[125][126][139,140]. Antibiotic use in agriculture is just one factor contributing to the rise in antimicrobial resistance. Modern factory farms are the ideal breeding grounds for antibiotic-resistant infections: many animals are raised in crowded, unsanitary conditions. The debate regarding the impact of antibiotic use in agriculture on the development of clinically relevant antibiotic resistance in human medicine is fuelled and perpetuated by the challenge of obtaining direct, quantitative data to assess the scale and nature of this contribution ^[125][139]. In fact, a substantial portion of antibiotics produced in the United States, are used in agricultural practices ^[127][10]. This use has undoubtedly played a role in the prevalence of antibiotic-resistant bacteria in the gut flora of food animals like chickens and swine. However, regulating the use of antibiotics in agriculture has been a contentious issue. Policymakers have been tasked with balancing the evident benefits of antibiotic use for animal health and economic advantages for food producers, pharmaceutical companies, and potentially consumers, against the nebulous threat to human health that is often challenging to precisely quantify ^[127][10]. It is important to note that antibiotic drugs and their bioactive breakdown products, while not technically classified as having “antibiotic resistance” characteristics themselves, are widely recognized as the primary driver of antibiotic resistance ^[128][141]. The presence of these drugs and their metabolites in the environment is a significant concern. Discussions aimed at addressing agricultural and environmental antibiotic resistance often revolve around the impact of anthropogenically controlled drugs in various environmental matrices. The release of antibiotics into the environment, whether through agricultural use, wastewater discharges, or other means, can contribute to the selection and proliferation of antibiotic-resistant bacteria ^[129][142]. These resistant bacteria can, in turn, pose a risk to human and animal health as they may enter the food chain, spread in the environment, and potentially transfer resistance genes to other bacteria. Efforts to address AMR must consider the environmental dimension and the role of antibiotics in driving resistance. It is essential to explore strategies for responsible antibiotic use in agriculture, effective wastewater treatment, and measures to mitigate the environmental impact of these drugs to help combat the growing challenge of AMR. As an example, significant is the use of ionophores in agriculture, with over 4 million kilograms sold in the United States in 2016 ^{[130][131][132]}[143,144,145]. Even if on one hand, ionophores are the second most widely used class of antibiotics in agriculture, on the other hand are not used in human medicine, there has been an assumption that their use in agriculture does not directly impact human health, leading in this way that have not been subjected to the same regulatory scrutiny as medically important antibiotics ^[131][132][144,145]. However, there is a growing concern that the current evidence base is insufficient to definitively conclude that ionophores do not contribute to antimicrobial resistance relevant to human health. Several factors contribute to this uncertainty:

(1)

Cross-Resistance: It is unclear whether resistance to ionophores in bacteria can lead to cross-resistance to medically important antibiotics. This is a significant concern, as cross-resistance could undermine the effectiveness of crucial human antibiotics ^[132][133][145,146].

(2)

Co-selection: Recent evidence suggests that the use of ionophores may have the unintended consequence of co-selecting for resistance to antibiotics used in human medicine. For example, there are indications that ionophore use might contribute to vancomycin resistance in some cases ^[132][145].

Given these concerns, there is a need for further research and investigation into the potential risks associated with ionophore use in agriculture. The goal is to better understand the implications of antibiotics, such as ionophore resistance and their potential impacts on human health. This ongoing examination is essential to inform responsible and evidence-based regulations and practices in the use of antibiotics in agriculture, ensuring the protection of both animal and human health.