AMR and Its Development

The quadripartite, composed of the World Health Organization (WHO), the Food and Agriculture Organization (FAO), the World Organization for Animal Health (WOAH) previously known as Office International des Epizoties (OIE), and the United Nations Environment Programme (UNEP), highlighted that antimicrobial resistance (AMR) occurs when bacteria, fungi, viruses, and parasites no longer respond to antimicrobial agents. As a result of drug resistance, antibiotics and other antimicrobials become ineffective and infections become difficult or impossible to treat, increasing the risk of disease spread, severe illness, and death. The development of resistance to antimicrobials is a natural phenomenon (Hall et al., 2020) but the excessive use of antimicrobials in livestock and human medicine accelerates this resistance (Kimera et al., 2020; Gahamanyi et al., 2021).

There are several mechanisms by which microorganisms resist antimicrobials. The two primary mechanisms of resistance are natural or intrinsic and acquired. Intrinsic resistance depends on the inherent structural/functional properties of the pathogen that stop the drug from reaching its target or produce enzymes that degrade it (Rosini et al., 2020). For instance, Campylobacter species are inherently resistant to β-lactams including ampicillin (Kashoma et al., 2016). On the contrary, acquired resistance is the ability of a previously susceptible pathogen to develop or gain resistance to antimicrobials in different ways (Blake & O’Neill, 2013; Blair et al., 2015).

Pathogens can be classified based on how they acquire resistance genes. Vertical gene transfer (VGT) occurs when resistome are passed from parent cells to offspring whereas horizontal gene transfer (HGT) occurs when microbes transfer resistome to neighboring cells in the environment (Wright, 2011). The HGT can take place between pathogens of the same or different species and thus complicating the efforts to control AMR. The HGT allows pathogens previously susceptible to become multidrug-resistant (MDR) by acquiring various resistance genes from the environment. The main mechanisms of acquired resistance are categorized into three groups: (i) those reducing the intracellular drug concentrations, membrane modification or developing efflux pumps, (ii) those leading to target modification, and (iii) those degrading the drug (Wright, 2011; Blair et al., 2015).