# The Invisible Threat of Antibiotic Resistance in Food
## 2 Antibiotics and Their Effects on Bacterial Resistance  
In recent decades, a number of mechanisms related to the behavior of bacteria in relation to antimicrobial compounds have been observed and described. It is essential to define these mechanisms in order to gain a precise understanding and to facilitate examination of bacterial responses. The mechanisms in question include resistance, tolerance, and persistence. At present, the published definitions of antibiotic resistance vary considerably. The term \"resistance\" is employed to denote various characteristics exhibited by bacteria, which can be categorized in accordance with their origin (intrinsic versus acquired resistance) or type (single, multiple, or cross-resistance). These characteristics encompass phenotypic traits, such as growth patterns, and genotypical traits, including the presence and/or expression of specific genes [18]. The term \"tolerance\" is used to denote the ability of microorganisms to survive transient exposure to high concentrations of an antibiotic without a change in MIC. This process is often achieved by slowing down an essential bacterial process. Tolerance confers upon bacterial cells the capacity to withstand transient exposure to antibiotic concentrations that would otherwise prove lethal [4]. The term \"persistence\" is employed to denote the ability of a subpopulation of a clonal bacterial population to survive exposure to high concentrations of an antibiotic [4]. As a consequence of antibiotic resistance, more than 2.8 million infections caused by antibiotic-resistant bacteria occur in the U.S. yearly, resulting in more than 35,000 deaths, and it was predicted that the number of deaths worldwide will reach up to 10 million per year by 2050 [19]. Antibiotics are becoming less effective due to the increasing number of pathogenic strains with MDR. A decline in the effectiveness of antibiotics used to treat infectious diseases has led to an increase in mortality rates and prolonged hospital stays [20]. Resistance to antibiotics has led to at least one million deaths each year since 1990, with increasing rates of drug-resistant infections expected to claim more than 39 million lives between now and 2050 without further policy action, according to a landmark study by the Global Research on Antimicrobial Resistance (GRAM) Project [21]. The emergence of ARB is a major issue around the world, particularly within Europe [22]. The continuous emergence of antibiotic resistance additionally has increased the financial burden on healthcare systems [23,24]. The main determinants of antibiotic resistance are ARGs. Available data provide growing evidence that ARGs are widely distributed in different environmental patterns. For example, one of the resistance-causing _mcr1_ genes was discovered in late 2015 [25]. Soon after its discovery, it was detected in 57 countries on 5 continents in almost all food-producing animals [19,26]. Health institutions have expressed concerns about the transmission of extended-spectrum \(\beta\)-lactamase (ESBL)-producing _E. coli_ isolates, particularly in meat products, from the food chain to humans [22,27]. A Dutch study reported the transfer of ARGs from poultry to the human gut microbiota [28]. Antibiotic residues can cause various adverse health effects. Many antibiotics can cause allergic reactions, including anaphylactic shock. Sulfamethazine, oxytetracycline, and furazolidone can result in carcinogenicity; gentamicin may produce mutagenicity and nephropathy; chloramphenicol may lead to hepatotoxicity, reproductive disorders, and bone marrow toxicity [29; 30]. Furthermore, therapeutic doses of antibiotics temporarily change both the composition of the human gastrointestinal microbiota and the immune and metabolic health of the host [31]. In addition to these serious consequences, low doses of antibiotics in feed or food and sub-lethal or sub-therapeutic doses also contribute to resistance and the emergence of ARB by promoting genetic and phenotypic variability in exposed bacteria [32].