# Antibiotic Resistance in the Farming Environment
## 5 Determinants of Antibiotic Resistance in Farming  
The phenomenon of drug resistance in the farming environment is determined by a number of factors related to both the characteristics of the organisms themselves and farming practices (Figure 3). The increase in the frequency of antibiotic-resistant bacteria in the environment is the result of phenomena such as horizontal gene transfer, genetic mutations, and recombination [34]. The location of antibiotic resistance genes within mobile genetic elements (plasmids, transposons, integrons) facilitates their dissemination in microbial communities [61; 62; 81]. Typical mechanisms for spreading drug resistance via horizontal gene transfer include conjugation involving plasmids, transduction caused by bacteriophages, and natural transformation, allowing the bacterium to acquire DNA in extracellular form [82]. The interspecies spreading of genes determining antibiotic resistance includes also the transfer of drug resistance traits to pathogenic species [36; 69]. For example, the transfer of drug resistance genes between strains of the _Salmonella_ and _Escherichia_ genera present in the gastrointestinal tract was observed [83]. In mixed microbial communities, \(\beta\)-lactam antibiotic-sensitive bacteria can survive due to their direct proximity to microorganisms that produce enzymes capable of hydrolyzing \(\beta\)-lactams [5]. There are also studies confirming that some avian pathogenic _Escherichia coli_ strains have developed resistance to antibiotics such as ampicillin, amoxicillin, and tetracycline [84]. As reported by [PERSON] et al. [16], resistance to cefepime, gentamicin, and chloramphenicol was detected in opportunistic pathogens _Pseudomonas corrugata_ and _Pectobacterium carnotorum_. It has also been confirmed that animals living in the wild in the immediate vicinity of livestock facilities show an increased presence of antibiotic-resistant microorganisms, suggesting the possibility of their transmission from livestock to the wild [85]. Potential vector species are indicated, among others, methicillin-resistant _Staphylococcus aureus_ (MRSA), vancomycin-resistant enterococci (VRE), extended-spectrum beta-lactamase (ESBL), carbapenemase-producing _Enterobacteriaceae_, as well as multidrug-resistant _Pseudomonas aeruginosa_ and _Clostridium difficile_. Importantly, vector strains do not need to be pathogens themselves; instead, it is sufficient that they have the ability to effectively colonize an organism [86]. One of the primary determinants of the development of antibiotic resistance in the farming environment is the fact that antibiotics are used in sub-therapeutic doses [34; 87; 88]. The presence of subinhibitory concentrations of antibiotics promotes the horizontal transfer of antibiotic resistance genes in the environment [81]. It was found that continuous exposure  
Figure 3: The main factors determining antibiotic resistance in farming environment. to low doses of pharmaceuticals creates a stronger selection pressure for antibiotic resistance than short-term use of higher doses [62]. Environmental stress is another factor that promotes the expression of antibiotic resistance genes and the development of multi-resistance [89]. Selective pressure toward antibiotic resistance can result not only from the presence of antibiotics themselves but also from other contaminants such as heavy metals or biocides [4; 34; 75]. Metal ions such as zinc or copper, used for growth promotion and high feed efficiency of cattle breeding, may contribute to the phenomenon of co-selection of antibiotic resistance genes in the case of bacteria inhabiting the digestive tract of animals [21]. Co-selection of antibiotic resistance and the resistance to other contaminants (heavy metals, pesticides, anti-fouling chemicals) is possible due to the close location of responsible genes in the same mobile genetic element [90]. It is also important to note the cross-resistance phenomenon between antibiotic-resistant microorganisms and those showing resistance to metals and biocides [69].