# Genomic and Metabolic Characteristics of the Pathogenicity in Pseudomonas aeruginosa
## 4 Metabolic Pathways Associated with Pathogenicity
### Antibiotic Resistance
#### 4.3.4 The Case of Multiple Colistin Response Mechanisms  
Polymyxin is a peptide antibiotic composed of five chemically different compounds (polymyxins A-E). In clinical practice, only polymyxin B and polymyxin E (colistin) are used [199]. For the treatment of some infections, colistin alone is still an effective antibiotic [200]. This antibiotic is used in human medicine, but it has also been widely used in veterinary medicine for the treatment of various infections or in farm animal production, to aid their growth [191; 201]. Since 2006 the use antibiotics as growth promoters is no longer allowed in the European Union [202].  
Colistin is polycationic and has both hydrophilic and lipophilic moieties. It causes destabilization of the outer membrane, leading to pore formation, thus increasing membrane permeability [203]. Colistin, which is positively charged, binds through electrostatic interaction to the negatively charged phosphate groups of lipid A subunits present in the structure of lipopolysaccharidesaccharide (LPS) [199]. The initial binding to an anionic LPS component displaces divalent calcium cations (Ca\({}^{2+}\)) and magnesium (Mg\({}^{2+}\)) in a competitive way, damaging the LPS three-dimensional structure. It is subsequently inserted at the hydrophobic terminal acyl fat chain, the external outer membrane (OM) monolayer is thus expanded, and membrane permeability is increased [204].  
Colistin resistance mechanisms can be accounted for by many different mechanisms [199; 205]. The transmissibility of a plasmid that contains the colistin resistance gene (_mcr-1_) is one of the resistance mechanisms adopted by _P. aeruginosa_[206]. The fast dissemination of this resistance gene may be due to the recurrent use of colistin in veterinary medicine, which has consequently accelerated the dissemination between animals and humans [207].  
Overall, two main mechanisms of resistance to colistin in _P. aeruginosa_ are chromosomal mutation and adaptation. Resistance caused by the chromosomal mutation is hereditary and is independent of the presence of antibiotics. One such example is the overexpression of efflux pumps (MexAB-OprM and MexXY-OprM), which are expressed in wild-type cells and contribute to the intrinsic resistance of several antibiotics [208].  
In _P. aeruginosa_, adaptive resistance to colistin occurs when extracellular concentrations of divalent cations Ca\({}^{2+}\) and Mg\({}^{2+}\) are limited and is controlled by the two-component regulators PmrA-PmrB and PhoP-PhoQ (Figure 6) [209; 210]. The PhoP/PhoQ system is a global regulatory system that autoregulates the _oprH-phoP-phoQ_ operon when there is a limitation of divalent cations in the environment. PhoP phosphorylation increases the transcription of several genes, such as _pmrD_, whose product binds to PmrA, when in the phosphorylated state [211]. In the PmrA/PmrB, the _pmrA/B_ also directly controls the _arnBCADTEF_ (also called _pmrHFIJKLM_) and _pmrCAB_ operons, involved in LPS modification. The _pmrCAB_ operon encodes PEtN and the _arnBCADTEF_ encodes the enzymes for covalent addition of 4-amino-4-deoxy-L-arabinose (L-Ara4N) to lipid A. After binding to lipid A, changes in the negative charge of the cell membrane take place, due to neutralization of the negatively charged phospholipids [205; 212; 213]. Regardless of the characterization of these loci, the evolutionary dynamics of colistin resistance is still poorly understood and the adaptive mutations reported to date are probably incomplete [205].  
Figure 6: Colistin resistance mechanism by LPS modification in _P. aeruginosa_.