Antibiotic Resistance in Pseudomonas.

Pseudomonas is a bacterial genus, with a bona fide environmental habitat that comprises different species, some of them causing diseases in humans and plants, as well as some strains with biotechnological potential. Amongst them, Pseudomonas aeruginosa is currently one of the most important nosocomial pathogens. In addition, this microorganism is a prevalent cause of chronic infections in cystic fibrosis patients and in people suffering from chronic obstructive pulmonary disease. The success of P. aeruginosa in colonising different habitats largely relies on its metabolic versatility and robustness. Besides, this bacterial pathogen harbours in its core genome a large set of virulence determinants that allows it to colonise/infect a variety of hosts, from unicellular organisms to humans. Nevertheless, these are not just the only conditions needed for infecting patients at hospitals. Taking into consideration that infected patients are regularly under antibiotic treatment, the ability to avoid antibiotics' action is also needed. In this sense, P. aeruginosa displays a characteristic low susceptibility to several antibiotics currently used in therapy. This is due to the reduced permeability of its cellular envelopes and the presence in its genome of an arrangement of genes encoding multidrug efflux pumps and antibiotic-inactivating enzymes that contribute to its resilience to antibiotics. Besides intrinsic resistance, P. aeruginosa is able to evolve towards antibiotic resistance through mutations (particularly relevant in the case of chronic infections) and via acquisition of antibiotic resistance genes. It is worth mentioning that acquired resistance is not the only venue that P. aeruginosa has for avoiding the action of antibiotics. Transient resistance can also confer this phenotype. Indeed, the induction of the expression of intrinsic resistance genes by conditions or compounds that P. aeruginosa could face during infection can compromise the effectiveness of antibiotics for treating such infections. In addition, tolerant cells able to survive during the exposure to bactericidal antibiotics without an increase in their antibiotic resistance phenotype are found as well in these patients, and they are the prelude of the evolution towards antibiotic resistance. Finally, P. aeruginosa biofilms, frequently encountered in the lungs of cystic fibrosis patients, in prostheses, or in catheters, present low antibiotic susceptibility and are associated with recalcitrance and disease worsening.