**1. Introduction**

There are many lung pathogens but one of the most studied is *Pseudomonas aeruginosa* because it cannot be eradicated under certain conditions. As an opportunistic pathogen, its interaction with the host has some particularities that we will explore in this chapter.

The *Pseudomonadaceae* comprise Gram-negative microorganism, nonsporulated, aerobic strict of wide distribution in the environment from the soil, water, and plants to humans; this is due to their nutritional versatility. Of this vast group, only *Pseudomonas cepacia*, *mallei*, and *aeruginosa* infect humans*,* of which *aeruginosa* is the more relevant because it is the most frequent cause of nosocomial infections [1].

It is often said that *Pseudomonas aeruginosa* does not infect healthy individuals but there are reports on the contrary, as swimmers otitis [2]. Because it is an opportunist pathogen, it does not need the host for its survival, and it may be lethal after becoming a chronic infection in susceptible patients with cystic fibrosis (CF) [3–5], cancer [6–8], hepatic cirrhosis [9], keratitis [10–13], or spondylodiscitis [14]. This bacterium is most feared by pulmonologist because when acquired by nosocomial patients [15, 16], it complicates any existing conditions, and when it invades immune-compromised patients, its eradication may become impossible.

Colonization with *P. aeruginosa* is observed in all stages of chronic obstructive pulmonary disease (COPD), but the prevalence significantly increases with disease severity from 0.7%, in stage 1 of the Global Initiative for Obstructive Lung Disease, to 1.5% for stages 2 and 3 up to 2.6% for stage 4 [17, 18]. This prevalence rises to 8–13% in acute exacerbations of COPD [19–21]. But still, the main susceptibility for the infection and death by *P. aeruginosa* [22, 23] are the mutations of the CF

transmembrane conductance regulator (CFTR) identified as F508, G542X, G551D, W1282X, R1162X, and N1303K [24, 25]. CF also has co-morbidity such as liver cirrhosis [26] with 18% prevalence [27, 28] of *P. aeruginosa* infection in this subset.
