**Abstract**

*Pseudomonas aeruginosa* lung infection is difficult to eradicate due to the multiple (intrinsic and acquired) antibiotic resistance of bacteria and to their ability to produce a thick biofilm. Antibiotic treatment is hampered by poor antibiotic diffusion, efflux pump overexpression and the development of a persistent subpopulation with low metabolic activity. This is a cause for special concern in Cystic Fibrosis (CF) patients, where *P. aeruginosa* lung infection is the chief cause of morbidity and mortality. Combined tobramycin-ciprofloxacin treatment is routinely adopted due to the low frequency of resistant strains and its ostensible ability to control the infection. Nevertheless, symptoms usually recur, mainly due to the antibiotic persisters, which are difficult to detect in routine cultural microbiological assays. This chapter describes the issues involved in the microbiological diagnosis of *P. aeruginosa* lung infection in CF patients and the possible role of subinhibitory antibiotic concentrations in persister development and infection recurrence.

**Keywords:** *Pseudomonas aeruginosa*, biofilms, antibiotic resistance, bacterial persisters, viable but non-culturable forms, infection recurrence

## **1. Introduction**

Infectious biofilms have long been recognized as a severe clinical problem due to their tolerance to antimicrobials and their successful evasion of host defenses [1]. Their eradication is hampered by a variety of factors that are related to the sessile lifestyle and high cell density typical of biofilms, chiefly the poor diffusion of antibiotics and immune cells, the selection of antibiotic-resistant mutants, the development of intrinsic antibiotic-resistant phenotypes, like small colony variants (SCVs), and the spread of resistance genes among the bacterial populations through horizontal gene transfer (HGT) events. The problem is compounded by the development of antibiotic-unresponsive dormant cells, which upon reaching the late stage of dormancy can become non-culturable [2–4], thus escaping detection by routine culture-based assays [5, 6]. The difficulty of eradicating bacterial biofilms is a key factor in recurrent and chronic infections [1, 7, 8].

The opportunistic pathogen *Pseudomonas aeruginosa* is one of the bacteria most frequently involved in biofilm-related infections. Although most strains are environmental, the pathogen can live in symbiosis with a variety of hosts including plants,

insects and animals. In humans it is an important nosocomial pathogen responsible for a variety of infections that have a strong tendency to recur, particularly in burn patients and in those with lung involvement. Like other opportunistic pathogens, it typically affects immunocompromised individuals [9]. However, the subjects most prone to develop *P. aeruginosa* infection are patients with cystic fibrosis (CF).
