## 1 Introduction  
Cystic Fibrosis (CF) is an autosomic recessive disease due to mutations in the cystic fibrosis transmembrane conductance regulator gene (CFTR) characterized by impaired chloride ion channel function, in which respiratory disease plays a prominent role in morbidity and mortality [1]. The lack of CFTR-mediated chloride secretion leads to the production of hyper-viscous mucus, impaired mucociliary clearance, and bacterial biofilm formation, which cause the failure of immune response within the lung [1]. Although the triple CFTR-modulator therapy has profoundly changed the CF scenario, significantly improving clinical parameters and the life quality of CF patients, the extent to which these therapies may reduce lung infections and restore a healthy-associated lung microbiome has yet to be fully determined [2]. Over the last two decades, inhaled antibiotics (tobramycin, colistin, and azirconam) have become the preferred therapeutic option, and studies have demonstrated improved pulmonary function and reduced exacerbation rates for inhaled versus oral antibiotics [3]. Despite the improvement of lung functions, current antibiotic treatments fail to eradicate _P. aeruginosa_ infection [3]. In addition to genetic resistance mechanisms, the difficulty of eradicating _P. aeruginosa_ infections is due to the acquisition of a mucoid phenotype by the infecting strains, the formation of biofilm, and the emergence of a subset of antibiotic-tolerant bacteria known as \"persisters\" [1,4,5]. Biofilm-residing bacteria exhibit significantly elevated antibiotic resistance, up to 1000 times more, than planktonic bacteria. The mechanisms for the antibiotic resistance of bacteria in biofilms are multiple. The first resistance mechanism is represented by the barrier effect played by the extracellular matrix of the biofilm, composed of exopolysaccharide, DNA, and proteins [6; 7]. Secondly, if an antibiotic can move through the superficial layers of the biofilm, it encounters a microenvironment rich in catabolites and poor in oxygen that could impact antibiotic activity [6; 7]. A third mechanism that contributes to the antimicrobial resistance of biofilm is the presence of persister cells in the deep biofilm layers [6; 7]. Persister cells are a subset of dormant cells, which lack proton motive force, transcription, and translation [8]. Although persister cells can arise stochastically in growing cultures, their formation can also be induced after exposure to antibiotic treatment, nutrient and oxygen deprivation, and oxidative stress [8]. The identification of new antimicrobial approaches able to act also against persister cells is needed. In this context, the possible use of probiotics to control respiratory infections in CF patients represents a recently emerged and particularly attractive strategy [9; 10; 11; 12]. To replace the use of live probiotics, thus taking into account safety concerns, different strategies have been developed, including the use of killed probiotics, microbial extracts, and cell-free supernatants [13; 14; 15]. Overall, these inanimate preparations of probiotics have been referred to as \"postbiotics\", but a debate still exists on the proper use of this term [16]. In postbiotics prepared from lactobacilli, several bioactive metabolites such as organic acids, short-chain fatty acids, and antimicrobial peptides have been described as [17]. Recently, we demonstrated a strong and fast antibacterial activity of cell-free supernatants from _Lacticaseibacillus rhamnus_ (LRM-CFS) in artificial sputum medium (ASM) towards clinical strains of _P. aeruginosa_ from CF lung, both in the planktonic and biofilm mode of growth, and a marked activity of the same supernatants against biofilms of wound pathogens in in vivo-like conditions [18; 19]. In this study, we tested in ASM the ability of LRM-CFS to kill persister cells of _P.