# Anti-Biofilm Agents to Overcome _Pseudomonas aeruginosa Antibiotic Resistance_
## 1 Introduction  
_Pseudomonas aeruginosa_ is a Gram-negative opportunistic bacterium able to survive in many environments and clinical settings. Its large genome (5.5-7 Mb) is composed of a conserved core and an accessory section with high plasticity. Therefore, the bacterium is extremely versatile and can easily adapt to harsh environmental conditions [1, 2, 3]. _P. aeruginosa_ is mostly responsible for pulmonary infections, but is also a major cause of urinary, skin, ear or eye infections, endocarditis and sepsis [4]. In particular, it is one of the leading causes of hospital-acquired infections (notably in intensive care units) and of chronic lung infections in patients suffering from cystic fibrosis (CF) or non-CF bronchiectasis, increasing mortality rates [5, 6]. The peculiar pathogenicity of this bacterium is strongly exacerbated by its innate virulence. Firstly, external components of _P. aeruginosa_ cells enable the establishment and persistence of infections. Notably, outer membrane lipopolysaccharidesaccharides (LPSs) protect the pathogen against phagocytosis and oxidative stress, but also promote as covering lectins the adhesion to host tissues. The type IV pili and flagellum allow planktonic cells to move from one infection site to another [7, 8]. In addition, the bacterium also secretes virulence factors. Their production is tightly regulated by quorum sensing (QS), the bacterial cell-cell communication network. QS is based on the production, release and perception ofsmall molecules called autoinducers (AIs) that coordinate the expression of specific genes involved in virulence behaviors. In _P. aeruginosa_, three inter-regulated QS systems, _las_, _rhl_ and _pqs_, are involved in the secretion of characteristic toxins (pyocyanin pigment, exotoxin A, HCN, exoenzymes ExoA, ExoU, ExoY and ExoS), siderophores (achromobactin, pyoverdines) and diverse proteolytic and lipolytic enzymes (type IV and alkaline proteases, elastases A and B, lipases A and C, phospholipase C) [7; 9; 10]. Not only can these virulence factors play a significant role in bacterial growth and survival towards immune system response, but they also cause critical damage to host tissues and worsen the clinical course of the infection. More importantly, QS regulates the production of biofilms. These protective structures provide favorable conditions for bacterial persistence, antibiotic (ATB) tolerance and recurrence of the infection. Biofilms are clusters of bacterial communities embedded in an extracellular matrix mainly composed of polysaccharides, nucleic acids, proteins, lipids and water [11]. This barrier offers protection against harsh external conditions (temperature, pH, physical stress) and against the immune system. The proximity between different bacterial species favors the transmission of genetic material including resistance-related genes [12]. These properties enable micro-organism dissemination in many environments including medical devices (catheters, implants, prosthetic valves or joints, endotracheal tubes) and in vivo tissues. Antibiotic resistance is also enhanced by this microenvironment in which sessile cells exhibit higher genotypic and phenotypic tolerance than planktonic forms. The MICs of antibiotics against bacterial biofilms are 10 to 1000 times higher than against planktonic cells [13; 14]. The matrix prevents the penetration of polar antibiotics such as tobramycin while the low oxygen and nutrient levels in deeper layers of the biofilm alter the effect of aminoglycos, quinolones or beta-lactams by slowing down bacterial metabolism, resulting in low protein, DNA or peptidoglycan synthesis [15; 16]. These dormant strains are able to tolerate high antibiotic concentrations and eventually repopulate the biofilm, leading to chronic infections [17]. _P. aeruginosa_ is one of the deadliest bacteria. In 2019, it was responsible for more than 500,000 deaths, which can be explained by its particular virulence, as well as the prevalence of multi-drug-resistant (MDR) strains [18].