Introduction of Pseudomonas aeruginosa
Overview
This lecture notes provide an overview of nosocomial infectious agent Pseudomonas aeruginosa.
Pseudomonas aeruginosa
Pseudomonas aeruginosa is a wide spread, gram negative, rod shaped, biofilm forming, pathogenic bacterium. It is found in a variety of habitats and has ability to infect both plants and animals including humans. Clinical significance of this bacterium is intensified due to phenomenon of natural tendency for acquiring drug resistance mechanisms and as a result arising of Multi-Drug-Resistant strains. P. aeruginosa causes chronic nosocomial infections in hospitalized, immune-compromised and transplant recipient patients. Infections from MDR strains of P. aeruginosa in immune-compromised and transplant patients are becoming a very serious healthcare issue.(Zhe-Xian Tian, Micheal Mac Aoga´in, Hazel F. O’Connor, Emilie Fargier, Marlies J. Mooij, Claire Adams, Yi-Ping Wang 2009; Burgess and Ii 2007; Levy and Marshall 2004)
Prominent infections by P. aeruginosa include skin burn wound infections, dermititis, eye infection, infections in transplant cancer and AIDS patients and lung infection and bacteremia in Cystic Fibrotic patients. In all these infections patients have weakened immune system and are unable to clear the infection effectively.
P. aeruginosa produces many primary and secondary metabolites with multiple bioactivities. Prominent of them all are Pyocyanin (PYO), Pyoverdine, Prorubin. PYO is a redox active blue Phenazine pigment. It has two benzene rings and a hetrocycle in the middle. It has structural similarity with Methylene Blue, which is a Thio-analog with a sulfur atom instead of Nitrogen (Gardner 1996; Byng et al. 1979; Watson et al. 1986).
Biosynthesis of PYO occurs under complete control and regulation of P. aeruginosa genome phz operons encode the genetic information required for PYO biosynthetic enzymes which convert a monocyclic molecule chorismate into tricyclic Phenazine-1-Carboxylic Acid (PCA) and then to Pyocyanin. Operons PhzA1B1C1D1E1F1G1 & PhzA2B2C2D2E2F2G2 encode enzymes which catalyze biosynthetic pathway and finally yield PCA. These operons are reported in multiple bacterial strains and aid in their pathogenicity while PhzM and PhzS modify PCA into PYO and are so far only reported in P. aeruginosa (Greenhagen et al. 2008; Gohain et al. 2006; Byng et al. 1979).
Being a redox active compound PYO alters the redox equilibrium inside a biological system. Upon detection of PYO inside a cell, cells utilize their NAD(P)H to avoid the damage caused by Reactive Oxygen Species (ROS)/ Oxidative Stress, NAD(P)H reduces PYO and get oxidized. The reduced PYO (PYOH2) then interacts with another PYO molecule both share a H atom each and become PYOH. Finally H atoms are transferred to Oxygen (the terminal electron acceptor in eukaryotic aerobic respiration at Electron Transport Chain) to regain their oxidized state PYO, and generate ROS (H2O2). As a result cells loss their stockpiles of energy to be generated in electron transport chain and become starved while the damage due to oxidative stress and ROS continues to mount (Price-whelan et al. 2007).
Bioactivities attributed to this pigment include antibacterial, antifungal, antiprotozoal, antiparasitic, antimalarial, immune-modulatory, pro-inflammatory, pro-apoptotic, enzyme inactivation and cytotoxicity. All these bioactivities are one or the other way related to generation of ROS and induction of oxidative stress (Hassan and Fridovich 1980; Muller 2006; Price-whelan et al. 2007; Muller 2002; Ra 2010; Bradley E Britigan et al. 1999; O’Malley et al. 2004; Bianchi et al. 2008; Denning et al. 1998; Cheluvappa et al. 2008; Fulfillment and Sinha 2008; Hashimoto et al. 2007).
PYO functions as a bio-control (antimicrobial) agent when P. aeruginosa faces competition over nutrients and / or habitat in environment, a signaling molecule in quorum sensing during and after the formation of biofilms, a virulence factor in infection, and one of the terminal electron acceptors when the bacterium respires anaerobically (Sadikot et al. 2005; Arai 2011; Winstanley and Fothergill 2009; Dietrich et al. 2006).
The lung infection by P. aeruginosa in Cystic Fibrotic patients is one of the few serious disease complications. Cystic Fibrosis (CF) is a genetic disorder in which a mutation in Cystic Fibrosis related Transmembrane conductance regulator protein effects all the body secretions and secretory organs as well. P. aeruginosa inside the lungs effects the host (lung and tracheal) tissues and cellular components of host immune system. PYO has been detected in higher quantities in sputum samples of P. aeruginosa infected CF patients, upto 100µM. These higher levels indicate the vital role PYO plays at the site of infection. Inside the lungs, studies have reported that PYO not only interacts with lung tissues but also with the cells of immune system. PYO induces apoptosis in Neutrophils, prevent the phagocytosis of these apoptotic neutrophils by Macrophages, increases the expression of inflammatory cytokines, inhibits ciliary beating of tracheal epithelial lining, depletes NAD(P)H stocks and induces apoptosis in the same. With all these actions this blue pigment tends to increase the survival chances and creates microenvironment inside lungs more favorable for P. aeruginosa growth (Cheluvappa et al. 2008; Bradley E Britigan et al. 1999; Muller 2002; Muller 2006; Winstanley and Fothergill 2009).
Cystic Fibrosis Related Liver Disease (CFRLD) is another important CF disease complication. In CFRLD liver suffers from cirrhosis and fibrosis due to abnormal secretions, but these symptoms also result from oxidative stress and ROS as witnessed in all the types of hepatitis including hepatitis A, hepatitis B, hepatitis C, Alcoholic hepatitis and Autoimmune hepatitis. In all these disease conditions multiple pathways and mechanisms are involved but the commonality is either favored generation of ROS or negative interactions with oxidative stress countering mechanisms of cell (Albano 2006; Namazi 2009; Lim et al. 2010; Colombo 2007; Muller 2009)