Genetic and environmental determinants of surface adaptations in Pseudomonas aeruginosa

Abstract

Pseudomonas aeruginosa is a well-studied Gram-negative opportunistic bacterium that thrives in markedly varied environments. It is a nutritionally versatile microbe that can colonize a host as well as exist in the environment. Unicellular, planktonic cells of P. aeruginosa can come together to perform a coordinated swarming movement or turn into a sessile, surface-adhered population called biofilm. These collective behaviours produce strikingly different outcomes. While swarming motility rapidly disseminates the bacterial population, biofilm collectively protects the population from environmental stresses such as heat, drought, toxic chemicals, grazing by predators, and attack by host immune cells and antibiotics. The ubiquitous nature of P. aeruginosa is likely to be supported by the timely transition between planktonic, swarming and biofilm lifestyles. The social behaviours of this bacteria viz biofilm and swarm modes are controlled by signals from quorum-sensing networks, LasI-LasR, RhlI-RhlR and PQS-MvfR, and several other sensory kinases and response regulators. A combination of environmental and genetic cues regulates the transition of the P. aeruginosa population to specific states. The current review is aimed at discussing key factors that promote physiologically distinct transitioning of the P. aeruginosa population.

Keywords: Pseudomonas aeruginosa; biofilm; environmental signals; quorum sensing; response regulators; sensor kinases; swarming motility.

Fig. 1.

Quorum-regulated lifestyle transitions in Pseudomonas aeruginosa. Hierarchical, inter-regulated quorum sensing circuits, Las, Rhl, and Pqs produce N-(3-oxo-dodecanoyl) l-homoserine lactone (C12-HSL), N-butanoyl l-homoserinelactone (C4-HSL), and 2-heptyl-3-hydroxy-4-quinolone (PQS), respectively. Las auto-inducer activates the expression of rhl and pqs operons; Rhl auto-inducer blocks the expression of pqs operon; Pqs auto-inducer activates the expression of rhl operon. These quorum systems collectively regulate the two major surface adaptations, swarming motility and biofilm formation.

Fig. 2.

Environmental factors inducing biofilm formation in Pseudomonas aeruginosa . Iron and phosphate sufficiency; magnesium scarcity; exposure to specific amino acids; host immune components; and antibiotic treatment trigger planktonic to biofilm transition in P. aeruginosa .