The structure-function relationship of Pseudomonas aeruginosa in infections and its influence on the microenvironment

Abstract

Pseudomonas aeruginosa is a human pathogen associated with both acute and chronic infections. While intensively studied, the basic mechanisms enabling the long-term survival of P. aeruginosa in the host, despite massive immune system attack and heavy antimicrobial treatment, remain to be identified. We argue that such infections may represent niche invasions by P. aeruginosa that influence the microenvironment by depleting host-derived substrate and activating the immune response. Bacteria embedded in cell aggregates establish a microenvironmental niche, where they endure the initial host response by slowing down their metabolism. This provides stable, lasting growth conditions with a constant, albeit slow supply of substrate and electron acceptors. Under such stable conditions, P. aeruginosa exhibits distinct adaptive traits, where its gene expression pattern reflects a life exposed to continuous attack by the host immune system and antimicrobials. Here, we review fundamental microenvironmental aspects of chronic P. aeruginosa infections and examine how their structural organization influences their in vivo microenvironment, which in turn affects the interaction of P. aeruginosa biofilm aggregates with the host immune system. We discuss how improving our knowledge about the microenvironmental ecology of P. aeruginosa in chronic infections can be used to combat persistent, hard-to-treat bacterial infections.

Keywords: biofilm; chronic infections; host–pathogen interactions; immune response; microenvironment; quorum sensing.

Figure 1.

Conceptual drawing of the microenvironment of infections in the lung (left) and wound (right). Colonization by bacteria leads to innate immune activation by recognition of pathogen-associated molecular patterns (PAMP) and biofilm-associated molecular patterns (BAMP) by pattern recognition receptors (PRR) and the release of proinflammatory cytokines. Immune cell activation leads to increased O₂ consumption for the respiratory burst which, along with bacterial respiration, leads to lowered O₂ tension. In wounds, bacteria are found as monospecies aggregates separated from each other where different species appear to inhabit different zones of the wound. In lungs of CF patients, bacteria are found intraluminally embedded in thickened sputum. Bacterial interactions occur if signaling molecules reach high enough concentrations to elicit a response. The quorum sensing (QS) system has been shown to be lost or inactive in late infection stages.

Figure 2.

(A) Modeling of the radius of aggregates at which the O₂ concentration in the aggregate center goes to zero depending on the growth rate of bacteria (divisions hour^–1) and the O₂ concentration at the surface of the aggregate using the expressions from Stewart (2003). We used a yield coefficient of biomass on O₂, = 0.85 mg mg^–1, a biomass density of bacteria in aggregates of 2.0⋅10⁵ mg l^–1, a diffusion coefficient of O₂ in water, D_aq = 2.0⋅10^–5 cm² s^–1, and an effective diffusion coefficient in the biofilm, D_e/D_aq = 0.2. (B) The two examples of the influence of the growth rate and surface O₂ concentration on the aggregate size where the center exactly becomes anoxic.

Figure 3.

Bacterial colonization can lead to acute infections, which in healthy individuals are usually cleared by the immune response and in some cases with the aid of antibiotics. In immunocompromised patients, the infection can progress into a chronic state characterized by a continuous inflammatory response with collateral tissue damage, hypoxic conditions, and low bacterial growth rates, resulting in low antibiotic susceptibility. Alternative antipathogenic strategies include the use of QS inhibitors or quorum quenching enzymes to decrease bacterial expression of virulence factors and biofilm formation. The QS system has been shown be lost or inactive in late infection stages so the efficacy of using QS inhibitors is most likely restricted to a certain time window. The low growth rates and high antibiotic susceptibility of bacteria in chronic infections can be reversed by treating with supplemental O₂ by breathing pure oxygen in either normo or hyperbaric conditions. The associated higher tissue concentrations of O₂ will lead to increased bacterial growth rates and higher susceptibility toward antibiotics targeting metabolically active bacteria. Alternatively, inhalation of NO can lead to upregulation of phosphodiesterases that break down the biofilm promoting molecule cyclic-di-GMP resulting in disaggregation.