Absence of Light Exposure Increases Pathogenicity of Pseudomonas aeruginosa Pneumonia-Associated Clinical Isolates

SIMPLE SUMMARY: An often-overlooked factor when designing an experiment is light exposure. Bacterial cultures are usually grown in an uncontrolled light environment, subject to natural daylight conditions. However, light as an abiotic factor has been described as a trigger for the switch from environmental to a pathogenic host-associated lifestyle. Pseudomonas aeruginosa possesses features that allow it to sense light and adapt its behavior accordingly. In this study, clinical isolates of P. aeruginosa from the sputum of pneumonia-diagnosed patients were assayed in two extreme light exposure conditions: constant illumination and intensity of full-spectrum light, and complete absence of light. The aim was to understand the influence of this factor on P. aeruginosa pathogenic potential, studied in interface with a human pulmonary epithelial cell line model. The preliminary findings here described evidenced that growth in the dark, which would more closely mimic the conditions in the lung microenvironment, was associated with a higher cytotoxicity against host cells, concordant with an increased pathogenic potential. Given the importance of accurately interpreting bacterial responses in in vitro studies, this study raises awareness of the need to control light exposure conditions to be as similar as possible to in vivo parameters. ABSTRACT: Pseudomonas aeruginosa can alter its lifestyle in response to changes in environmental conditions. The switch to a pathogenic host-associated lifestyle can be triggered by the luminosity settings, resorting to at least one photoreceptor which senses light and regulates cellular processes. This study aimed to address how light exposure affects the dynamic and adaptability of two P. aeruginosa pneumonia-associated isolates, HB13 and HB15. A phenotypic characterization of two opposing growth conditions, constant illumination and intensity of full-spectrum light and total absence of light, was performed. Given the nature of P. aeruginosa pathogenicity, distinct fractions were characterized, and its inherent pathogenic potential screened by comparing induced morphological alterations and cytotoxicity against human pulmonary epithelial cells (A549 cell line). Growth in the dark promoted some virulence-associated traits (e.g., pigment production, LasA proteolytic activity), which, together with higher cytotoxicity of secreted fractions, supported an increased pathogenic potential in conditions that better mimic the lung microenvironment of P. aeruginosa. These preliminary findings evidenced that light exposure settings may influence the P. aeruginosa pathogenic potential, likely owing to differential production of virulence factors. Thus, this study raised awareness towards the importance in controlling light conditions during bacterial pathogenicity evaluation approaches, to more accurately interpret bacterial responses.