Motility, virulence and biofilm formation by the human pathogen Acinetobacter baumannii are affected by blue light

MUSSI M. A.; GADDY, J.A.; CABRUJA, M.; VIALE A. M.; RASIA R.; ACTIS L.A.

Buenos Aires

Congreso; XII Congreso Argentino de Microbiología; 2010

Introduction: Light is a ubiquitous environmental signal thatmany organisms sense and respond to by modulating theirphysiological responses accordingly. While this is an expectedresponse among phototrophic microorganisms, the ability ofchemotrophes to sense and respond to light has become apuzzling and novel issue in bacterial physiology, particularly amongbacterial pathogens. In bacteria, response to light has beenascribed to the production of photoreceptors, with those harboringthe blue-light sensing using flavin (BLUF), the light, oxygen orvoltage (LOV) or the photoactive yellow protein (PYP) domainsbeing the most prevalent. Objectives: The unexpected findingmade in our laboratory that A. baumannii strain ATCC 17978motility is regulated by blue light at 24 °C prompted us to furthercharacterize the microorganism response to this signal.Specifically, since motility affects biofilm formation in other bacteria,we decided to study whether blue light also affects biofilmformation on abiotic surfaces. Moreover, we tested the effect ofblue light on the ability of A. baumannii to interact with eukaryoticcells using the Candida albicans model. Finally, we evaluatedwhether the BLUF domain-containing product of A1S_2225 geneis involved in photoresponse both by genetics and biophysicalmeans. Materials and Methods: Motility was evaluated in swimmingplates containing 0.3% agarose, which were inoculated on thesurface with bacteria lifted from LB agar overnight cultures usingsterile sticks, and incubated in the dark or blue light. For biofilmassays, one milliliter of LB broth was inoculated in glass tubes with0.01 ml of an overnight shaking culture, and incubated for fourdays stagnantly at 24 °C either in darkness or blue light. AA1S_2225 mutant was generated by insertion of a kanamycinresistance cassette. Besides, a His6-tagged derivative of theproduct of this gene was overexpressed in E. coli, purified andused to obtain spectra in the dark or light. Results: Motility andformation of biofilms and pellicles were observed only whenbacterial cells were incubated in darkness, while inhibited underblue light. In contrast, the killing of Candida albicans filaments wasenhanced when co-cultured with bacteria under light. Thesebacterial responses depend on the expression of the A. baumanniiATCC 17978 A1S_2225 gene, which codes for an 18.6-kDaprotein that contains an N-terminal BLUF domain and lacks anydetectable output regulatory signals. Spectral analyses of purifiedrecombinant protein showed its ability to sense light by a red shiftupon illumination. Therefore, the A1S_2225 gene, which isconserved among several members of the Acinetobacter genus,was named blue-light sensing A (blsA). Interestingly, we show thattemperature plays a role in the ability of A. baumannii to senseand respond to light via the BlsA photoreceptor protein.Conclusions: This work shows the unexpected ability of theopportunistic pathogen A. baumannii to sense and respond to bluelight by differentially expressing cellular functions that could playa role in its virulence properties.