Impact of the Type VI Secretion System and AmpDh3 amidase on Pseudomonas aeruginosa's capacity to compete with cystic fibrosis-related non-fermenting Gram-negative bacilli

Abstract

[eng] The need for novel antipseudomonal therapies is driven by the severe clinical- epidemiological threat posed by Pseudomonas aeruginosa's rising levels of acquired antibiotic resistance, in the field of nosocomial opportunistic infections and in the unique context of Cystic Fibrosis (CF)-associated chronic infections. Identifying therapeutic targets is the first essential step in the process of drugs development, and in this regard, to deeply understand how different species can coexist and compete to occupy the CF niche, where P. aeruginosa is an extraordinarily successful species, could be an interesting source of therapeutic targets. For this reason, this study sought to better understand the importance of an enzyme, the peptidoglycan amidase AmpDh3, recently described to be essential for P. aeruginosa competition capacity against certain species not related to chronic niches. Thus, the role of AmpDh3 toxin but also of the type VI secretion system per se (T6SS, the mechanism through which this toxin is injected in competitor cells to degrade their peptidoglycans) were analyzed in terms of their importance for the competition capacity of P. aeruginosa versus typical rival species in the CF niche. To do so, in this work simple mutants deficient in ampDh3 and in retS (repressor of T6SS), and a double KO strain were used, in order to analyze the resulting P. aeruginosa phenotypes in the competition carried out against Stenotrophomonas maltophilia, Achromobacter xylosoxidans and Burkholderia multivorans (gram-negative species described to be a notable cause of chronic infection in CF patients) in solid medium. Our results indicate a variable importance of T6SS per se in the competition capacity of P. aeruginosa against the mentioned species: greater against S. maltophilia, much milder against A. xylosoxidans and despicable against B. multivorans. Meanwhile, although our results suggest that AmpDh3 slightly contributes to P. aeruginosa competitiveness against these species when the T6SS is de-repressed, its importance seems much minor compared to previous works in which it was shown to be essential to challenge species not related with CF. Future work will be needed to fully understand these facts and their potential therapeutic implications.