“Indirect pathogenicity” in cystic fibrosis during pulmonary exacerbations: Stenotrophomonas maltophilia modulates Pseudomonas aeruginosa virulence in a Galleria mellonella model.

Introduction During cystic fibrosis (CF) airway disease, periods of stability are punctuated by acute pulmonary exacerbations (PEs), whose physiology is not yet completely understood. Although Pseudomonas aeruginosa is considered the primary cause of PEs, it is often co-isolated with Stenotrophomonas maltophilia, whose pathogenic role is not yet clear. It is, therefore, plausible to hypothesize the occurrence of interspecies interactions able to affect P. aeruginosa virulence and persistence. In this study, we evaluated the interactions between P. aeruginosa and S. maltophilia in a Galleria mellonella systemic infection model, using strains co-isolated from the same lung of five different CF patients. Materials and Methods Eleven P. aeruginosa (RR8, BR1, DIN1, DIN2, DAT8, DAT9, DAT10, AC12a, AC12b, AC13a, AC13b), and six S. maltophilia (RR7, BR2, BR3, DIN3, DAT7, AC8) strains were co-isolated, during PE episode, from the sputa of five CF patients (RR, BR, DIN, DAT, AC). The in vivo effects of S. maltophilia on P. aeruginosa host response were evaluated in a wax moth model. Overnight cultures of S. maltophilia and P. aeruginosa grew in Trypticase Soy broth were washed and resuspended in PBS. Each larva (n=20/group) was inoculated, directly into the hemocoel via the right proleg, with 103 CFU of each strain, alone or in combination, or PBS only (control). Larvae were incubated in the dark at 37°C until 96h, and checked daily for survival. Results Survival curves of “RR” and “BR” strains showed no differences between groups of larvae infected with P. aeruginosa alone (RR8 and BR1) and in combination with the respective strains of S. maltophilia (RR7, BR2 and BR3, respectively). With regard to “DIN” strains, all S.maltophilia-P.aeruginosa combinations exhibited a comparable mortality rate compared to P. aeruginosa tested alone, but significantly higher (p<0.0001) than S. maltophilia. The same trend was observed for the combination AC8+AC12a with respect to S. maltophilia tested alone (p<0.0001). In “DAT” strains, no differences were observed between groups except for DAT7+DAT10 combination where mortality was significantly reduced (p<0.05) when compared to that observed in larvae infected with P. aeruginosa DAT10 alone (mortality at 24h: 5 vs 30%, respectively). The same trend was observed in larvae infected with AC8+AC13b when compared to larvae infected with the respective strains tested alone (p<0.05). Conclusions Overall, our results showed that S. maltophilia could indirectly contribute to lung disease, modulating P. aeruginosa host response. Particularly, the presence of S. maltophilia seems to mitigate P. aeruginosa virulence in G. mellonella model in a strain-dependent way. Further in vivo studies are needed to gain new insights in the pathogenic significance of microbial cooperation.