Strain-dependent and indirect effect of Stenotrophomonas maltophilia on Aspergillus fumigatus in mixed biofilm

Elise Melloul1, Lolita Roisin1, Paul-Louis Woerther1,2, Jacques Guillot1,3, Jean-Winoc Decousser1,2, Francoise Botterel1,2*

1 EA Dynamyc 7380 UPEC, ENVA, Faculté de Médecine de Créteil, Créteil, France 
2 Département de Microbiologie, DHU VIC, Hôpital Henri Mondor, AP-HP, Créteil, France 
3 EnvA, Alfort, France

Objectives: Aspergillus fumigatus (Af) and Stenotrophomonas maltophilia (Sm) colonize the respiratory tract of immunocompromised patients or with chronic pulmonary diseases and form biofilms. We recently discovered an antibiosis effect of Sm on Af in an in vitro mixed biofilm realised at 37°C in RPMI + FBS medium. The bacteria inhibited the fungal growth and induced an increase of fungal cell wall thickness, which led to abnormal mycelium with highly branched hyphae. Our last study showed that all the Af strains were susceptible to Sm effects. The aim of the present study was to assess the capacity of Sm strains to induce antibiosis effect on Af.

Methods: The effect of 9 Sm strains from different origins (human, animal and hospital environment) and genogroups (MLST profiles) was evaluated on Af ATCC 13073-GFP strain (Af13073). The antibiosis effects of these strains were checked after co-inoculation of fungal and bacterial inocula (105 conidia/mL and 106 or 108 cells/mL, respectively) on polystyrene supports in RPMI + FBS medium at 37°C for 24 h. The inhibition of fungal growth and the modification of Af phenotype were analysed by qPCR method and CLSM.

Results: The 10 Sm strains tested were not able to induce antibiosis effects on Af13073 in the conditions of biofilm formation (37°C, in RPMI + FBS medium). Strains belonging to genogroup 6 (from human clinical cases) were able to grow at 37°C and always induced inhibition of fungal growth and a modification of Af phenotype. The environmental strains were only able to grow at 25°C and did not induce antibiosis effect on Af at 37°C with 106 cells/mL of bacteria. However, with higher inoculum concentration (108 cells/mL), these strains had an effect on fungal growth and phenotype. A strain from genogroup 2 (Sm_2-1) was not able to induce phenotype modification even with a high concentration of bacteria. Both effects, fungal inhibition and phenotype modification, were due to different molecules secreted by bacteria in planktonic and biofilm cultures. 

Conclusion: In this study, the antibiosis effects of S. maltophilia seem to be strain-dependent and concentration-dependent, related to the bacterial growth rate.