Two are better than one: increased biofilm formation in dual-type compared to single-type communities of Cutibacterium acnes

Cutibacterium acnes is a known opportunistic pathogen in orthopedic implant-associated infections (OIAIs). The species of C. acnes comprises distinct phylotypes. Previous studies suggested that C. acnes can cause single- as well as multi-typic infections, i.e. infections caused by multiple strains of different phylotypes. However, it is not known if different C. acnes phylotypes are organized in a complex biofilm community, which could constitute a multicellular strategy to increase biofilm strength and persistency. Here, the interactions of two C. acnes strains representing phylotypes IB and II were determined in co-culture experiments. No adverse interactions between the strains were observed in liquid culture or on agar plates; instead, biofilm formation in both microtiter plates and on titanium discs was significantly increased when combining both strains. Fluorescence in situ hybridization (FISH) showed that both strains co-occurred throughout the biofilm. Transcriptome analyses revealed strain-specific alterations of gene expression in biofilm-embedded cells compared to planktonic growth, in particular affecting genes involved in carbon and amino acid metabolism. Overall, our results provide first insights into the nature of dual-type biofilms of C. acnes , suggesting that strains of different phylotypes can form biofilms together with additive effects. The findings might influence the perception of C. acnes OIAIs in terms of diagnosis and treatment.