Re-discovering Bacterial Biofilm Heterogeneity with MALDI Imaging Mass Spectrometry

Significance of the Topic

Biofilms represent structured microbial communities whose spatial heterogeneity underpins survival, virulence and resistance in infections. Understanding the distribution of proteins and nutrient metals within biofilms is critical for elucidating host–pathogen interactions, in particular the mechanisms of nutritional immunity during infection.

Objectives and Overview of the Study

The study aimed to employ MALDI imaging mass spectrometry (IMS) complemented by bottom-up proteomics to:

• Characterize spatial protein and metal distributions in Pseudomonas aeruginosa biofilms grown in vitro and in cystic fibrosis (CF) lung tissue.
• Identify distinct bacterial subpopulations associated with nutrient gradients.
• Assess the effect of the host antimicrobial protein calprotectin on biofilm architecture and protein profiles.

Methodology and Instrumentation

Biofilm culture and sectioning:

• In vitro biofilms were grown for 5–6 days in a drip flow reactor on glass slides, embedded in 25% OCT, and cryosectioned at 12 µm.
• Human CF lung tissue sections (10 µm) were prepared by sequential solvent washes to remove lipids and salts.

MALDI IMS sample preparation:

• Matrix application (DHB/CHCA or DHA) using an automated HTX TM-Sprayer.
• ITO slides washed through graded ethanol series prior to matrix coating.

Imaging instrumentation:

• Bruker rapifleX TOF/TOF in linear positive ion mode.
• Parameters: 50 µm pixel size, 50 shots/pixel for biofilms; 50 µm, 500 shots/pixel for CF lung images.

Bottom-up proteomics:

• Gross dissection of biofilm center and edge regions followed by protein extraction (80% ACN, 5% FA).
• SDS-PAGE separation, in-gel reduction, alkylation, tryptic digestion.
• LC-MS/MS on Thermo LTQ with database search using Sequest and Scaffold analysis.

Main Results and Discussion

CF lung imaging revealed inflamed regions co-localized with P. aeruginosa/S. aureus and host calprotectin, with specific m/z signals (e.g., 4132.222, 4048.831, 2534, 2451) marking microbial and host proteins. In vitro biofilms displayed heterogeneous spatial patterns of protein ions; calprotectin treatment unveiled metal-sensitive subpopulations confined to distinct biofilm layers. Proteomic profiling identified hundreds of proteins differentially abundant in biofilm centers versus edges, reflecting nutrient availability and responding dynamically to calprotectin challenge.

Benefits and Practical Applications of the Method

• Unbiased, multiplexed spatial mapping of proteins and metals without prior target selection.
• Integration of imaging and proteomics provides a comprehensive view of biofilm microenvironments.
• Relevance to clinical microbiology for understanding infection progression and host defense mechanisms.

Future Trends and Applications

• Adoption of higher-resolution IMS platforms for single-cell biofilm analysis.
• Multi-omics integration (lipidomics, metabolomics) to further dissect biofilm complexity.
• Extension to polymicrobial biofilms and application in diagnostic imaging and treatment monitoring.

Conclusion

MALDI IMS paired with proteomics offers a powerful approach to dissect biofilm architecture and host interactions at molecular resolution. The insights into nutrient-driven heterogeneity and calprotectin response advance our understanding of biofilm-associated infections and support the development of targeted therapeutic strategies.

References

• Brockhurst MA, Hochberg ME, Bell T & Buckling A. Current Biology, 2006;16:2030–2034.
• Sagel SD et al. Impact of Pseudomonas and Staphylococcus infection on inflammation and clinical status in young children with cystic fibrosis. J. Pediatr., 2009;154:183–188.
• Wakeman CA et al. The innate immune protein calprotectin promotes Pseudomonas aeruginosa and Staphylococcus aureus interaction. Nat. Commun., 2016;7:11951.