Imaging mass spectrometry reveals complex lipid distributions across Staphylococcus aureus biofilm layers

Abstract

Introduction: Although Staphylococcus aureus is the leading cause of biofilm-related infections, the lipidomic distributions within these biofilms is poorly understood. Here, lipidomic mapping of S. aureus biofilm cross-sections was performed to investigate heterogeneity between horizontal biofilm layers.

Methods: S. aureus biofilms were grown statically, embedded in a mixture of carboxymethylcellulose/gelatin, and prepared for downstream matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS). Trapped ion mobility spectrometry (TIMS) was also applied prior to mass analysis.

Results: Implementation of TIMS led to a ∼ threefold increase in the number of lipid species detected. Washing biofilm samples with ammonium formate (150 mM) increased signal intensity for some bacterial lipids by as much as tenfold, with minimal disruption of the biofilm structure. MALDI TIMS IMS revealed that most lipids localize primarily to a single biofilm layer, and species from the same lipid class such as cardiolipins CL(57:0) - CL(66:0) display starkly different localizations, exhibiting between 1.5 and 6.3-fold intensity differences between layers (n = 3, p < 0.03). No horizontal layers were observed within biofilms grown anaerobically, and lipids were distributed homogenously.

Conclusions: High spatial resolution analysis of S. aureus biofilm cross-sections by MALDI TIMS IMS revealed stark lipidomic heterogeneity between horizontal S. aureus biofilm layers demonstrating that each layer was molecularly distinct. Finally, this workflow uncovered an absence of layers in biofilms grown under anaerobic conditions, possibly indicating that oxygen contributes to the observed heterogeneity under aerobic conditions. Future applications of this workflow to study spatially localized molecular responses to antimicrobials could provide new therapeutic strategies.

Keywords: Biofilms; CASI, continuous accumulation of selected ions; CFU, colony forming units; CL, Cardiolipin; CMC, carboxymethylcellulose; DAN, 1′,5′-Diaminonaphthalene; DAPI, 4′,6-diamidino-2-phenylindole; DG, diacylglycerol; DGDG, digalactosyldiacylglycerol; DHA, 2′,5′-Dihydroxyacetophenone; DsRed, red fluorescent protein from Discosoma; EFG, electric field gradient; FWHM, full-width half max; GFP, green fluorescent protein; IMS, imaging mass spectrometry; ITO, indium-tin oxide; Imaging Mass Spectrometry; L-PG, lysyl-phosphatidylglycerol; Lipids; MALDI, matrix-assisted laser desorption/ionization; OCT, optimal cutting temperature; PC, phosphatidylcholine; PG, phosphatidylglycerol; PIP, phosphatidylinositol phosphate; S/N, Signal-to-noise; TIC, total ion current; TIMS, trapped ion mobility spectrometry; TSA, tryptic soy agar; TSB, tryptic soy broth; Trapped Ion Mobility; XIM, extracted ion mobilogram.