Tissue images and MS data were provided courtesy of Jessica Moore, Mass Spectrometry Research Center, Department of Chemistry, Vanderbilt University School of Medicine, Nashville, TN, USA.
Application & Background
Biofilms are multicellular microbial communities where cells stick to each other and often adhere to a surface (1). The study of biofilms is important given that most infections in the human body are caused by biofilms originated from one, or a combination of bacterial pathogens (1). Biofilms exhibit tissue-like heterogeneity due to not only the different bacterial subpopulations, but also the spatially-defined differentiation of cells that supports the overall survival of the biofilm (2, 3). During the process of infection, the concentration and distribution of nutrient metals fluctuate due to the efforts by the host to sequester essential minerals to limit pathogenicity in a process known as nutritional immunity (4).
Identification of a particular bacterial subpopulation within a biofilm has been previously achieved by RNA fluorescence in situ hybridization (FISH) and reporter gene fusion techniques (5, 6). The biggest limitation of these methods, however, is that they only support targeted experiments, requiring prior knowledge of the molecule of interest. Furthermore, the number of targets that can be assessed in a single experiment is limited. MALDI mass spectrometry imaging (MALDI MSI) has proven a powerful technology to detect analytes in tissue while preserving their spatial distribution, and thus comprises a unique approach to study bacterial biofilm heterogeneity.
P. aeruginosa is a biofilm-forming bacterium associated with pulmonary infections, particularly in patients with cystic fibrosis (CF). P. aeruginosa infects the respiratory tracts of 60-70% of CF patients by the age of 20 (7). Here we aimed to study biofilm architecture by visualizing protein distributions within P. aeruginosa biofilms grown both in vitro and developed in CF human lung. In addition, the effect of calprotectin, a key player in nutritional immunity, on biologically-relevant protein profiles was examined.
workflow for in vitro grown biofilms
Bacterial bioﬁlms were grown over 5 – 6 days, in a Drip Flow Bioﬁlm Reactor (DFR) (BioSurface Technologies) using glass microscope slides (VWR, USA; #48300-025) as growth surfaces. Figure 2 depicts a diagram of the experimental workflow, from biofilm culture, to MSI data acquisition. Biofilm sections were washed with 70% EtOH for 30 sec, 90% EtOH for 30 sec, 95% EtOH for 30 sec. 3:1 CHCA:DHB matrix was applied in 90% ACN using an HTX TM-Sprayer as described in Table 1. MS imaging of biofilms were performed at 50 µm step size, in linear positive ion mode, with 50 x 50 µm pixels and laser on single beam mode. 50 laser shots were collected in random-walk mode at each pixel.
workflow for biofilms on cystic fibrosis explants
Biopsies from explanted CF lungs were mounted onto chilled ITO coated glass. Tissue was sequentially washed with 70% EtOH (30s), 100% EtOH (30s), 6:2:1 EtOH / chloroform / acetic acid (2min), 100% EtOH (30s), H2O (30s), and EtOH 100% (30s). 3:1 CHCA:DHB matrix was applied in 90% ACN using an HTX TM-Sprayer as described in Table 2. The MALDI MS was operated at 50 by 50mm pixels with the laser in single beam mode. 500 laser shots were collected per pixel, in 50 shot increments.
Figure 3A shows an H&E stain of CF human lung tissue presenting significant inflammation. Gram-staining further revealed that bacteria with features consistent with P. aeruginosa and S. aureus co-colonized the infected airspace in the lung. A series of m/z protein ions were observed to localize in the inflamed airspace (Figure 3B). Calprotectin was also found to co-localize in these areas (8).from a CF patient. Selected protein ion images collected with ultra-high speed MALDI-TOF IMS are shown in Figure 3B.
Figure 4A shows MALDI MSI of four different cross-sections of a control P. aeruginosa biofilm. MALDI images highlight the heterogeneity of m/z spatial distributions along, and within, the biofilm. Analysis of P. aeruginosa biofilms challenged with the host protein calprotectin an antimicrobial protein that chelates essential nutrient metals) revealed metal-sensitive bacterial subpopulations within the biomass (Figure 4B).
Bottom-up proteomics analysis of grossly-dissected biofilms allowed for the identification of hundreds of proteins related to metabolism, stress, protein synthesis, and other biological processes. MALDI MSI signals identified using bottom-up proteomics (Figure 5) revealed distinct protein profiles in the center compared to the edge of the biofilm, as predicted by the presence of nutrient gradients (i.e. proximal vs. distal to nutrient influx).
These protein profiles were also found responsive to calprotectin exposure (Figure 5A-C). The application of MALDI MSI to the study of structural microbiology significantly furthers our understanding of how microbial communities respond to the host-imposed nutrient limitations during the process of infection.
The combination of MALDI MSI, LA-ICP IMS and proteomics provides a powerful tool for the study of protein profiles (i.e. identification and spatial localization) and metal homeostasis within a bacterial biofilm, granting us the opportunity to re-discover biofilm architecture and heterogeneity, which seems tightly correlated with patterns of nutrient(s) availability. The information and data collected by these technologies has high clinical implications given the frequent occurrence of bacterial co-infections in human disease.
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