Article Bacillus subtilis Biofilms Confocal microscopy Gene expression MALDI-FTICR Mass spectrometry imaging Metabolomics Multimodal imaging alanine arginine aspartic acid biofilm cell differentiation chemical cue confocal microscopy controlled study extracellular matrix fluorescence microscopy gene expression gene expression level germination glutamine iron mass spectrometry nonhuman phenotype sporogenesis surfactin tricarboxylic acid