Some microorganisms can respire with extracellular electron acceptors using an extended electron transport chain to the cell surface. These organisms apply flavin molecules as cofactors to facilitate one-electron transfer catalysed by the terminal reductases and as endogenous electron shuttles. In the model organism Shewanella oneidensis, riboflavin production and excretion triggers a specific biofilm formation response that is initiated at a specific threshold concentration, similar to canonical quorum sensing molecules. Riboflavin-mediated messaging is based on the overexpression of the gene encoding the putrescin decarboxylase speC which leads to posttranscriptional overproduction of proteins involved in biofilm formation. We performed a mass spectrometry-based analysis of cells with and without speC overexpression to identify the effect of SpeC overexpression on the cell proteome.