Gel-based proteomics of Gram-positive bacteria: a powerful tool to address physiological questions.

In this review, we demonstrate the power of gel-based proteomics to address physiological questions of bacteria. Although gel-based proteomics covers a subpopulation of proteins only, fundamental issues of a bacterial cell such as almost all metabolic pathways or the main signatures of stress and starvation responses can be analyzed. The analysis of the synthesis pattern of single proteins, e.g., in response to environmental changes, requires gel-based proteomics because only this technique can compare protein synthesis and amount in the same 2-D gel. Moreover, highly sophisticated software packages facilitate the analysis of the regulation of the main metabolic enzymes or the stress/starvation responses, PTMs, protein damage/repair, and degradation and finally protein secretion mechanisms at a proteome-wide scale. The challenge of proteomics whose panorama view shows events never seen before is to select the most interesting issues for detailed follow up studies. This "road map of proteomics" from proteome data via new hypothesis and finally novel molecular mechanisms should lead to exciting information on bacterial physiology. However, many proteins escape detection by gel-based procedures, such as membrane or low abundance proteins. The smart combination of gel-free and gel-based approaches is the "state of the art" for physiological proteomics of bacteria.