The importance of being in or out

Phagocytosis is an essential function of host defenses against invading microbes, and the study of phagocytosis relies on knowing if a target object is bound to the outer surface of a cell, or has been internalized by it. In the inaugural year of FEMS Microbiology Letters, Dr Jan Hed published a novel technique for discriminating between intracellular and extracellularly bound bacteria and yeasts. The original method used crystal violet to quench the fluorescence emission of extracellular fluorescein-labeled bacteria and yeasts while leaving intracellular particles largely unaffected. The technique of fluorescence quenching was a major catalyst for phagocytosis research using living cells, in part due to its accuracy, flexibility and adaptability to a variety of experimental systems.