Abstract: The present invention provides an indicator for the in-situ detection of the presence of a lysozyme or a microbe producing lysozyme at a location. The indicator comprises a layer (8) which is susceptible to degradation by lysozyme and a signaling layer (7) which is adapted to produce a detectable signal which indicates the presence of the lysozyme or the microbe producing lysozyme which is located at substantially the same location as the lysozyme or microbe. In use, the signaling layer is at least initially protected from contact with the lysozyme or microbe which is located at substantially the same location as the lysozyme or microbe by the degradable layer.