Abstract: A method, a sensor and apparatus for detecting biological activities in a specimen, for example in a blood sample, are provided in which a sealable container is sealed with a culture medium therein into which the sample is introduced, metabolic processes are enhanced in the presence of microorganisms in the sample and changes taking place in the concentrations of the substances. Such processes are detected and monitored with an excitation and detection assembly assigned to concentration sensors, in the form of optodes which are optically coupled to the excitation and detection assembly and to thereby to an evaluation unit for determining concentration changes of the substances over time as indications of the presence of microorganisms.