A system for determination of a value for at least one parameter describing microbial activity of individual biological organisms in a liquid sample, said system comprising: - an optical detection assembly comprising at least one image acquisition device adapted to acquire images wherein individual biological organisms may be identified, - at least one sample device comprising at least one sample container for holding a sample in liquid form, - at least one translating unit arranged to move said sample device and said image acquisition device relative to each other, - a control unit, wherein said control unit is a computing unit adapted to control said image acquisition device and said translating unit and adapted to sequentially acquire optical sectionings from said sample, comprising at least a first optical sectioning and at least a second optical sectioning of biological organisms in said liquid sample where the optical sectionings of biological organisms in said liquid sample are along a scanning path with an angle to the optical axis of the image acquisition device which is larger than zero and where each optical sectioning comprises one or more object focus stacks consisting of several images of an object imaged in different focus planes, - an image analysing device for analysing images and optical sectionings acquired from the sample container, wherein said image analysing device is programmed to identify individual biological organisms and to determine said value for said at least one parameter describing microbial activity of said individual biological organisms in each sample container and for each optical sectioning based on in- and out-of-focus images of the individual biological organisms for each sectioning and determining if a change in the value of the at least one parameter has occurred between the acquisition of the two optical sectioning; said parameter is selected from the group of cell division rate, cell viability, living/ dead rate, Brownian movements, metabolic rate, morphology, growth factor, kinetics, object mobility or focus behaviour. - an optical detection assembly comprising at least one image acquisition device adapted to acquire images wherein individual biological organisms may be identified, - at least one sample device comprising at least one sample container for holding a sample in liquid form, - at least one translating unit arranged to move said sample device and said image acquisition device relative to each other, - a control unit, wherein said control unit is a computing unit adapted to control said image acquisition device and said translating unit and adapted to sequentially acquire optical sectionings from said sample, comprising at least a first optical sectioning and at least a second optical sectioning of biological organisms in said liquid sample where the optical sectionings of biological organisms in said liquid sample are along a scanning path with an angle to the optical axis of the image acquisition device which is larger than zero and where each optical sectioning comprises one or more object focus stacks consisting of several images of an object imaged in different focus planes, - an image analysing device for analysing images and optical sectionings acquired from the sample container, wherein said image analysing device is programmed to identify individual biological organisms and to determine said value for said at least one parameter describing microbial activity of said individual biological organisms in each sample container and for each optical sectioning based on in- and out-of-focus images of the individual biological organisms for each sectioning and determining if a change in the value of the at least one parameter has occurred between the acquisition of the two optical sectioning; said parameter is selected from the group of cell division rate, cell viability, living/ dead rate, Brownian movements, metabolic rate, morphology, growth factor, kinetics, object mobility or focus behaviour.