Monitoring biotechnological processes involves acquiring information about key metabolic events and, ideally, single cell states should be determined to obtain comprehensive data on the physiological status of the surveyed population. In this paper, growth stages of the strain Acinetobacter calcoaceticus 69-V were characterised at the single cell level using flow cytometry. Four methods for analysing bacterial cellular characteristics by fluorescence were compared with respect to their sensitivity to changes in the physiological states induced by changing micro-environmental conditions. DNA analysis was confirmed to be highly informative with regard to the multiplication activity of the population. Measuring the membrane potential related fluorescence intensity (MPRFI) and the rRNA content were found to be useful for describing high-active cell states. A method for the measurement of the fluidity related fluorescence intensity (FRFI) was developed, since it allowed changes in the fluidity of the bacterial membrane to be detected, and thereby provided a valuable means of tracking adaptation of the population to micro-environmental deviations from optimal growth conditions.