In this paper we demonstrate that conventional absorption spectroscopy in the ultraviolet, visible and near-infrared spectral range facilitates characterization of Escherichia call (E. colt) suspensions. Two kinds of samples have been studied: (1) Untreated E. coli suspensions with systematically varied cell concentration and (2) E. coli treated by different inactivation procedures. For the purpose of inactivation the bacteria have been treated by either heat at elevated temperature as an established method or by hydrostatic or dynamic high pressure. The results show that at cell concentrations above a certain threshold extinction measurements in the ultraviolet region can yield a quantitative measure of the cell number density with optimal sensitivity and precision. Furthermore, examining suitable spectral regions the absorption spectra reveal characteristic features hence allowing identification of the treatment procedure later on. In conclusion, this study establishes a simple and cost-efficient approach for online and in-situ monitoring of processes for the inactivation of microbiological organisms. Moreover, the method provides a tool for the investigation of the inactivation mechanisms.