The invention provides a composition containing an aqueous liquid phase, bacterial cells, and an ionic compound dissolved in the liquid phase. The ionic compound is selected from the group consisting of 1-butyl-3-methyl-imidazolium-thiocyanate, 1-butyl-3-methyl-imidazolium-2(2-methoxy-ethoxy)ethylsulfate, 1-methyl-1-[4-(3-methyl-3H-imidazol-1-ium)-but-1-yl]-3H-imidazolium-di(toluylsulfate), and 1-butyl-3-methyl-imidazolium-octylsulfate. The compositions of the invention are advantageously used for preparing lysates of biological cells, particularly bacterial cells.