A process for treating a microorganism-containing stream includes the steps of passing the stream through a chamber, pressurizing the stream in the chamber to a pressure greater than 14.7 p.s.i.g., introducing a feed gas into the pressurized stream such that the feed gas is soluable within the microorganisms, and depressurizing the stream so as to cause the soluablized feed gas to expand within the microorganisms so as to rupture the cell walls of the microorganisms. The feed gas can be carbon dioxide, air, nitrogen, methane, or mixtures thereof.