The Membrane Filter in Marine Microbiology

The advent of the membrane filter has opened a new path for the study of bacteria. The apparatus and procedures have been described by Clark et al. (1951) and Goetz and Tsuneishi (1951). The membrane filter used in these experiments employs a filter membrane consisting of a mixture of cellulose esters which can be carefully controlled to provide different effective pore sizes (Goetz, 1951). The effective pore size used in the following experiments is referred to as "Z " value which describes the specific flow resistance. Z is determined by the time required for one ml of dis-tilled water to pass through one cm of filter area at a pressure of 70 cm of Hg. A filter of Z = 0.8 to 1.0 seconds has an effective pore size of approximately 0.4 microns. The filter type Z = 1.5 to 2.0 seconds (table 1) was able to retain marine bacteria while a filter type Z = 0.8 to 1.0 seconds capable of complete retention of fresh water-born microorganisms proved to allow certain marine bacteria to pass. The filter membranes were sterilized with ethylene oxide vapor for four hours in a closed system. The abundance and kinds of bacteria retained