A comparison of the membrane filter with the most probable number method for coliform determinations from several waters.

At present, the most probable number (MPN) method, as described in "Standard Methods for the Examination of Water and Sewage" (1946), is used extensively for the determination of coliform populations in water and sewage. Briefly, this technique consists of planting three, five, or more replicate samples of water in at least three dilutions, in lactose broth. Tubes in which there is gas production after incubation must be confirmed for the presence of coliforms by use of brilliant green lactose bile broth or streaked on some differential medium for confirmation. This procedure' is time consuming, expensive, and laborious, and the results obtained leave much to be desired. With the advent of the molecsular, or membrane, filter there is offered a possible method for the bacterial analysis of water that may result in considerable savings in labor, time, and expense, and it may yield more accurate results. The molecular filter, as described by Goetz and Tsuneishi (1951), is a cellulose acetate membrane suitable for the removal by filtration of bacteria and other particulate materials from either liquids or aerosols. Subsequent to filtration the membrane is removed from the filtering unit and transferred to an absorbent pad saturated with the desired medium contained in a small Petri dish. The dish is then inverted and incubated in a saturated atmosphere for a predetermined time and temperature. The membrane can be transferred from an enrichment medium to a selective or differential medium, thus allowing the development of the organisms sought. The advantages offered by such a device are numerous. Goetz et al. (1951),, Clark, Geldreich, Jeter, and Kabler (1951) have demonstrated that coliforms could be detected and enumerated within 16 to 24 hours after filtration with the molecular filter when grown on a modified Endo broth resulting in a saving of time and labor, and probably producing more accurate results. Quite often the bacterial count of a water sample is so low as to make detection of organisms by ordinary plating and planting technique impractical. In clear waters the volume of water that can be sampled by filtration is limited only by the bacterial population of the water and the amount of debris that collects on the membrane. Obviously, waters with appreciable insoluble content tend to clog the filters. Considerable work has been done at the United States Public Health Service Environmental Health Center by Clark et al. (1951) in developing various selective and differential media for members of the enteric group of organisms. They describe the use of an enrichment medium, Albimi M lactose broth, in conjunction with a modified Endos broth for the detection of coliforms. Comparisons of the membrane filtration procedure to standard plate counts made with nutrient agar revealed that growth of coliforms in the membrane was at least equivalent to that obtained by the nutrient agar plate counts. Coliform densities of domestic sewage and waters in various stages of the treatment process were determined comparatively by the MPN procedure and the membrane filter technique. Their results indicated that there was no great difference between the two methods of coliform determination. However, they suggested that "a final answer will depend on the data obtained after trials in different treatment plants with waters of different degrees of pollution, varying chemical composition, and treatment methods. " This work was an attempt to compare the two aforementioned methods of coliform determination for several xvaters polluted initially with two different concentrations of Escherichia coli.