A Delayed Incubation Membrane Filter Test for Coliform Bacteria in Water

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Abstract
A delayed incubation coliform test on membrane filters is described for field application where shipment of refrigerated fluid samples cannot be made or during periods of natural or wartime emergency. This procedure consists of (1) filtration of sample at collection site; (2) placement of membrane on the preservative medium, i.e., benzoated Endo medium; (3) wrapping of individual Petri dishes in squares of "Parafilm"; followed by (4) unrefrigerated shipment to a laboratory for completion of the bacterial examination. This delayed incubation test was studied in parallel with MPN determinations on stored liquid samples and results from both were compared with results from an initial 5 tube, 3 dilution MPN test. The comparison was made on samples from 6 sources, consisting of 3 rivers, 2 farm wells, and a lake. From each source 3 samples were examined in a winter and in a summer series. Storage of liquid samples was at 5[degree]C, room temperature (13-32[degree]C) and at 35[degree]C for periods of 24, 48, and 72 hours. MPN coliform counts made on these liquid samples held at 5[degree]C for periods up to 72 hours were quite-variable but more closely approximated the initial MPN results than did the counts from liquid samples stored at 13-32[degree]C or at 35[degree]C. Estimations of coliform densities in water by use of the delayed incubation MF technique at 13-32[degree]C indicated good agreement with the MPN on stored samples at 5[degree]C but tend to be less than the initial MPN. The MF storage results at room temperature and at 35[degree]C were superior to results of MPN procedures on liquid samples stored more than 24 hours at room temperature or 35[degree]C.