Comparison of the MF and MPN Techniques in Examining Sea Water

Abstract

RESULTS OF A STUDY comparing coliform density values of sea water obtained by the membrane filter (MF) and the most probable number (MPN) procedures establish the former as a reliable technique if due regard is given to turbidities and bacterial densities in determining the volume of sample for filtration. The two techniques gave results 87.1 percent in agreement. The timeand material-saving features of the MF method as compared with the MPN technique (1) make it particularly desirable for examining the frequently large number of samples required for determining sanitary quality of waters in shellfish-producing areas. The Manual of Recommended Practice for the Sanitary Control of the Shellfish Industry (2) stipulates that in making bacteriological examinations of shellfish-producing areas the number of samples from each station should be sufficient to give a true picture of the number of coliform organisms present in the water under the various tide and weather conditions occurring during the shellfish-harvesting period. The minimum number of samples required varies, but, where examinations are made to define the line between approved and restricted areas, at least 15 samples per station during the period of survey appear necessary. The advantages of the membrane filter in the bacteriological examination of fresh water sources have been indicated (3, 4). It has also been suggested that the membrane filter method gives coliform densities comparable to those obtained by the MPN procedure, as applied to fresh water (5). Goetz (6) reports that in an ocean water survey results obtained by the MF method were more consistent and more accurate than data gathered by standard MPN procedures. Considering the advantages of the membrane filter method in the bacteriological examination of fresh water, it was deemed advisable to initiate a study to demonstrate the comparative performance of the membrane filter in the examination of sea water. Consequently, the present study, conducted in the Public Health Service Shellfish Sanitation Laboratory at Woods Hole, Mass., was undertaken to determine: 1. The comparison of coliform density values obtained by the membrane filter and the standard MPN procedures. 2. The effect of bacterial densities on the agreement of results by the two methods. 3. The influence of turbidity on the agreement of results by the two procedures.