Ordinarily not less than three portions of each sample should be tested, the portions being even decimal multiples or fractions of a cubic centimeter; for example, 10 cc., 1 cc., 0.1 cc., .01 cc., etc. It is essential that the dilutions should be such that the largest amount gives a positive test (unless the water is such as to give negative tests in 10 cc.), and the smallest dilution, a negative result. To insure this result, it is often necessary to plant four or five dilutions, especially in the examination of a sample of entirely unknown quality. The quantitative value of a series of tests is lost, unless all or at least a large proportion of the smallest dilutions tested have given negative results.
In reporting a single test, it is preferable merely to record results as observed, indicating the amounts tested and the result in each, rather than to attempt expression of the result in numbers of B. coli per cc. In summarizing the results of a series of tests, however, it is desirable, for the sake of simplicity, to express the results in terms of the numbers of B. coli per cc., or per 100 cc. To convert results of fermentation tests to this form, the result of each test is recorded as indicating a number of B. coli per cc. equal to the reciprocal of the smallest decimal or multiple fraction of a cubic centimeter giving a positive result. For example, the result: 10 cc. +; 1 cc. +; 0.1 cc. -; would be recorded as indicating one B. coli per cc. An exception should be made in the case where a negative result is obtained in an amount larger than the smallest portion giving a positive result; for example, in a result such as: 10 cc. +; 1 cc. -; 0.1 cc. +. In such case, the result should be recorded as indicating a number of B. coli per cc. equal to the reciprocal of the dilution next larger than the smallest one giving a positive test, this being a more probable result.
Where tests are made in amounts larger than 1 cc., giving average results less than one B. coli per cc., it is more convenient to express results in terms of the numbers of B. coli per 100 cc.
The following table illustrates the method of recording and averaging results of B. coli tests:
| Result of Tests in Amounts Designated. | Indicated Number of B. coli. | ||||
|---|---|---|---|---|---|
| 10 cc. | 1 cc. | 0.1 cc. | .01 cc. | per cc. | per 100 cc. |
| + | − | − | − | 0.1 | 10. |
| + | + | − | − | 1.0 | 100. |
| + | + | + | − | 10.0 | 1,000. |
| + | + | + | + | 100.0 | 10,000. |
| + | + | − | + | 10.0 | 1,000. |
| Totals (for estimating averages) | 121.1 | 12,110. | |||
| Average of 5 tests | 24.0 | 2,422. | |||
The above method of expressing results is not mathematically altogether correct. The average number of B. coli per cc., as thus estimated, is not precisely the most probable number calculated by application of the theory of probability.[[220]] To apply this theory to a correct mathematical solution of any considerable series of results involves, however, mathematical calculations so complex as to be impracticable of application in general practice. The simpler method given is therefore considered preferable, since it is easily applied and the results so expressed are readily comprehensible.
In order that results as reported may be checked and carefully valuated, it is necessary that the report should show not only the average number of B. coli per cc., but also the number of samples examined; and, for each dilution, the total number of tests made, and the number (or per cent.) positive.
10. INTERPRETATION OF RESULTS.
While it is not within the province of this report to suggest the proper interpretation of results obtained by the use of the methods herein specified as standard, the committee feels that a word of caution should be given regarding the significance of the presence in a water of members of the B. coli group as defined in this report. Recent work seems to indicate that the B. coli group as herein defined consists of organisms of both fecal and non-fecal origin. Therefore care must be exercised in judging the sanitary quality of a water solely from the determination of the presence of members of the group.