Analytical Methods - "Total coliform bacteria (most probable number, MPN, method) presumptive test" and "Total coliform bacteria (most
probable number, MPN, method) presumptive field test" by Gary G. Ehrlich, WRD, Menlo Park, Calif.


                                        December 18, 1974

QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 75.11

Subject: WATER QUALITY: Analytical Methods - "Total coliform 
        bacteria (most probable number, MPN, method) 
         presumptive test" and "Total coliform bacteria (most 
         probable number, MPN, method) presumptive field 
         test" by Gary G. Ehrlich, WRD, Menlo Park, Calif.

Attached is a copy of each of the subject provisional 
methods. The first, "presumptive test," produces results that 
are directly comparable with the multiple-tube fermentation 
technique described in Standard Methods (American Public 
Health Association and others, 1971, p. 662-678). The 
"Presumptive Field Test" is similar to the "Presumptive 
Test," except that provision is made for the incubation of 
samples under field conditions. Both tests are approved for 
use within the Water Resources Division when circumstances 
warrant.

The WRD's standard procedure for determining total coliform 
concentrations in water is the "Membrane Filter Method" 
described in Methods for the Collection and Analysis of 
Aquatic Biological and Microbiological Samples (Slack and 
others, 1973, p. 35-39). Under those conditions where the 
membrane filter (MF) method cannot be used (for example, with 
large concentrations of suspended sediment), it will be 
permissible to use the MPN method.

Two points should be noted. First, the results obtained from 
the MPN method are not directly comparable with results 
obtained with the MF method. Second, the precision of the 
presumptive MPN procedure is poor; for example, when 5 
aliquots of each dilution are incubated, the precision of 
results at the 95 percent confidence limit is 31 to 259 
percent of the most probable number of coliform bacteria.

For more precise results, it is preferred to confirm the 
actual presence of total coliform bacteria determined in the 
presumptive tests. For positive confirmation, use the 
procedure in Methods for the Collection and Analysis of 
Aquatic Biological and Microbiological Samples (Slack and 
others, 1973, TWRI, Bk. 5, Chap. A4, p. 44-45) starting with 
step #7.8 (p. 45). Calculations should be based upon the 
number of positive tubes determined in the confirmation test. 
Data obtained from the MPN confirmation test may be stored 
under parameter code 31505, "Coliform, Total, MPN, Confirmed 
Test,.35 deg. C."  No parameter code exists for results 
obtained with the presumptive test.

Questions or comments pertaining to the provisional 
procedures should be directed to the Chief, Quality of Water 
Branch (mail stop 412, Reston, Virginia).


                              R.J. Pickering
                              Chief, Quality of Water Branch

Attachment

WRD Distribution:  A, B. FO-LS, PO