EXAMINATION OF SHELL FISH 487 
The sum of these values for the five oysters gives the total value 
for the sample and this figure shall be taken as the “‘ rating for B. col.” 
The results shall be expressed in the following tabular form: 
RESULTS OF TESTS FOR B. COLI IN DILUTIONS INDICATED 
Oysters 10ce. 0.1 c.c. 0.01 ce. Numerical Value. 
1 + + 0 10 
2 + + 0 10 
3 + 0 0 1 
4 se 0 0 1 
5 + 0 0 1 
Total, or rating for B. colt.. |... 0... ccc cee le cece cece cee ele wens 23 
Snaumaniteeel 
+ =Presence of bacteria of the B. colt group in fermentation tube test with lactose bile. 
0=Failure to demonstrate presence of bacteria of the B. colz group. 
Estimated Number of B. coli per Cubic Centimeter. If the stand- 
ard B. coli rating above described is divided by 5 or, in general, if the 
rating is divided by the number of oysters tested, the result will be 
approximately the number of B. colt per cubic centimeter of shell water. 
Partly because it does not do this exactly, but also for simplicity 
and the avoidance of fractions, the method of stating results as an 
arbitrary rating is preferred by the committee. Practical experience 
with the method also has appeared to justify this preference. 
Illustrations of the Application of the Method of Rating Oysters 
for B. coli. Sometimes results similar to the following are obtained, 
that is, one or more oysters may show positive results in small quan- 
tities of shell water, while an equal number may show negative results 
in larger quantities. In this case the next lower numerical value shall 
be given to the positive results in the high dilutions, and such positive 
results shall be considered as being transferred to a lower dilution giving 
negative results in another oyster. This is done on the theory that 
inconsistent results, mathematically considered, may follow naturally 
from an uncqual distribution of the bacteria in the shell water. This 
recession of the assigned values, however, shall not be carried beyond the 
point where the number of such recessions is greater than the number of 
instances where other oysters in the series failed to give positive B. colz 
results. 
As examples of the method of obtaining the rating for B. coli, the 
following illustrations are given. They represent results that may be 
met in practice. 
