BACTERIOLOGY OF THE OYSTER. 



87 



Results of B. Coli Test in Duplicate Tubes in Dilutions Indicated. 



The question now arises as to what numerical value we shall assign 

 to the dilutions which are positive in one tube and not in the other. 

 Is it proper to assign full value to these dilutions? Would it not be 

 fairer to assign one-half the value recommended by "Standard 

 Methods" to these dilutions, because basing our calculation upon both 

 tubes there are only one-half the B. coli present that would be indicated 

 by the positive result alone? 



Suppose now we wanted to transfer the positive result in the 1-100 

 c.c. dilution of oyster No. 4. Should it be transferred to the negative 

 tube in oyster No. 5, or to the 1-10 c.c. dilution of the same oyster? 

 Further, what shall we do with the positive result in the 1-100 c.c. 

 dilution of oyster No. 3. Shall it remain where it is or shall it be 

 transferred to the negative tube in the 1-10 c.c. dilution of oyster 

 No. 1 or 2? Again suppose in oyster No. 3 in the 1-100 c.c. dilution 

 both the tubes should be positive and there were only one tube negative 

 in the 1-10 c.c. dilution of any of the oysters, should these two positive 

 tubes be separated and transferred to the negative tubes in two of the 

 other oysters? 



But whatever method we use for transferring, shall we assign the 

 full value recommended by "Standard Methods" to the dilutions 

 which are positive in one dilution and negative in the other, or shall 

 we assign the better value, i. e., one-half the value recommended by 

 " Standard Methods? " By taking advantage of the various possibili- 

 ties we can obtain ratings varying between thirty, the lowest possible, 

 and one hundred and forty, the highest possible. In the first case 

 the oysters would be very near the permissible standard, while in the 

 other, the oysters would be considered badly polluted. If we use 

 three tubes in each dilution the matter is still further complicated. 



