148 BACTERIOLOGICAL METHODS 



6. Determining Bacteria of the Colon Bacillus Group. — Meas- 

 ured quantities of the shell liquor of each of five oysters selected 

 from the dozen shall be placed in fermentation tubes containing 

 lactose-peptone-bile. The measured quantities shall be i cc, 

 o.io cc, and o.oi cc, or such other quantities or corresponding 

 dilutions as may be desired. The fermentation tube inoculations 

 thus prepared shall be incubated for 3 days at a temperature of 37° 

 C, and the presence of gas noted daily. From 10 to 85 per cent, 

 of gas during this period shall be considered a positive test indi- 

 cating a presumption of the presence of at least one bacterium of 

 the colon bacillus group in the quantity of the water used in the 

 test. But no final colon bacillus rating shall be made unless con- 

 firmatory tests for the presence of organisms of the colon bacillus 

 group shall have been obtained from the tube of highest or next 

 highest dilution from each oyster showing the presence of gas. 

 These confirmatory tests shall be begun immediately upon noting 

 the formation of gas and shall be carried out in conformity with 

 the procedure recommended by the Committee on Standard 

 Methods of Water Analysis. 



7. Statement of Results. — The results of the bacterial counts 

 shall be expressed as the number of bacteria per cc. The results 

 of the colon bacillus test shall be expressed either in the form of an 

 arbitrary numerical system or in estimated number of colon bacilh 

 per cc. of the sample. 



It is suggested that the arbitrary numerical method proposed 

 by the American Health Association be given the preference. The 

 following are the rating valuations according to this method. 



Colon bacillus in i .00 cc. but not in o. lo cc, a value of i 

 Colon bacillus in o. 10 cc. but not in o.oi cc, a value of 10 

 Colon bacillus in o.oi cc. but not in o.ooi cc, a value of 100, etc. 



The sum of these values for five oysters gives the total value of 

 the sample examined and this figure indicates the rating for 

 Bacillus coli. According to this system the highest (best) rating 



