Quantitative Bacteriological Examination. 37 



equal distribution of the bacteria. If the number of 

 bacteria present is probably not greater than 200, i c.c. is 

 then withdrawn with a sterile i c.c. pipette and delivered 

 into a sterile Petri dish of 10 cm. diameter. To this is 

 added 5 c.c. of standard 10 per cent gelatin at a tempera- 

 ture of about 30 C. or standard agar (7 c.c.) at 4o-42 C. 

 Should the number of bacteria per c.c. probably exceed 

 200, dilution is necessary. This is best accomplished by 

 adding i c.c. of the water in question to 9, 99 or 999, 

 etc., c.c. of sterile tap water according to the amount of 

 dilution required. The diluted sample is then shaken 

 thoroughly and i c.c. taken for enumeration. In order 

 to determine the number of bacteria originally present 

 it is only necessary to multiply by the factor 10, 100, or 

 1000, etc. 



When a sample of water from an unknown source is to 

 be examined it is generally desirable to make two check 

 plates at each of the above dilutions, selecting those dilu- 

 tions which give nearest to 200 colonies on the plates after 

 incubation as the ones on which to rely for the count. 

 A much smaller number will not give average figures, 

 and if more than 200 colonies are present on a plate many 

 bacteria will be checked by the waste products of those 

 which first develop and the count obtained will be too low. 

 After the addition of the diluted sample and the nutrient 

 medium, their thorough mixture in an even layer on the 

 bottom of the plate is obtained by careful tipping and 

 rotation. 



