286 



VETERINARY BACTERIOLOGY 



liquefying species are present, the count on the gelatin must be 

 made before all the slower-growing species have had a chance to 

 develop. 



Methods. Various dilutions of the water to be tested are 

 placed in a series of sterile Petri dishes, and the melted medium 

 to be used is poured in and thoroughly mixed. After the medium 

 has solidified, the plates may be kept at room-temperature, or, 

 better, placed in a thermostat which maintains a temperature of 

 about 20 to 22. The number of colonies developing upon a 

 given plate, multiplied by the dilution introduced, will give 

 approximately the number present per cubic centimeter in the 

 original sample. The final count should be made only after the 

 lapse of several days or a week. Discrepancies will usually be 

 detected between the numbers, as determined from the plates 

 containing the lesser and the greater dilutions. It is customary 

 to use the plate having the nearest to 200 colonies in making the 

 final estimation. Where more than this number of colonies are 

 present, it is probable that many more have failed to develop at 

 all, or to a size that can be readily detected, on account of the over- 

 crowding. The numbers of bacteria can, of course, be determined 

 only approximately by the higher dilution; it is, therefore, cus- 

 tomary to follow the mode of expression suggested by the committee 

 on standard methods of the American Public Health Association: 



NUMBERS OF BACTERIA FROM 



1-50 shall be recorded to the nearest unit. 



51-100 



100-250 



251-500 



501-1000 



1001-10,000 



10,001-50,000 



50,001-100,000 



100,001-500,000 



500,001-1,000,000 



1,000,001-5,000,000 



Interpretation of Results. No standard for the number of 

 bacteria that may be present in potable water can be set because 

 of the various factors which may determine a high count. Stern- 

 berg, however, has suggested a standard that is in general ap- 



