i 4 4 



PATHOGENIC BACTERIA. 



thin layer, so that all the colonies which develop may 

 be counted. 



The method, which originated with Koch, may be per- 

 formed with the Koch plates or with Petri dishes or 

 with Bsmarch rolls. It is always best to make a num- 

 ber of these plate-cultures with different amounts of the 

 water to be examined, using, for example, o.oi, o.i, 0.5, 

 and i.o c.cm. added to a tube of gelatin, agar-agar, or 

 glycerin agar-agar. 



The exact method must depend somewhat upon the 

 quality of the water to be examined. If the number of 

 bacteria per cubic centimeter is small, large quantities 

 may be used, but if there are millions of bacteria in 

 every cubic centimeter, it may be necessary to dilute the 

 water to be examined in the proportion of i : 10 or i : 100 

 with sterile water, mixing well, and making the plate- 

 cultures from the dilutions. 



FIG. 41. Heyroth's instrument for counting colonies of bacteria in Petri dishes. 



It is best to count all the colonies if possible, but when 

 there are hundreds or thousands scattered over the plate, 



