582 A MANUAL OF BACTERIOLOGY 



AMOUNTS TO BE PLATED, SIZE OF DISHES, etc. Gelatin. 

 For an ordinary water amounts of Ol, 0*2 and O3 c.c. 

 may be plated in Petri dishes of about 10 cm. diameter, 

 preferably done in duplicate. 



Agar. Two plates may be made with 0*1 and O2- 

 O3 c.c., and are preferably duplicated. 



The desired volume of water should be run into the sterile Petri 

 dish by means of a sterile 1 c.c. pipette graduated in hundredths, 

 The tubes of gelatin should be melted in a water-bath at a low 

 temperature (40 C.). A tube is taken from the water-bath, wiped 

 to prevent the adherent water running down into the Petri dish, 

 its mouth is singed in the Bunsen flame to sterilise it, and the 

 contents are then quickly poured into the dish and mixed with the 

 water by tilting the dish several times. 



The agar tubes must first be boiled, then cooled to about 45 C., 

 and similarly treated, or surface plates may be made. 



If waters are constantly being examined, it saves trouble to have 

 the gelatin and agar in small flasks, 30-60 c.c. of the former and 

 20-40 c.c. of the latter ; a flask of each will then be used for an 

 examination. 



In dealing with an unknown water, and in all cases of 

 doubt, additional plates should be prepared with a dilution 

 of the water (made with sterilised tap-water) of ten or 

 hundred fold, according to circumstances. 



The amount of the medium in a plate should be 10 c.c. 



The counting is done with the naked eye, preferably in 

 daylight, any doubtful colony being determined with the 

 aid of a lens or low power objective. The number of 

 liquefying colonies in the gelatin plates should also be 

 noted. The plates should be inspected daily, in order 

 that the count may be made earlier should liquefaction 

 render this necessary. 



In examining an ordinary drinking-water there is no need ever 

 to dilute. As 1000 or 1500 colonies can be counted in a plate, and 

 if the number on a plate should be, owing to crowding, uncountable, 

 ipso facto this would be sufficient to condemn without an actual 

 count. Dilution is necessary when dealing with river or other 



