1 86 GENERAL BIOLOGY OF MICRO-ORGANISMS 



card with a smooth black surface divided into squares by white 

 lines may be placed under a Petri dish and will be found to serve 

 very well. For the mere examination of the colonies no better 

 surface can be devised than the ferrotype plate used by pho- 

 tographers. The examination of the colonies will be easier if a 

 small hand-lens be used. Care must be taken not to mistake 

 air-bubbles or particles of dirt for colonies of bacteria. 



In any case, if possible, all the colonies in the plate should be 

 counted. But if this is not possible, the number contained 

 within several squares may be counted and the average taken; 

 knowing the size of the squares and the area of the plate, the 

 number contained in the whole plate may be calculated. 



The plating may be done by rolling the medium after the 

 manner of Esmarch. When the number of colonies is not large 

 this may serve very well. Counting may be assisted by drawing 

 lines with ink on the outer surface of the test-tube. It is obvious 

 that the character of the bacteria is of prime importance; that 

 pathogenic organisms may occasionally be present, even when 

 the number of bacteria to the cubic centimeter is small. But 

 knowing the number usually found in a good water-supply, any 

 sudden variation above that number is to be looked upon with 

 suspicion, as indicating a possible contamination. 



The bacteriological examination should always be accom- 

 panied by a chemical examination, and by an inspection of the 

 surroundings. A large number of bacteria is to be expected 

 when the water has been subjected to unusual agitation from 

 winds or currents which stir up the bacteria from the bottom. 



The Detection of Intestinal Bacteria. Bacillus coli is the 

 organism ordinarily sought as a proof of pollution of a water- 

 supply. Various quantities of the water, o.oi c.c., o.i c.c. and 

 i c.c. may be inoculated into three series of fermentation tubes 

 containing glucose broth or lactose bile. These are incubated 

 at 39 C. Plates are made from the tubes in which gas is pro- 

 duced and pure cultures obtained from the colonies for further 

 study and identification. The number of tubes in which gas is 





