QUANTITATIVE BACTERIOLOGICAL ANALYSIS. 11 



divide up the outer surface of the bottom of the Petri dish into 

 sections by straight lines running from the centre to the 

 periphery ; this can easily be done with a wax pencil. A small 

 hand magnifying glass will be found useful in differentiating 

 masses of granules in the gelatine from true colonies. The 

 result of the enumeration should be stated as so many aerobic 

 colonies or organisms which have developed at 20 to 22 C. after 

 from forty-eight hours to five days incubation, as the case may be. 

 Neisser recommended that the plates should be counted under the 

 microscope, and showed that the number obtained by this method 

 after twenty-four hours incubation would equal the number 

 obtained by the ordinary microscopic method after seventy-two 

 hours incubation. In tropical climates it is often impossible to use 

 gelatine, for during many months of the year the air temperature 

 is above the melting-point of this medium, consequently the plates 

 remain fluid and the bacteria are not kept apart so as to permit 

 a proper isolation of the colonies. Lender these conditions agar- 

 agar must be used as the nutrient medium. Agar tubes are 

 melted and seeded with the water after they have been cooled 

 down to 40 C. ; the mixture of water and agar is rapidly poured 

 out, as before described, into Petri dishes and allowed to 

 incubate at the prevailing temperature. A simpler method is 

 to pour out the melted agar into a Petri dish and allow it to 

 set ; the calculated amount of water is then introduced by means 

 of a pipette and the fluid evenly distributed over the surface of 

 the agar by means of a platinum spreader. The high tempera- 

 tures which obtain in tropical climates are prejudicial to the 

 development of water- organisms, so that the counts obtained on 

 the agar plates are smaller than those obtained from the same 

 water seeded in gelatine during the colder months. 



The interpretation of the results of a quantitative bacterio- 

 logical analysis is a matter of considerable difficulty. In order to 

 give a reliable opinion on the results obtained, it is necessary to 

 study the causes of the multiplication of micro-organisms and the 

 way in which they are influenced by light, temperature, movement, 

 and the chemical composition of the water. The bacterial contents 

 of the various sources of supply must also be examined to see if 

 they maintain, under natural conditions, a fairly constant 

 numerical composition. 



