BACTERIA IN WATER 693 



may be broken off by means of the wire, and water will be forced into 

 the bulb to satisfy the vacuiun. 



After the water has been collected it is important to plate it before 

 the bacteria in it have a chance to increase. The changes taking place 

 during transportation, even when packing in ice has been resorted to, 

 have been found by Jordan and Irons * to be considerable. It is impera- 

 tive, therefore, that plating of the water, if possible, shall not be delayed 

 for longer than one or two hours after collection. 



Before plating, the bottle containing the sample should be shaken 

 at least twenty-five times in order to distribute the bacterial contents 

 evenly. The quantities to be plated will depend to a certain extent upon 

 the probability of there being a large or small number of bacteria. 

 If less than two himdred are suspected, 1 c.c. of the water should be 

 taken out of the bottle with a sterile pipette and placed in a sterile Petri 

 dish. To this is added 10 c.c. of gelatin at a temperature of about 30° C, 

 or of agar at a temperature of not over 40° C. The water is thoroughly 

 mixed with the medium by repeated tilting of the plate, and finally al- 

 lowed to harden in the regular way. If unusual pollution or other data 

 lead to a suspicion that the bacterial count is apt to be extremely high, 

 it is advisable to dilute the sample of water with sterile water before 

 plating. The reason for mixiug water and medium in the Petri dish, 

 rather than in the tube, as was formerly done, is the fact that in the 

 pouring of the mixture from the tube a certain amount of residuum is 

 left which natm-ally leads to a diminution in the actual nimiber of 

 colonies developing in the plate. The gelatin' is incubated at 20° C 

 the agar at 37.5° C. 



Neither the gelatin nor the agar alone can give an accurate estima- 

 tion of the total bacterial contents of water. A better estimate can of 

 course be made when both are used. However, since aerobic and 

 anaerobic bacteria may be present and since many water organisms may 

 require nutritive conditions and a reaction other than that of the stand- 

 ard media, it is quite likely that none of the methods in use can give 

 an accurate total of organisms originally in the water. For sanitary 

 purposes, however, the agar counts, in which the plates are incubated at 

 37.5° C. or body temperature, are by far the most important since the 

 bacteria which grow at this temperature are the ones hkely to possess 

 pathogenic significance, or at any rate to emanate from animal and 

 human sources. It has been suggested by Rosenau^ and others that es- 



' Jordan and Irons, Reports of the Amer. Pub. Health Assn., xxv, 1889. 



^ Rosenau, "Preventive Medicine and Hygiene," 755. D. Appleton & Co., 1913. 



