148 BACTERIOLOGICAL EXAMINATION OF MILK. 



to the plates may be done for four or eight samples, then the agar 

 poured, one tube to each plate, then another eight samples diluted, etc. 



INCUBATION. 



Methods. Concerning incubation two methods are at present in use. 

 Three-fifths of the laboratory workers consulted recommended incuba- 

 tion at 37 C. for twenty-four hours with saturated atmosphere, the 

 remaining two- fifths allowed varying lengths of time at different degrees 

 of room temperature and at whatever degree of humidity happened to 

 obtain. 



When considering these two methods many ad vantages. of the method 

 of incubation at 37 C. are evident, including the ease of maintaining 

 this temperature in any laboratory, the evident uniformity of counts so 

 obtained in different places as compared with those obtained by the 

 varying methods of technique, as to temperature, and incubation period, 

 where room temperature is employed, and the quickness with which 

 results are obtained, doing away with large accumulations of uncounted 

 plates. 



Forty-eight hour plates grown at 37 C. give a slightly higher count 

 (11), not enough higher to materially change the report, while the loss 

 by "spreaders" is increased and the count delayed. 



To secure saturation of the atmosphere the incubator should be made 

 with a shallow depression over the whole bottom surface, which may be 

 kept filled with water, or in default of this a large shallow pan of water 

 may be kept on one of the lower shelves. 



Much work will be done on comparison of 37 C. and ' ' room temper- 

 ature " during the coming year. 



Extract from 1908 report. Summarizing up the work on incubation, 

 we ma}- say that the weight of evidence is against 24 hours incubation at 

 37 C. and two day incubation at or around 21 C., the number of col- 

 onies obtained being too small for a fair idea of the number of bacteria 

 in the sample. 



Bight day and ten day incubations at 21 C., while giving a slightly 

 higher average count, have disadvantages, such as delay in reports, 

 accumulation of plates and drying out of media, which render them 

 undesirable for routine work. 



The lines of highest efficiency on a working basis would seem to rest 

 on a 48 hour incubation at 37 C. and a five day incubation at 21 C. 



It would seem advisable to recognize as standard both of these 

 methods of incubation (it being understood that carefully regulated 

 incubators should be used). 



Board of Health regulations governing the number of bacteria allow- 

 able in milk, should state the method to be used in examination and in 

 all reports, papers, etc. , on the bacterial count of milk this factor should 

 be explicitly stated. 



