QUANTITATIVE BACTERIAL ANALYSIS 259 



-where possible it is well to have one man's work occasionally checked 

 "by another. 



Unless duplicate plates show as a rule approximately the same 

 count, the worker should see if there is error in his technique. 



Racks are very useful for stacking the plates and to prevent 

 breakage. 



Plating should be done always in a place free from dust or 

 ■currents of air. 



In order that the colonies may have sufficient food for proper 

 •development, 10 c.c. of agar shall be used for each plate. In plat- 

 ing a large number of samples at one time the dilution and transfer 

 of diluted 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.* 



Concerning incubation two methods are at present in use. 

 Three-fifths of the laboratory workers consulted recommended in- 

 cubation at 37" C. for twenty-four hours w'ith saturated atmos- 

 phere, 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 advantages of the 

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

 maintaining this temperature in any laboratory, the evident uni- 

 formity of counts so obtained in dfferent places as compared with 

 those obtained by the varying methods of technique, as to temper- 

 ature 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, not enough higher to materially change the report, while 

 the loss by " spreaders " is increased and the count delayed. 



* The lines of highest efficiency on a working basis would seem to rest on a 48 hour 

 incubation at 37" C. and a 5 day incubation at 21° C. It would seem advisable to recog- 

 nize as standard both of these methods of incubation. (Extract from 1908 Report of 

 American Public Health Association.) 



