ABSTRACTS 



375 



the stain, stained, decolorized and cleared. When this dried and 

 stained plate culture is viewed under the microscope, the little colonies 

 are definitely stained and appear highly colored on a colorless or slightly 

 colored background. These colonies can be readily counted and the 

 number of bacteria per cubic centimeter calculated." — G. H. S. 



Counting Bacteria by Means of the Microscope. R. S. Breed and J. D. 



Brew. (N. Y. State Sta. Tech. Bui. 49, pp. 31, pis. 2, figs. 5.) 



This bulletin contains the results of tests of this method which have 

 been made since those published in an earlier bulletin of the station 

 (N. Y. Dept. Agr. Expt. Sta. Bui. 373, pp. 1-38 (1914) ). A general 

 description is given of the technique employed in applying this method 

 to milk, the various processes involved being discussed with reference 

 to possible errors. The result of this investigation are indicated by 

 the following quotations from the authors' summary: 



'The results obtained from the examination of samples of milk 

 collected in clean test tubes containing preservatives indicate that just 

 as accurate counts of the number of bacteria present can be made from 

 such samples as can be made if the samples are collected in sterile 

 tubes and iced. . . . 



"Capillary pipettes have been found to be more satisfactory for the 

 measurement of 0.1 cc. quantities of milk than standardized wire loops. 



"Faulty calibration of pipettes has been found to be a serious cause 

 of error. Allowance must be made for the adhesion of a certain quantity 

 of milk to the pipette if accuracy of measurement is to be secured. 



"It has been found that sterihzation of pipettes is an unnecessary 

 refinement of technique and that a single pipette may be used for mak- 

 ing preparations from a long series of samples, provided it is carefully 

 cleaned in glass-cleaning solutions after each day's use and also cleaned 

 by rinsing in fresh, clean tap water after using in each sample and 

 before passing to the next sample. Carelessless in cleaning pipettes 

 causes marked errors in counts. 



"Growth of bacteria has been found to take place in the drops of 

 milk as they dry so that it is important that these be prepared either 

 from samples containing preservatives or that the milk be dried quickly. 

 No growth was detected in the dried films even after incubation in a 

 moist, 37°C. incubator for one to four days. 



"The claim made by some that bacteria are removed when the fat 

 drops are dissolved by solvents does not seem to have any foundation 

 in fact. The dried milk-solids-not-fat appear to act as a practically 

 perfect fixative, no detectable mechanical loss of bacteria taking place 

 when the fat Vops are removed. On the other hand, serious errors 

 in count are introduced where the bacteria are stained in the milk 

 before the dried films are prepared, because in this way the bacteria 

 are not always sufficiently stained to make it possible to detect the 

 full number present. Where the fat drops are left in the fibns, even 

 though these be spread out so as to be in a very thin laj^er, they tend 

 to obscure bacteria and so lower the count. . . . 



