VERIFICATION AND RESEARCH METHODS 



19 



same; but roughly, when a 10 x eye- 

 piece is used, one colony under a 

 16 mm. lens means 4,000 colonies per 

 cc. of milk, and the colonies in an 

 average field of a 4 mm. lens should be 

 multiplied by 100,000, and under a 

 1.9 mm. or oil immersion lens by 

 400,000. At least five representative 

 fields should be counted and aver- 

 aged. 



"Dilutions may be avoided with milks 

 expected to contain several million 

 bacteria per cc. by using 0.01 cc. A 

 drop of sterile milk should be added 

 before mixing with the agar in order 

 that the composition and consistency 

 of the medium may not vary greatly 

 from that used with the ordinary 

 dilution. Some special apparatus has 

 been found to be desirable."* 



This is essentially an agar plate 

 technique in which the count of colonies 

 can be secured within a shorter period 

 than that used in the standard plating 

 technique. This possibility makes it 

 highly desirable that further studies 

 be made with it. Some comparative 

 counts have already been made by the 

 author^^ and others who have used the 

 method report to the Committee that 

 they find counts similar to those 

 obtained by the author. The fact 

 that a microscope is used in making 

 the counts has caused some to con- 

 fuse this technique with that just 

 described as the Breed method. The 

 two are essentially different in that 

 in the one case, the actual bacteria 

 are counted as they exist in the milk, 

 while in the other case the count is a 

 count of colonies of bacteria which 

 have grown either from isolated bac- 

 teria or from clumps of bacteria. The 

 counts obtained by the Frost technique, 

 like those from the standard technique 

 ought not to be described as showing 



*Listed by the Central Scientific Co., Chicago. 



the number of individual bacteria 

 originally present in the milk. 



Because the counts obtained with 

 the Frost technique do not agree 

 exactly with standard counts, this 

 technique cannot be recommended as 

 a standard technique at present. This 

 fact ought not, however, to be inter- 

 preted as expressing a view on the part 

 of the Committee that the counts 

 obtained by experienced workers are any 

 less accurate than those obtained by the 

 standard technique. Very little data 

 has thus far been gathered even in com- 

 parative studies, and none is of sufficient 

 extent or accuracy to warrant making a 

 mathematical analysis of it in order 

 to establish the true accuracy of the 

 counts. 



(d) verification and RESEARCH 



METHODS 



Because of the fact that the Com- 

 mittee on Technique of the Society of 

 American Bacteriologists has under- 

 taken the study of methods of making 

 bacterial counts for research purposes, 

 it is not necessary to discuss further 

 the use of standard methods as research 

 methods. The standard methods are 

 designed for use in routine analytical 

 work and should also be used in those 

 cases where investigations involving 

 routine milk control are under con- 

 sideration. They may also be suitable 

 for use in other cases, but ordinarily 

 will not be found to give the grade of 

 accuracy expected in research work. 



There is in all routine laboratories 

 a very important use for methods giv- 

 ing more accurate data than can be 

 obtained from the use of the routine 

 count. These may be termed verifica- 

 tion methods; and they should be 

 used in all cases where administrative 

 actions are taken which depend upon 

 the analytical results^^ 



