METHODS DESCRIBED 



do not represent the highest count 

 that can be obtained.* 



It has been abundantly demonstrated , 

 that no plate count, with any technique 

 yet devised, gives the total number of 

 bacteria in milk. Hence, the standard 

 plate method should not be expected 

 to show all the bacteria possible, but 

 rather to furnish an artificial means 

 by which different laboratories can 

 get reasonably accurate comparative 

 results. Because some method gives 

 a higher count than the standard 

 technique does not prove that it is 

 better for control work. The best 

 count for this purpose would be the 

 one giving the most uniform per- 

 centage of the actual number of bac- 

 teria. Until the evidence is forth- 

 coming to show which method is best 

 from this standpoint, there is ample 

 justification for recommending media 

 that can be prepared as easily and 

 inexpensively as possible, although 

 other media may give higher counts. 



2. Verification and Research Methods. 

 For certain specific purposes it is 

 recognized that the routine methods 

 here described do not give the 

 full information or the grade of 

 accuracy required. For investigational 

 purposes or wherever it may become 

 necessary in a routine laboratory to 

 verify routine analyses for legal pur- 

 poses or for other reasons, more 

 accurate methods should be used. 

 These uses are likely to become even 

 more distinct than they are at present 

 as the grading of milk comes to be 

 more generally observed in com- 

 mercial practice. In the first report 

 issued^ these different uses for bacterial 

 counts from milk samples were not 

 recognized because the bacteriological 



*The most notable exceptions to this state- 

 ment are the ordinances for Boston and several 

 nearby cities. 



control of the milk supply had scarcely 

 started and no one could anticipate 

 what it might demand. 



METHODS DESCRIBED 



In spite of numerous attempts to 

 find satisfactory and simple biochemical 

 tests or tests for specific bacteria that 

 will give results as valuable as those 

 secured from bacterial counts none have 

 been found of such value that they 

 have come into general use. For this 

 reason they are not included in the 

 present report; but this conservatism 

 should not be used to discourage 

 investigation along these lines. 



The sediment test has been included 

 because investigations have made it 

 clear that the relation between milk 

 containing visible dirt and milk contain- 

 ing large numbers of bacteria is not 

 necessarily a close one. If sole reliance 

 is placed on bacterial counts in the grad- 

 ing of milk, it occasionally happens that 

 dirty milk, or milk that is undesirable 

 for other reasons, is given a stamp of 

 approval. Even the use of the sediment 

 test in addition to the bacterial analysis 

 does not certainly prevent this, as dirt 

 is easily removed from milk by straining 

 or clarification. Moreover, neither the 

 ordinary bacterial analysis nor the sedi- 

 ment test reveal the presence of patho- 

 genic bacteria, or undesirable oders or 

 flavors. 



The most important new develop- 

 ments in the five years since the last 

 report was issued, have been the in- 

 creasing use of modifications of the agar 

 plate technique intended still further to 

 simplify and shorten it, in improvements 

 in adjusting the reaction of agar media, 

 and in using microscopic examinations 

 of fresh milk as a means of controlling 

 its quality. The newer developments 

 in the technique of counting bacteria 

 have resulted in making it possible to 



