142 HACTKKIOI.OC1ICAI. EXAMINATION OF MILK. 



are used, the count is always less than the actual number of bacteria 

 present, for the following reasons : 



() Many bacteria in process of multiplication are held together bv 

 aclhesive membranes in pairs, chains or masses. It is for the purpose of 

 separating bacteria thus joined, as well as to obtain an even mixture, 

 that the sample itself and the diluted sample when plating are shaken. 

 This shaking, while it breaks up larger masses and shortens long chains, 

 does not to any great extent break apart the shorter chains, diplococci, 

 etc. Each of these groups of bacteria, when caught in the solid medium, 

 develops as a single colony. 



() It is impossible to obtain a medium suited to the food require- 

 .ments of all species or races of bacteria (2). (See foot-note.) 



It has been found by experiment that a medium consisting chiefly 

 of a watery extract of raw meat, alkaline to litmus and slightly acid to 

 phenolphthalein, will furnish the best food for the greatest number (3). 



(c} These varying forms of minute vegetable life require varying 

 temperatures for their best development. Many forms which will de- 

 velop at room temperature will not grow at bod}' temperature. Some 

 require a very high temperature for their best growth. 



(d) Some bacteria develop in an atmosphere free from oxygen, 

 some only where oxygen is present ; many are facultative growing under 

 either condition. Bacteria which require an oxygen -free atmosphere do 

 not develop in plates as generally prepared. Bacteria requiring oxygen, 

 if deep in the medium, develop but slowly, as they obtain oxygen only 

 by diffusion. 



(e) Many forms are slow in developing into visible colonies, some 

 requiring three or four days. On the other hand, in plates grown for 

 several days many small colonies are obscured in the growth of larger 

 ones (4). 



(/) Each bacterium requires a certain amount of nourishment for 

 development (5). There are also antagonistic forms which will not 

 develop in close proximity to each other. ' It therefore follows that in a 

 crowded plate, i. e., over two hundred colonies, many will not develop 

 (6). This is easily proven by making a higher dilution. 



Of) Spreaders and molds, by their rapid surface growth, merge with 

 other surface colonies and obscure deeper ones. 



(#) Samples kept in the collecting case at 34 F. for varying periods 

 have shown a tendency to decrease in the number of bacteria which will 

 develop into colonies (7). Samples kept in dilution water for several 

 hours have shown a marked decrease in the number of bacteria which 

 will develop into colonies (8). 



( )n account of these reasons strict adherence to standard procedure is 

 of especial importance, since there are so many points where disagree- 

 ment may result if uniform technique is not followed. 



NOTK. The reference numbers occurring in the Committee report refer to the bibli- 

 ography accompanying the same, appearing on puye I5n. 



