142 liACTEKIOLOGICAT^ ]iXAMINATION OF MILK. 



are vised, the count is alwaj-s less than the actual nunil:er of bacteria 

 present, for the following reasons : 



. (a) Many bacteria in process of multiplication are held together by 

 adhesive 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 b3- e.xperiment 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 varj-ing forms of minute vegetable life reqvtire varying 

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

 velop at room temperature will not grow at body temperature. Some 

 require a ver3' high temperature for their best growth. 



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

 some onl}' where oxj-gen 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 slowlj-, as they obtain oxygen only 

 by diffusion. 



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

 requiring three or four da3-s. On the other hand, in plates grown for 

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

 ones (4). 



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

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

 develop in close proximit}- 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 diUition. 



(,?■) Spreaders and molds, by their rapid surface growth, merge with 

 other surface colonies and obscure deeper ones. 



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

 have shown a tendency to decrease in the number of Ijacteria 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). 



On account of these reasons strict adherence to standard procedure is 

 of e.special importance, since there are so man\- points where disagree- 

 ment ma_\' result if uniform technique is not followed. 



NOTi:. — The rt-ffrence munber.s occiirriiiir in the Coininittee report refer to the bibli- 

 ography iieconip.'uiyiiig" the .same, appearing on pa^e 1.50. 



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