264 CLEAN MILK 



follows that in a crowded plate, i. e., over two hundred colonies, f 

 many of the weaker forms will not develop. This is easily proven 

 by making a higher dilution. (See p. 254.) 



(g) Spreaders * and molds, by their rapid surface growth, 

 merge with other surface colonies and obscure deeper ones. 



(h) Samples kept in the collecting case at 34 F. for vary- 

 ing periods have shown a tendency to decrease in the number of 

 bacteria which will develop into colonies. Samples kept in dilution 

 water for several hours have shown a marked decrease in the 

 number of bacteria which will develop into colonies. 



On account of these reasons strict adherence to standard pro- 

 cedure is of especial importance, since there are so many points 

 where disagreement may result if uniform technique is not fol- 

 lowed. (See p. 241.) 



SCHEDULE FOR MAKING THE PRINCIPAL ARTIFICIAL MEDIA. 



(Arranged by H. W. Hill in accordance with recommendations 

 of the committee on standard methods: A. P. H. A.) 



Plain nutrient agar ; plain nutrient gelatin ; plain nutrient broth; 

 and their modifications (?'. c, glycerine agar, plain dextrose agar, 

 muscle-sugar-free dextrose (lactose, maltose, etc.) agar; muscle 

 -sugar-free dextrose (maltose, etc.) litmus agar; glycerine broth; 

 plain dextrose broth; sugar-free broth; muscle-sugar-free dex- 

 trose (maltose, etc.) broth; muscle-sugar-free dextrose (maltose, 

 etc. ) litmus broth, and other similar combinations, may all be made 

 by these methods. 



* " Spreaders " are, properly, intensely vigorous and motile bacteria; these tend to 

 travel over the surface of the agar, instead of remaining within the bounds of a def- 

 inite colony. But any surface colony may be spread over the surface of a plate, if 

 water, separated from the medium, condenses on the agar, or drops down from conden- 

 sation on the cover of the Petri dish. The use of earthenware (or " porous ") tops for 

 Petri dishes, in place of glass, excludes very largely spreading from this cause, reducing 

 the total of plates spoiled by spreading from nearly 40,^ to less than 3^. 



