C'ultivation and Identification of Bacteria 129 



of an organism, but these data do add weight to the other factors 

 considered in the identification process. 



Slant Cultures 



Media containing a soHdifying agent such as agar can be dis- 

 pensed in tubes while the media are still hot and in a liquid state. 

 By placing these tubes at an angle during the solidifying process, 

 the material will harden into what is termed a slant. Such a 

 preparation provides more surface area for inoculation. Care must 

 be exercised to avoid wetting the cotton stoppers with the medium 

 during the slanting operation, otherwise the natural filtering ability 

 of the cotton will be lost and contamination of the tube's contents 

 might well be expected. 



In addition to furnishing information as to the form, con- 

 sistency, and optical characters of the bacterial growth, the agar 

 slant also provides information relative to the color-producing 

 capacity ( chromogenesis ) of the species. Some media are better 

 than others for stimulating pigmentation of cultures; meat infusion 

 agar is one such medium. If the color produced by organisms is 

 water soluble, the pigment will diffuse throughout the agar. Most 

 bacterial pigments, however, are of the water-insoluble type, and 

 fail to leave the cell, at least not in detectable amounts. 



Nutrient Broth 



A young broth culture, in addition to providing information 

 about motility and natural arrangement of organisms, also displays 

 cultural characteristics, such as surface growth, clouding of the 

 medium, and sediment formation. A sudden jarring of the tube 

 will suspend the sediment, and the type of sediment can be de- 

 termined and recorded under the appropriate heading on the de- 

 scriptive chart. 



Selective Media 



This has been discussed earlier in this chapter and needs little 

 more elaboration than to point out that organisms which do grow 



