Cultivation and Identification of Bacteria 123 



amount of inoculum is used, isolated colonies representing even 

 some of the minorities in mixed cultures can be examined for dif- 

 ferences in their size, shape, elevation, and pigmentation— the com- 

 mon criteria for identifying different species on a streak plate. At 

 times, however, it becomes necessary to employ enrichment (selec- 

 tive diet) technics to encourage the growth of certain organisms 

 found in small numbers in a given mixed culture. Final identifica- 

 tion of these isolated species involves a systematic elimination 

 procedure to be discussed later in this chapter. 



SELECTIVE AND ENRICHMENT MEDIA 

 The studies of Churchman and others have been instrumental in 

 the development of selective media for the isolation of specific 

 bacteria from mixtures. Certain dyes have been found to exert 

 a growth-inhibitory effect upon gram positive bacteria as a group, 

 while other dyes act similarly toward gram negative organisms. 

 In general, the gram positive species are more susceptible to dye 

 action than is true for gram negative bacteria, and diflFerences within 

 a given staining group also exist. By incorporating one or more 

 dyes into a medium, the task of isolating selected bacteria is sim- 

 plified. This principle has decided value in clinical laboratory 

 work, particularly in the detection of intestinal (enteric) patho- 

 gens, including typhoid and dysentery organisms. Such dyes as 

 malachite green, brilliant green, gentian violet, and others, can be 

 added in low concentration to various media for retarding growth 

 of selected organisms. 



Escherichia coli is one of the more numerous gram negative 

 species found in feces of warm-blooded animals, and because of 

 its aggressive characteristics, it might readily overgrow the enteric 

 pathogens one is trying to isolate by cultivation of feces organisms 

 in a clinical laboratory. Selective media provide a valuable means 

 of retarding the growth of Escherichia coli. Another gram negative 

 species, Proteus vulgaris, has the peculiar characteristic, especially 

 on primary isolation, of spreading over the entire surface of a cul- 

 ture plate, and this may cover up the growth of other organisms 

 being sought in clinical material. By adding chloral hydrate to 



