Selective Media

Selective media inhibit the growth of organisms other than the one being sought. For example, Thayer-Martin agar is used to isolate Neisseria gonorrhoeae from clinical specimens. This is chocolate agar to which three or more antimicrobial drugs have been added. The antimicrobials inhibit fungi, Gram-positive bacteria, and Gram-negative rods. Because these drugs do not inhibit most strains of N. gonorrhoeae, they allow growth with little competition from other organisms.

Table 4.6 Ingredients in Two Representative Types of Media that Support the Growth of E. coli

Nutrient Broth


(complex medium)

(defined medium)



Meat extract

Dipotassium phosphate


Monopotassium phosphate

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