Final answer:
The primary purpose of using Bismuth sulfite agar in microbiology, especially when isolating Salmonella typhi from feces, is to selectively promote the growth of Salmonella typhi while inhibiting the growth of other bacteria.
Step-by-step explanation:
The primary purpose of using Bismuth sulfite agar in microbiology, especially when isolating Gram-negative Salmonella typhi from feces, is to selectively promote the growth of Salmonella typhi while inhibiting the growth of other bacteria.
Bismuth sulfite agar contains bismuth and sodium sulfite, which create a toxic environment for most bacteria. However, Salmonella typhi is able to tolerate these inhibitory substances and will grow as black colonies on the agar.
This selective medium allows for easy identification and isolation of Salmonella typhi from a mixed microbial population in feces.