Question

An epidemic of milk-borne gastroenteritis caused by Salmonella was reported in an Indian city in 1985. Faecal samples from suspected individuals were inoculated into Selenite broth (high Selenium concentration) and incubated. After a thorough investigation, it was found that only Salmonella, and not the common gut bacterium Escherichia coli could grow in this medium. The basis of growth of Salmonella in Selenite broth is _______.

• A. Amensalism
• B. Synergism
• C. Adaptation
• D. Selection

Answer 1

Answer: (D)

Selection

Answer 2

Selenite broth is a selective medium. It contains sodium selenite, which inhibits the growth of most Gram-positive bacteria and many Gram-negative bacteria (like E. coli) but allows for the growth of Salmonella and Shigella. This differential growth is due to the selective pressure exerted by the medium, which favors the survival and proliferation of organisms like Salmonella that can tolerate the toxic effects of selenite, while inhibiting others. This process is known as selection.