Which of the following is a non-lactose-fermenting bacterium?

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Multiple Choice

Which of the following is a non-lactose-fermenting bacterium?

Explanation:
Lactose fermentation on MacConkey agar differentiates enteric Gram-negative bacteria by their ability to acidify the medium. Lactose-fermenters such as Escherichia coli, Klebsiella pneumoniae, and Enterobacter cloacae convert lactose to acid, causing pink colonies on MacConkey. A non-lactose-fermenter won’t acidify the medium, so its colonies appear colorless. Among the options, Salmonella enterica does not ferment lactose, so its colonies stay colorless on MacConkey agar, making it the non-lactose-fermenting bacterium. (Salmonella may also produce hydrogen sulfide on other media, which helps distinguish it from lactose fermenters.)

Lactose fermentation on MacConkey agar differentiates enteric Gram-negative bacteria by their ability to acidify the medium. Lactose-fermenters such as Escherichia coli, Klebsiella pneumoniae, and Enterobacter cloacae convert lactose to acid, causing pink colonies on MacConkey. A non-lactose-fermenter won’t acidify the medium, so its colonies appear colorless. Among the options, Salmonella enterica does not ferment lactose, so its colonies stay colorless on MacConkey agar, making it the non-lactose-fermenting bacterium. (Salmonella may also produce hydrogen sulfide on other media, which helps distinguish it from lactose fermenters.)

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