Which organism is lactose fermenting and hydrogen sulfide positive?

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Multiple Choice

Which organism is lactose fermenting and hydrogen sulfide positive?

Lactose fermentation and hydrogen sulfide production are two differential traits used to identify enteric bacteria on media like TSI or Hektoen. A lactose fermenter turns the medium yellow due to acid from lactose, while H2S production creates a black precipitate. Citrobacter freundii shows both traits: it ferments lactose and produces hydrogen sulfide, so you’d expect the characteristic yellow with blackening on the differential medium. E. coli also ferments lactose but typically does not produce hydrogen sulfide, so it lacks the black precipitate. Propionibacterium acnes isn’t a lactose-fermenting enteric and isn’t an H2S producer in this context. Hafnia alvei can ferment lactose but is usually H2S-negative. Therefore, the organism with both properties is Citrobacter freundii.

Lactose fermentation and hydrogen sulfide production are two differential traits used to identify enteric bacteria on media like TSI or Hektoen. A lactose fermenter turns the medium yellow due to acid from lactose, while H2S production creates a black precipitate. Citrobacter freundii shows both traits: it ferments lactose and produces hydrogen sulfide, so you’d expect the characteristic yellow with blackening on the differential medium. E. coli also ferments lactose but typically does not produce hydrogen sulfide, so it lacks the black precipitate. Propionibacterium acnes isn’t a lactose-fermenting enteric and isn’t an H2S producer in this context. Hafnia alvei can ferment lactose but is usually H2S-negative. Therefore, the organism with both properties is Citrobacter freundii.

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