In the lauryl tryptose broth test, which observation corresponds to a positive result?

• A. Gas production with purple-stained, spore-forming, filamentous bacteria
• B. Gas production with red-stained, nonspore-forming, rod-shaped bacteria
• C. Gas production with pink-stained, spore forming, cocci shaped bacteria
• D. Gas production with red-stained, nonspore-forming, filamentous bacteria

Answer: (C)

Lauryl tryptose broth is used to presumptively detect coliform bacteria by lactose fermentation that produces gas. The main signal of a positive result is gas production in the inverted Durham tube, indicating that the organism ferments lactose with gas as a byproduct. Coliforms are typically Gram-negative, non-spore-forming rods, so the crucial clue is the presence of gas rather than any staining color or a specific shape or spore status. If you see gas in the Durham tube, that points to a positive lactose-fermenting result in this medium, which aligns with identifying a probable coliform. The other observations that mention different staining colors, spore formation, or cocci shape aren’t consistent with the typical characteristics of coliforms, so gas production remains the decisive feature.