In the Presumptive Phase, what observation indicates bacterial presence?

• A. Gas bubbles
• B. Color change
• C. Sedimentation
• D. Turbidity

Answer: (A)

Gas bubbles signal lactose fermentation by bacteria in the Presumptive Phase. The setup uses a Durham tube inside lactose broth; if coliforms or other lactose-fermenting organisms are present, they metabolize lactose and generate gas that gets trapped in the Durham tube, producing visible bubbles. This gas production is interpreted as a positive sign of bacterial presence. Color change indicates acid production, turbidity shows growth, and sedimentation isn’t the standard indicator for this phase, so they’re not as reliable as gas formation for signaling bacterial activity.