Microbiology Lab Practical Practice Exam 2026 - Free Microbiology Practice Questions and Study Guide

Catalase activity is being tested. When an organism makes catalase, it rapidly breaks down hydrogen peroxide into water and oxygen. The release of that oxygen gas shows up as visible bubbles or effervescence right after you apply hydrogen peroxide to the colony, making bubbles the telltale sign of a positive result. If the organism lacks catalase, no gas is formed and you won’t see bubbling.

The other observations describe different reactions: a color change to pink would come from a pH indicator or a different test, clear halos around a colony come from diffusion of enzymes or antimicrobial effects rather than hydrogen peroxide breakdown, and precipitate formation points to a precipitation reaction, not catalase activity.