Cross contamination has occurred when:

• A. A sterile environment is maintained throughout an experiment.
• B. All microbial cultures are destroyed using an autoclave.
• C. Microorganisms are unintentionally transferred from one object or surface to another.
• D. A single species of microbe is isolated in pure culture.

Answer: (C)

Verified step by step guidance

1. Understand the definition of cross contamination in microbiology: it refers to the unintentional transfer of microorganisms from one object or surface to another, which can compromise sterile conditions or experimental results.
2. Analyze each option given in the problem to see if it matches this definition:
3. 1. 'A sterile environment is maintained throughout an experiment' means no contamination occurs, so this is not cross contamination.
4. 2. 'All microbial cultures are destroyed using an autoclave' describes sterilization, which prevents contamination rather than causing it.
5. 3. 'Microorganisms are unintentionally transferred from one object or surface to another' directly matches the definition of cross contamination.
6. 4. 'A single species of microbe is isolated in pure culture' refers to successful isolation, not contamination.