Cross-contamination can occur during which of the following activities?

• A. Using the same pipette for different samples without sterilization
• B. Labeling petri dishes before inoculation
• C. Storing sterile media in a sealed container
• D. Observing cultures under a microscope without touching them

Answer: (A)

Verified step by step guidance

1. Understand the concept of cross-contamination: it refers to the unintentional transfer of microorganisms from one sample or environment to another, which can lead to inaccurate experimental results or contamination of cultures.
2. Analyze each activity to determine if it involves a potential transfer of microorganisms between samples or environments.
3. Consider 'Using the same pipette for different samples without sterilization' — this can transfer microbes from one sample to another, causing cross-contamination.
4. Evaluate 'Labeling petri dishes before inoculation' — this is a preparatory step that does not involve contact with samples and thus does not cause cross-contamination.
5. Review 'Storing sterile media in a sealed container' and 'Observing cultures under a microscope without touching them' — both maintain sterility and do not involve transferring microbes between samples, so they do not cause cross-contamination.