Preserving pure cultures: Which practice(s) are used for proper maintenance and long-term preservation of pure microbial cultures?

Introduction / Context:
Maintaining pure cultures without genetic drift or contamination is central to reproducible microbiology. Different time horizons (short-term vs long-term) require different preservation strategies to maintain viability and phenotype.

Given Data / Assumptions:

• The options list common maintenance and preservation techniques.
• Goal: identify whether each is valid.
• Organisms vary in survivability with each method.

Concept / Approach:

Periodic transfer to fresh medium maintains active cultures for short-term work but risks contamination and selection for fast growers. Mineral oil overlay reduces oxygen diffusion and evaporation, extending shelf life of slants at ambient or refrigerated temperatures. Lyophilization (freeze-drying) provides long-term storage by removing water under vacuum after freezing; cultures remain stable for years when stored properly.

Step-by-Step Solution:

Verification / Alternative check:

Culture collections (e.g., ATCC protocols) document these methods among others (cryopreservation at -80°C or in liquid nitrogen). Mineral oil overlays and lyophilization are classic, widely taught approaches.

Why Other Options Are Wrong:

Choosing any single method disregards the others’ validity and complementary uses.

Rejecting all contradicts standard laboratory preservation practices.

Common Pitfalls:

Too-frequent subculturing causing genetic drift; improper sealing of lyophilized ampoules; using mineral oil with obligate aerobes that require higher oxygen transfer.

Final Answer:

All of the above