What are the common storage techniques used to store the microbials samples for later experiments?

When storing microbial samples for later experiments it’s important to use the correct techniques so that the samples remain free from contamination and genetic changes until they are ready to be used. There are two main storage techniques. Short-term storage techniques are used to preserve samples that are used regularly. Long-term storage techniques are the better option for preserving samples that will not be used as frequently.  

Short-Term Storage

Microbes can be cultivated for brief periods on an appropriate growth medium containing agar, a solidifying agent. When stored at 4°C in laboratory fridges, agar plate cultures stay viable for several weeks.

Stab culture is another short-term but slightly longer term storage technique that involves inserting a single colony from an agar plate into a mix of agar and growth media. The duration of stab cultures ranges from weeks to a year, depending on the microbe species and genus.

These cultures use nutrients from the culture media to grow continuously, allowing them to survive for longer periods on available nutrients. 

Wrapping the plates securely with laboratory film helps prevent agar desiccation and contamination. Storing the plate with the agar side up helps to prevent condensation. 

The downside of short-term storage is the eventual desiccation of agar when stored for extended periods. Another drawback is the potential for microbial demise and unwanted genetic mutations as microbes continue consuming nutrients from the culture media and toxic metabolic waste continues accumulating. 

Long-Term Storage

Long-term storage involves cryopreserving microbes at temperatures below freezing at about -20°C or -80°C. Cells are either directly placed in a freezer or rapidly frozen using liquid nitrogen. The extremely low temperatures substantially reduce enzymatic activity, curtailing microbial metabolic processes. This method is suitable for storing all types of microbes for extended periods. 

Suspending cells in growth media containing cryoprotectants such as ~30% v/v glycerol before freezing helps prevent damage from ice crystal formation as ice expands when it freezes. Skimmed milk can serve as an alternative to glycerol.

For maximal recovery rates, it's advisable to freeze cultures during the stationary growth phase when cell concentration peaks.

Cultures stored at -20°C remain stable for approximately a year, whereas those stored at -80°C or in liquid nitrogen can remain viable for decades. To retrieve frozen microorganisms, gently thaw them at 37°C and inoculate cells into fresh growth medium.