Second only to media, safeguarding the stock cultures is the most important component of a successful microbiology laboratory. These must be handled carefully at all times to avoid contamination.
The care of the cultures starts upon receipt. A careful stock culture curator will confirm the identity of the received cultures, even if they come from as respected a source as a national culture collection. Mistakes can happen. The use of an incorrect strain in a compendial test could bring the results of weeks or months of work into question.
The chapter reinforces the compendial preference for the “seed lot technique” in culture maintenance. Critical to this is the need to go into your containers of stock culture only once, and in restricting the number of passages. Now, it must be stated that there is nothing magic about the number 5. This number gained popularity in the compendia through its use in the Sterility Test, and has been maintained for consistency. The point to the practice is that a careful lab will safeguard the purity and identity of their stock cultures by limiting the potential for “drift” due to excessive transfers.