A collection of strains is a set of living microorganisms stored securely and according to specific scientific procedures. 

Researchers may deposit the strains they have isolated during their research or used in their projects in a collection. These are then given a unique number and are referenced worldwide in a database. 
The main purpose is to make this material available to the scientific community and industry for research, teaching or development purposes. The collections also make it possible to ensure the traceability of the strains, and thus the continuation of further research.