6.1 At room temperature or in refrigeration

Isolates used daily or weekly (for example, control strains) can be preserved in refrigeration at 2–4°C. It is hard to put a time limit, but Salmonella and S. aureus might remain viable for up to 2–3 months in refrigeration and for shorter terms at room temperature.

The isolates to be stored are grown on an appropriate solid storage medium slant to reduce desiccation. Strict aseptic techniques during subculture and manipulations of the isolate are important to prevent bacterial and fungal contamination.

Frozen or freeze-dried ‘copies’ (see Sections 6.2–6.3) that can be resuscitated when needed, should be prepared and kept in reserve.

A final consideration concerns successive subcultures. These should be kept to a minimum due to the risk of mutations and phenotypic changes. For example, when repeatedly passaged in the laboratory, some strains may lose plasmids harbouring antibiotic resistance genes.