High-quality culture
This page explains the definition of a high-quality bacterial culture.
Key takeaways
A high-quality culture is one where total cell counts and CFU/mL agree within a practical tolerance (e.g. ±25%).
When CFU/mL and direct cell counts align, it means that all cells in a culture are viable.
Poor-quality cultures occur when total direct cell counts are higher than CFU/mL, often in late stationary or death phase.
Non-culturable cells can still influence experiments or affect downstream propagation, so cultures with misaligned counts add uncertainty and risk.
What we mean by high-quality culture
A high-quality culture is one in which all cells are able to form colonies on solid agar under your defined plating protocol. In other words, BactoBox cells/mL and plate CFU/mL align within an agreed tolerance at the time of use. We suggest adopting a practical window such as ±25 percent to account for variability in the plating technique.
Why high-quality cultures matter
When BactoBox cells/mL[1] and CFU align, you remove ambiguity about the state of the cells you are working with. In contrast, a culture is of poor quality when BactoBox cells/mL and CFU do not align, most commonly with cells/mL results being significantly higher than CFU/mL. This commonly occurs in late stationary or death phase, i.e. when a fraction of the cell population is stressed, dormant, injured, dead, or perhaps temporarily non-culturable under the plating conditions. Failure to form colonies does not prove cell death[2]. Such cells may still proliferate in liquid culture or interact with your system. Because their behavior is uncertain, these mixtures add noise and risk to experiments or subsequent growth dynamics and can confound interpretation of results.
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[1] Using the default gating setting “BacTotal-v2024-10”
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