Kuolemavaihe

Kuolemavaihe, translated as the death phase, is a term used in microbiology to describe the final stage of a closed microbial growth curve. It follows the stationary phase and is characterized by a net decline in viable cell numbers, because the death rate exceeds the birth rate.

The phase arises as resources are exhausted and environmental conditions become increasingly unfavorable. Contributing factors include depletion of nutrients, accumulation of toxic metabolic byproducts, pH changes, and waste buildup. In aerobic systems, oxygen limitation can further stress cells. The initial decline is often rapid, but the rate can vary and may slow as some cells persist in a state of dormancy, become viable but nonculturable, or when resistant subpopulations emerge. In certain species, more durable survival strategies such as sporulation or formation of persister cells can occur, enabling persistence beyond the death phase.

In practice, the death phase is most evident in batch or closed systems where resources are finite. In continuous culture systems, like chemostats, steady-state growth can be maintained to delay or mitigate the death phase if nutrients are supplied and waste products are removed accordingly. Viability is typically assessed by plate counts and other viability assays, as optical density may continue to rise or plateau even when cells are no longer viable.

Understanding the death phase has implications for food safety, fermentation, and clinical microbiology, informing sterilization, preservation, and treatment strategies. It is distinct from the stationary phase, which involves little net change in viability, and from exponential growth, where birth rates exceed death rates.