In Section 11.7.2 we described the batch growth curve based on biomass (Figure 11.22). If cell numbers are used instead, the curve will have a similar shape. However, cell number does not decrease because of decay. It will, on the other hand, decrease as a result of cell death. Thus, this final phase is also sometimes referred to as the death phase.

Note that as decay does not necessarily lead to death, so death, or loss of viability, does not necessarily lead to decay. Also like decay, death does not necessarily occur only in the final phase; rather, it becomes more apparent there because it is not masked by high growth rates. Despite these similarities, the mechanisms of death (such as predation, lysis, acute toxicity, and lethal mutations) and decay are potentially very different.

When using biomass as a measure of microorganism concentration, viability may be of importance (although it is often ignored). In this sense, viability refers to the fraction or percent of the biomass that is still "alive," or capable of growth. Thus, if we consider XL as the living biomass and XD as the dead biomass, the total biomass (XT) and viability (v) will be

The "growth" equation for each fraction can be considered using the same subscripts for the coefficients (although only mL is a real growth rate). First, the rate of change of the concentration of the living organisms is dependent on their rate of growth minus their rate of death:

The rate of change of the concentration of the dead biomass is dependent on the rate of formation from the death of living cells:

What is typically observed is the rate of change of the total biomass:

However, the rate of change of the total biomass can also be expressed as the sum of the rates for the two fractions:

dXT dXL dXD

dt dt dt L

The right-hand side of equation (11.14) also makes sense, in that only the living organisms can grow to produce more total biomass. However, this means that equations (11.13) and (11.14) can be set equal:

Substituting in equation (11.12) yields

Thus, the growth rate observed for the total biomass of a culture with a 10% viability is only 10% of the actual growth rate of the living organisms. This means that the living organisms are in fact growing 10 times faster than the apparent overall growth rate, in order to produce all of both the living and the dead biomass.

If cells die and lyse (or lyse and die), their cell constituents become available as substrates for other microorganisms. This is sometimes referred to as cryptic growth.

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