BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. DIVERSITY OF LIFE ON EARTH

2.3. Prokaryotes

2.3.5. Bacterial Population Growth

2.1. Let us consider a scenario where a single bacterial Cell is placed in a nutrient medium under conditions optimal for growth. Copy Table 2.4 and fill it in, assuming that this cell and all its descendants divide every 20 min.

Using the data from the completed table, plot the graphs.

On the vertical axis, plot the number of Bacteria (Curve A) and the common logarithm of this number (Curve B), and on the horizontal axis, plot time.

What can be said about the shape of these curves?

When the number of Cells increases, as seen in the completed Table 2.4, it is referred to as logarithmic, exponential, or geometric growth. In this case, we obtain an exponential series of numbers. This is much easier to understand by looking at row C in Table 2.4, where the number of bacteria is expressed as the number 2 raised to the corresponding power. The exponent can be called the logarithm or power of the number 2. The Logarithms, or exponents, form a linear series 0, 1, 2, 3, etc., corresponding to the number of generations.

Class="center">Table 2.4. Growth of a model bacterial population

Returning to Table 2.4, instead of the numbers located in row A, we can write their base-2 logarithms as follows:

Compare rows C and D. However, common logarithms (base 10) are usually used (see row B). In this case, 1 = 10⁰, 2 = 10^0,3, 4 = 10^0,6, etc.

The curve obtained from the data in row A (Table 2.4) is called a logarithmic or exponential curve. Such a curve can be transformed into a straight line by plotting The change in cell number over time on a logarithmic scale. Under ideal conditions, bacterial growth should theoretically be exponential.

Let us compare this mathematical model with the growth curve of a real bacterial population shown in Fig. 2.15. Four distinct phases of growth are clearly visible.

Fig. 2.15. Typical bacterial population growth curve.

1. During the lag phase, bacteria adapt to their new environment, and therefore growth does not yet reach its maximum rate. During this period, bacteria may, for example, synthesize new Enzymes required to assimilate the nutrients present in the new medium.

2. The logarithmic (exponential) phase is the period when bacteria grow at maximum speed, The Cell number increases almost exponentially, and the growth curve is practically a straight line.

3. Eventually, the growth of the colony begins to slow down, and the culture enters the stationary phase, where the growth rate drops to zero and competition for food resources sharply increases. The formation of new cells slows down and then stops entirely. The increase in cell number is offset by the simultaneous death of other cells, so the Number of viable cells remains constant (Table 2.5). The transition to this phase is driven by several factors: a decrease in nutrient concentration in the medium, the accumulation of toxic metabolic products, and, in the case of aerobic bacteria, a reduction in the oxygen content of the medium.

4. During the final phase — the death phase (decline phase) — cell death accelerates while reproduction ceases. Methods for counting cells are described in the practical exercises at the end of Ch. 12.

Table 2.5. Bacterial culture at 30 °C