BIOLOGY Volume 2 - A Guide to General Biology - 2004

12. MICROBIOLOGY AND BIOTECHNOLOGY

12.5. Bacterial Growth

12.5.1. Population Growth

When bacterial Cells reach a certain size, they undergo asexual reproduction known as binary fission, in which The Cell divides into two identical daughter cells (Fig. 2.11). The details of this process are described in Section 2.5.3. In this chapter, we will look closely at the growth of an entire population.

If a single bacterium is placed in a nutrient medium under optimal growth conditions, it and its descendants will divide every 30 minutes, as shown in Table 12.2.

Class="center">Table 12.2. Growth of a model bacterial population

12.5. If you have not yet answered Question 2.1 in Chapter 2, use the data from Table 12.2 to plot graphs of the number of bacteria (Graph A) and the lg of the number of bacteria (Graph B) against time. What can be said about the shape of such graphs?

The increase in cell number, as shown in Table 12.2, is characteristic of logarithmic or exponential growth. This is easily explained by considering row C in Table 12.2, where the number of bacteria is expressed as 2 raised to a certain power. The exponent can be called a logarithm (log) or an exponent. Logarithms, or exponents, form a linearly increasing sequence 0, 1, 2, 3, etc., corresponding to the number of divisions.

The numbers in Column A of Table 12.2 can be represented as base-2 logarithms, as shown in row D. (Compare row C with row D.) However, it is standard practice to use base-10 logarithms, as in row B. Thus, 1 is 10⁰, 2 is 10^0,3, 4 is 10^0,6, and so on.

The curve in Graph A in Question 12.5 represents a logarithmic or exponential curve. Such growth curves can be converted into straight lines by plotting them on a semi-logarithmic scale. Thus, under ideal conditions, bacterial growth is exponential. During exponential growth, the time required for the bacterial population to double remains constant. This is called the doubling time, or generation time, and it can be calculated from the graph.

The ideal model of Bacterial population growth can be compared to the growth of a real population in a closed vessel where there are no external influences, such as The addition of nutrients (Fig. 12.8). Note that two curves are shown, one of which reflects the total number of bacteria, including dead ones. In practice, this is the easiest number to determine (Section 12.6.2). The second curve, which is of the greatest interest for study, reflects the Number of viable bacteria (Section 12.6.1). This curve exhibits four phases. The first is the lag phase, during which bacteria adapt to their new environment and maximum growth rate is not yet achieved. During this period, bacterial cells may, for instance, synthesize new Enzymes required to assimilate the nutrients present in the new environment.

Fig. 12.8. Typical growth of a bacterial population.

The next phase is the logarithmic phase, when bacteria grow at their maximum rate and the number of bacteria increases almost exponentially, i.e., the growth curve is nearly a straight line. During this phase, the doubling time remains constant and reaches its minimum value. Over time, the growth of the colony begins to slow down, the doubling time increases, and the culture enters the stationary phase, where the population growth rate drops to zero and competition for food resources sharply intensifies. The formation of new cells slows down and then stops altogether. Any increase in cell number is offset by the simultaneous death of other cells, so the total number of living cells remains constant. The transition to this phase is determined by several factors: the depletion of essential nutrients, the accumulation of toxic waste products such as alcohol, and, in the case of aerobic bacteria, oxygen limitation. Bacterial growth also slows down with changes in pH.

During the final phase—the decline phase (or death phase)—The rate of cell death increases and exceeds the rate of reproduction. Eventually, the cells stop reproducing altogether. Methods for counting bacteria are described in the next section.

12.6. What would the growth curve look like if a bacterial sample were taken at the beginning of the culture's stationary growth phase, inoculated into fresh medium, and population growth was measured?

12.7. A bacterial culture was inoculated into a nutrient medium and maintained at 30 °C. Starting from point 0, at the specific time intervals listed in Table 12.3, the number of bacterial cells in the culture was counted. Using the data in Table 12.3, draw growth curves. Explain, using your graphs, what caused the changes in cell numbers.

12.8. What is the minimum doubling time (generation time) of the bacteria in Question 12.7?

Table 12.3. Bacterial culture at 30 °C

The same principles apply to the growth of any population, even human populations. Theoretically, any population can achieve exponential growth as long as the doubling time remains constant. sooner or later, however, limiting factors come into play, and The Study of these factors forms The basis of population ecology (Section 10.7).