The kinetics of microbial growth

Unicellular organisms divide by binary fission; each cell grows to full size, replicates its genetic material then divides into two identical daughter cells. By identical means, two cells divide into four, four into eight and so on, leading to an exponential increase in cell numbers:

If we were to plot the number of cells in a population against time, we would get an exponential curve (Figure 5.8a). It is more convenient when plotting a growth curve

Cell numbers

Time (min)

Time (min)

Figure 5.8 (a) Under ideal physicochemical conditions, the number of cells in a population of a unicellular organism increase exponentially. (b) A plot of the log of cell numbers against time during exponential growth gives a straight line

Time (min)

Figure 5.8 (a) Under ideal physicochemical conditions, the number of cells in a population of a unicellular organism increase exponentially. (b) A plot of the log of cell numbers against time during exponential growth gives a straight line

phase

Figure 5.9 A microbial growth curve. The four phases of a typical growth curve are shown. See text for details phase

Figure 5.9 A microbial growth curve. The four phases of a typical growth curve are shown. See text for details to plot the logarithm of cell numbers of against time, giving us a straight line (Figure 5.8b). Such exponential growth cannot continue indefinitely, however, and growth usually slows down due to either the supply of nutrients becoming exhausted, or because metabolism leads to an accumulation of harmful waste substances. Unicellular growth usually occurs in a series of different phases (Figure 5.9):

1 Lag phase. When an inoculum of bacteria is first introduced into some growth medium, it will probably require a period to adapt to its new surroundings -the less familiar these are, the longer the period of adaptation (see also Box 5.5). If, for example, the carbon source in the medium is unfamiliar, the cells will need time to synthesise the necessary enzymes for its metabolism. The length of the lag phase will also depend on the age and general health of the cells in the inoculum. During this period, there is no net increase in bacterial numbers, however the cells are metabolically active.

2 Log (exponential) phase. When the bacteria have acclimatised to their new environment and synthesised the enzymes needed to utilise the available substrates, they are able to start regular division by binary fission. This leads to the exponential increase in numbers referred to above. Under optimal conditions, the population of cells will double in a constant and predictable length of time, known as the generation (doubling) time. The value for the widely used laboratory bacterium E. coli is 20 min, and for most organisms it is less than an hour. There are some bacteria, however, whose generation time is many hours. Thus, during exponential growth, the number of cells can be expressed as:

Inoculum is the term given to the cells used to 'seed' a new culture.

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