Reproduction and Growth of Microorganisms

The below mentioned article provides a study-note on the method of reproduction and growth of microorganisms.

The ability of microbes to self-multiply is known as “Reproduction”. There will be increase in the number of individuals per unit volume during reproduction. Due to the synthesis of cellular material, the mass of bacterial cytoplasm is increased indicating the growth of microorganisms.

Method of Reproduction:

Bacteria reproduce by simple binary division; vegetative reproduction occurs in different planes and produces many kinds of cells (clusters, chains, pairs, packets etc.). They also reproduce by budding, by conjugation, fusion of the chromatin matter of two related species of bacteria.

Actinomycetes and many fungi reproduce by sporulation. One mother bacterial cell divides by the binary division in two equal daughter cells, representing a constant separation of daughter cells from the mother cell. After a certain number of generation, the mother cells age and die. This explanation does not support the concept of ‘bacterial immortality’.

The rate of cell division varies among bacteria and depends on:

(a) The species of bacteria,

(b) The age of the culture,

(c) The nutrient medium,

(d) Temperature,

(e) Carbon dioxide concentration, and

(f) Other factors.

The length of the generation of Escherichia coli, CI.welchii, Streptococcus faecalis is 15 minutes, while for cells of an animal tissue culture it is 24 hours.Thus bacteria reproduce almost 100 times faster than a tissue culture.

The increase in the number of cells can be expressed in the following way:

1 – 2- 4- 8-16 -32- N number of cells.

0-1-2-3-4-5-n number of generations.

The total amount of bacteria (N) after n generation will be equal to 2ⁿ per cell of seeded material. If we take the original amount of bacteria inoculated into the nutrient medium as a single individual and the time for one division as 30 minutes then theoretically the total amount of bacteria produced for 24 hours would be equal to N = 2⁴⁸. Reproduction of bacteria conforms to certain laws: The rate of reproduction the size of the bacterial population expressed as logarithm of the numbers of live cells per milliliter of the medium.

There are eight principal phases of repro­duction:

1. An initial stationary phase indicates the time from the moment of seeding the bacteria on the nutrient medium. Reproduction does not occur at this phase. The length of the initial stationary phase after seeding is 1 – 2 hours.

2. The lag phase of reproduction. Bacterial reproduction is not intensive during this phase, but the growth rate is accelerated. This phase lasts almost two hours.

3. Phase of logarithmic growth. In this phase there is a maximal division rate and decrease in cell size. The length of this period ranges from 5 to 6 hours.

4. Phase of negative growth acceleration. The rate of bacterial reproduction ceases to be maximal and the number of dividing cells diminishes. This phase lasts almost two hours.

5. Maximal stationary phase during which the number of newly produced bacteria is almost equal to the number of dead organisms. This phase continues for two hours.

6. Accelerated death rate. Here the equilibrium between the stationary phase and the bacterial death rate is interrupted. This continues for 3 hours.

7. Logarithmic death rate during which the cells die at a concentrate. This continues about 5 hours.

8. Decelerated death rate phase. In this phase cells which remain alive enter a dormant state.

The lengths of these periods are variable. The coli bacilli divide every 15-17 minutes; salmonellae of enteric fever – every 23 minutes; pathogenic streptococci every 30 minutes; diphtheria bacilli — every 34 minutes and tubercle bacilli — every 18 hours.