FUNDAMENTALS OF MICROBIOLOGY - V. M. Samygin - 2015

CHAPTER 9. MICROBIAL GROWTH AND REPRODUCTION

Living organisms differ from the inanimate world primarily in their ability to grow and reproduce. A distinction is made between Cell growth and population growth. Cell growth refers to The process of an increase in the size, volume, and mass of each individual Organism. Growth begins after Cell Division, following which The Cell rapidly reaches maturity and then proceeds either to reproduce or enter a resting stage. Population growth is the increase in living matter (biomass) resulting from an increase in both the mass and the number of Cells per unit volume of the nutrient medium. Microbial reproduction is understood as the process of self-Replication that ensures the preservation of the species. Microbes employ a variety of reproductive Methods, including budding, sporulation, and sexual processes, but the majority of Bacteria reproduce via binary fission—that is, asexual division yielding two daughter cells.

Growth depends primarily on Temperature and environmental pH, the availability of nutrients, and ion concentrations. Obligate aerobes additionally require oxygen, whereas obligate anaerobes, conversely, require its complete absence. Bacterial cell growth is not unlimited. Upon reaching a certain size, a bacterial cell ceases to grow and initiates binary fission. In the fastest-growing bacteria (such as Escherichia coli or Vibrio cholerae), division occurs every 20 minutes. Cells originating from a mother cell constitute a new generation, and the time interval between divisions is termed the generation time. With a generation time of 20 minutes, a single microbial cell under favorable conditions could yield a microbial mass of about 400 tons after 36 hours, and after 48 hours its progeny would weigh 2.2 • 10³¹ g, which is approximately 4 thousand

times greater than the weight of the Earth. However, both in natural and artificial environments, bacterial reproduction is constrained by A number of environmental factors.

Bacterial reproduction follows certain regularities. Back in 1942, J. Monod, a scientist at the Pasteur Institute in Paris, graphically depicted bacterial reproduction using a "growth curve," which illustrates changes in microbial population size over time. A typical Microbial growth curve is shown in Fig. 9.

Class="center">Fig. 9. Growth curve of microorganisms during batch cultivation: I - lag phase; II - acceleration phase; III - exponential growth phase; IV - deceleration phase; V - stationary phase; VI - death phase

When bacteria are inoculated into fresh nutrient medium, they adapt to the new conditions and initially do not reproduce; this period is called the lag phase (I) (lag meaning delay). The duration of the lag phase averages 4-6 hours and depends on external conditions, species Specificity, and the physiological state

of the inoculated culture. Throughout the lag phase and the acceleration phase (II), the synthesis of relevant Enzymes and a significant increase in RNA content take place.

This is followed by the rapid reproduction phase (III), characterized by a logarithmic dependence of cell number on cultivation time, which is represented by an exponential curve. Accordingly, this portion of the growth curve is referred to as the logarithmic, exponential, or log phase. In this phase, cells divide at a maximum constant rate and exhibit peak biochemical and biological activity. The rate of cell reproduction during the exponential phase depends on the bacterial species as well as the nutrient medium used. Enterobacteria divide every 15-30 minutes, many soil species achieve a rate of 60-150 minutes, and Nitrobacter even takes 5-10 hours. During the exponential phase, Cell size and protein content also remain constant in many bacteria; that is, the bacterial culture consists of "standard cells." Because the rate of cell division is relatively constant in the exponential phase, this phase is most convenient for determining the suitability of various substrates, studying growth rates, and assessing the effects of physicochemical environmental factors (pH, redox potential, temperature, aeration, etc.). Biotechnologically valuable products (enzymes, Vitamins, NUCLEOTIDES) are synthesized during the exponential phase—these are the so-called primary metabolites of the bacterial cell.

Gradually, nutrients are depleted from the medium, while the medium becomes enriched with Metabolic waste products from the bacteria that inhibit reproduction (IV - deceleration phase), after which the bacteria enter the stationary phase (V), marked by an equilibrium between resting, dying, active, and newly formed cells. Toward the end of the logarithmic phase and the beginning of the stationary growth phase, secondary metabolites of the bacterial cell—such as Antibiotics, toxins, and pigments—are synthesized.

Growth rate depends on Substrate Concentration; as this concentration decreases, even before the substrate is fully exhausted, the growth rate begins to decline. Consequently, the transition from the exponential phase to the stationary phase occurs gradually. Growth rate can drop not only due to substrate scarcity, but also owing to high bacterial population density, low oxygen partial pressure, or the accumulation of toxic metabolic products; all these factors drive the transition to the stationary phase.

In the early stationary phase, bacterial cell size reaches a minimum. In the late stationary phase (as well as during the death phase), distorted or swollen cells, known as "involution forms," are frequently observed. This is associated either with cell Damage caused by Cell wall or Plasma Membrane enzymes, or with poor Regulation of cellular component synthesis. In Gram-positive bacteria, the stationary phase is typically accompanied by a loss of Gram-staining ability. Bacterial resistance to Physical and Chemical stresses (such as hypotonic environments or sudden temperature shifts) is higher in the stationary phase than in the log phase. Certain bacteria are characterized by The formation of endospores or exospores during the stationary phase.

The Number of viable bacteria present in a population during the stationary phase is referred to as the maximum concentration and is designated by the symbol M-concentration. This value represents the maximum concentration of viable cells per unit volume of nutrient medium. In other words, the M-concentration is an index characteristic of a given bacterial species under specific environmental conditions, changes in which lead to alterations in the M-concentration value.

The next phase is the death phase (VI). Cells cease division, perish, and disintegrate. The number of living cells drops sharply. Their death results from the action of several factors, one of which is the depletion of energy reserves within the cell.