If a fixed volume of liquid medium is inoculated with microbial cells taken from a culture that has previously been grown to saturation and the number of viable cells per milliliter is determined periodically and plotted, a curve of the type shown in Figure 1 is usually obtained. The phases of the bacterial growth curve shown in Figure 1 are reflections of the events in a population of cells, not in individual cells. This type of culture is referred to as a batch culture. The typical growth curve may be discussed in terms of four phases (Table 1). Batch culture is a closed system with finite resources; this is very different from the environment of the human host where nutrients are metabolized by bacteria and human cells. Nonetheless, understanding growth in batch culture pro vides fundamental insight into the genetics and physiology of bacterial replication, including the lag, exponential, stationary, and death phases that comprise this process.

Fig1. Idealized bacterial growth curve plotting the log viable cell concentration versus time. Noted in the figure are the lag, log, stationary, and death phases with the approximate rates of increase or decrease representing what one would see upon inoculating a single bacterial colony in a closed batch culture system.

Table1. Phases of the Microbial Growth Curve

Lag Phase

 The lag phase represents a period during which cells, depleted of metabolites and enzymes as the result of the unfavorable conditions that existed at the end of their previous culture history, adapt to their new environment. Enzymes and inter mediates are formed and accumulate until they are present in concentrations that permit growth to resume.

If the cells are taken from an entirely different medium, it often happens that they are genetically incapable of growth in the new medium. In such cases, a long lag in growth may occur, representing the period necessary for a few variants in the inoculum to multiply sufficiently for a net increase in cell number to be apparent.

Exponential Phase

 During the exponential phase, the cells are in a steady state and grow as modeled in equations 5–7. New cell material is being synthesized at a constant rate, but the new material is itself catalytic, and the mass increases in an exponential manner. This continues until one of two things happens: either one or more nutrients in the medium become exhausted or toxic metabolic products accumulate and inhibit growth. For aerobic organisms, the nutrient that becomes limiting is usually oxygen. When the cell concentration exceeds about 1 × 10⁷/mL, the growth rate decreases unless oxygen is forced into the medium by agitation or by bubbling in air. When the bacterial concentration reaches 4–5 × 10⁹/mL, the rate of oxygen diffusion cannot meet the demand even in an aerated medium, and growth is progressively slowed.

Stationary Phase

 Eventually, the exhaustion of nutrients or the accumulation of toxic products causes growth to cease completely. In most cases, however, cell turnover takes place in the stationary phase: There is a slow loss of cells through death, which is balanced by the formation of new cells through growth and division. When this occurs, the total cell count slowly increases, although the viable count stays constant.

Death Phase

After a period of time in the stationary phase, cell viability begins to decrease at a defined rate. This varies with the organism and with the culture conditions; the death rate increases until it reaches a steady level. The mathematics of steady-state death is discussed as follows. In most cases, the rate of cell death is much slower than that of exponential growth. Frequently, after the majority of cells have died, the death rate decreases drastically, so that a small number of survivors may persist for months or even years. This persistence may in some cases reflect cell turnover, a few cells growing at the expense of nutrients released from cells that die and lyse.

 A bacterial culture phenomenon referred to as viable but not culturable (VBNC) cells, is thought to be the result of a genetic response triggered in starving, stationary phase cells. Just as some bacteria form spores as a survival mechanism, others are able to become dormant without changes in morphology. When the appropriate conditions are available (eg, passage through an animal), VNBC microbes resume growth.

مواضيع ذات صلة

Immunologic Detection of Microorganisms

Identification of Bacteria

Growing Bacteria in Culture

Cultivation of Listeria, Corynebacterium, and Similar Organisms

Pathogenesis and Spectrum of Disease for Listeria, Corynebacterium, and Similar Organisms

The Measurement of Microbial Concentrations

Bacterial Pathogenesis

Distribution of Normal Flora in The Body

Cultivation of Bacillus and Similar Organisms

Diagnostic Laboratory Tests for Borrelia Burgdorferi

Bacterial Cell wall

Bacillus cereus