Growth and reproduction of bacteria

Growth curve in a batch culture

If microorganisms are placed in a closed liquid medium (without adding fresh nutrients and removing toxins), their development will be limited by resources. The graph, which plots time on the abscissa and the logarithm of the cell count on the ordinate, clearly distinguishes four stages:

1. Lag phase (2–5 hours). The adaptation period after inoculation. Cells do not divide yet, but their metabolism works at peak capacity—intensive synthesis of new enzymes is underway to break down the substrates of the specific medium.
2. Exponential (logarithmic) phase. The period of maximum and constant division rate. All cells during this period have standard sizes and a stable protein content.
3. Stationary phase. Growth ceases. An equilibrium is reached: the number of newly formed cells equals the number of dead cells. This occurs due to nutrient depletion, a drop in oxygen levels, high population density, and the accumulation of toxins.
4. Death phase. Microorganisms die en masse under the influence of their own metabolic acid products or as a result of autolysis (self-destruction by enzymes). This stage can last from ten hours to several weeks.

Features of growth on nutrient media

Manifestations of vital activity depend on the physical state of the substrate:

• Liquid media. Most often, bacteria cause diffuse turbidity of the liquid, form a surface pellicle, or settle into a bottom sediment. An exception is Leptospira (Leptospira), whose growth does not visually change the transparency of the medium.
• Solid media. Here, isolated colonies are formed—the progeny of a single parent cell (clones). The surface of the colonies can be smooth and shiny (S-forms, from smooth) or rough and matte (R-forms, from rough). The size, color, and consistency of such clusters are called cultural properties and serve as a key to identifying the microbe.

Classification of bacterial pigments

Some microorganisms are able to synthesize pigments that protect them from ultraviolet radiation, neutralize toxic oxygen radicals, and can act as antibiotics to suppress competitors. Pigments are classified by chemical composition and solubility:

• Carotenoids: fat-soluble pigments of red, orange, or yellow color. Characteristic of the genera Mycobacterium and Micrococcus.
• Pyrroles: soluble in alcohol. For example, red prodigiosin produced by Serratia marcescens.
• Phenazines: water-soluble, which causes them to stain the nutrient medium itself. A typical example is the blue-green pyocyanin secreted by the pyocyanic bacillus (Pseudomonas aeruginosa).
• Melanins: insoluble black or brown pigments found in bacteria of the genus Porphyromonas.

Alternative modes of reproduction and dormancy

Binary fission is not the only pathway of prokaryotic reproduction. In the course of evolution, other mechanisms have also emerged:

• Actinomycetes reproduce by fragmentation of filaments (hyphae).
• In mycoplasmas, pronounced polymorphism is observed: they can divide transversely, bud, or fragment to form elementary bodies.
• Chlamydia have a complex biphasic cycle. A small, non-dividing extracellular elementary body infects a cell, transforms inside it into a large reticulate body, which actively divides in two and forms a new generation of elementary bodies.
• Spirochetes (for example, the causative agent of syphilis Treponema pallidum) under adverse conditions roll up into cysts, which then break down into small granules—the basis for new cells.

Viable but non-culturable forms (VBNC) If environmental conditions become critical, non-spore-forming bacteria can enter a special state of dormancy. They stop growing on standard media (division halts) but retain metabolic activity. They can only be diagnosed using molecular genetic methods. Upon contact with protozoa, soil inhabitants, or phytohormones, reversion occurs—the cells emerge from hibernation, restoring their ability to multiply and cause infections.

Class and exam questions

What else to study on this topic

• Dependence of lag phase duration on the age of the inoculum
• Specifics of spore formation in the family Streptomycetaceae
• Mechanisms of cell autolysis in the death phase
• Application of oxidation-reduction potential indicator dyes
• Relationship between S- and R-forms of colonies (bacterial dissociation)

Other topics in the "Microbial Physiology" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.