Bacterial Growth Curve in Batch Culture
When microorganisms are placed in a closed liquid medium (without adding fresh nutrients or removing toxins), their development is resource-limited. On a graph plotting time on the abscissa against the logarithm of cell number on the ordinate, four distinct phases are observed:
- Lag phase (2–5 hours). The adaptation period following inoculation. Cells do not divide yet, but their metabolism operates at maximum capacity—intensive synthesis of new enzymes occurs to break down specific substrates in the medium.
- Exponential (logarithmic) phase. The period of maximal and constant division rate. All cells during this phase have standard dimensions and a stable protein content.
- Stationary phase. Growth ceases. Equilibrium is reached: the number of newly formed cells equals the number of dying cells. This occurs due to nutrient depletion, a drop in oxygen levels, high population density, and the accumulation of toxins.
- Death (decline) phase. Microorganisms die en masse under the influence of acidic metabolic waste products or as a result of autolysis (self-destruction by enzymes). This stage can last from ten hours to several weeks.
Growth Characteristics on Culture Media
Manifestations of vital activity depend on the physical state of the substrate:
- Liquid media. Most commonly, bacteria cause diffuse turbidity of the liquid, form a surface pellicle, or settle into a bottom sediment. An exception is leptospires (Leptospira), whose growth does not visually alter medium transparency.
- Solid media. Here, isolated colonies form—the progeny of a single parent cell (clones). Colony surfaces may be smooth and glistening (S-forms, from smooth) or rough and matte (R-forms, from rough). The size, color, and consistency of these aggregations are called cultural characteristics and serve as a key to microbial identification.
Classification of Bacterial Pigments
Some microorganisms 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 red, orange, or yellow pigments. Typical for the genera Mycobacterium and Micrococcus.
- Pyrroles: soluble in alcohol. For example, the red pigment 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 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. Evolutionary pressures led to other mechanisms:
- Actinomycetes reproduce via fragmentation of filaments (hyphae).
- Mycoplasmas exhibit marked polymorphism: they can divide transversely, bud, or fragment to form elementary bodies.
- Chlamydiae have a complex biphasic cycle. A small, non-dividing extracellular elementary body infects a cell, transforms intracellularly into a large reticulocyte body (reticulate body), which actively divides by binary fission to generate a new generation of elementary bodies.
- Spirochetes (e.g., the syphilis causative agent Treponema pallidum) coil into cysts under unfavorable conditions, which then disintegrate into small granules—the basis for new cells.
Viable but Non-Culturable (VBNC) Forms When environmental conditions become critical, non-spore-forming bacteria can enter a specialized dormant state. They stop growing on standard media (cell division halts) but maintain metabolic activity. They can only be diagnosed using molecular genetic methods. Upon contact with protozoa, soil inhabitants, or phytohormones, reversion occurs—cells wake up, restoring their ability to multiply and cause infections.