Introduction to Bacterial Growth:
Bacterial growth is a complex process that involves biosynthetic reactions including synthesis of cell constituents and metabolites and anabolic reactions that involves the breakdown of cell constituents and metabolites.
Bacteria growing in a closed system typically go through four distinct stages: the lag phase, exponential (log) phase, stationary phase, and decline (death) phase.
Bacterial Growth Curve:
Lag phase
During the lag phase, bacterial cells are not dividing rapidly but are instead adapting to the new conditions, preparing their internal systems for active growth. The age of the culture, the amount of the inoculum, and how different the settings are from the ones the bacteria originated in can all affect how long the lag phase lasts.
Molecular Events in Lag Phase
- One significant aspect of the lag period is the acquisition of metals. As a crucial component of membrane phospholipids, nucleic acids, and nucleotides, as well as for phosphorylation processes inside cells, phosphate is a necessary element for bacterial growth.
- During the lag period, phosphate intake is high. S. enterica either absorbs or excretes metals when it enters the lag phase. During the lag phase, intracellular concentrations of cobalt, nickel, sodium, and molybdenum dropped while those of iron, manganese, and calcium increased.
- The lag phases of S. enterica and E. coli exhibit the induction of genes involved in iron recruitment and metabolism. Fe-S formation is significantly higher in the lag phase. In the lag phase, calcium, which is necessary for chemotaxis, cytoskeleton rearrangement, and motility, is significantly absorbed.
- Genes cannot be translated into proteins without ribosomes. As the lag phase comes to an end, the number of ribosomes per cell and the translation rate must increase from the low stationary-phase concentration at inoculation to greater (exponential-phase) concentrations.
- During the lag period, the metabolism of carbohydrates is rearranged to optimize the flow of carbon. The fact that metabolic pathway intermediates quickly build up during the lag phase supports this.
- Overflow metabolism, which redirects carbon flux from the TCA cycle, is high in the log phase. Fis protein peaks in the late lag and early log phases and is crucial for processes including rRNA transcription and DNA replication start.
- Aerobic respiration is indicated by the upregulation of genes for terminal cytochrome oxidase and the consumption of oxygen during the lag phase.
- Enzymes of gluconeogenesis, TCA cycle, Glyoxylate cycle are down regulated.
Log phase
When a cell has acquired all the resources it needs to expand, it begins to divide. Under ideal conditions, the exponential phase of expansion is marked by fast population growth, during which the population doubles. All of the cells are expanding geometrically and dividing frequently by binary fission throughout the exponential phase of growth.
Molecular Events in Log Phase
- Acquiring metals is an important feature of log phase.
- In log phase, phosphate incorporation is low.
- Metals are absorbed or expelled from S. enterica upon entry into lag phase. Iron is in decreased concentrations. Iron, Calcium and Manganese are in low concentrations in mid exponential phase because of low gene induction of genes encoding these. The Fe-S cluster formation is also not significant in log phase.
- Calcium is required for motility, cytoskeleton rearrangement and chemotaxis. Calcium uptake is high in log phase.
- The concentrations of nickel and cobalt are low in the log phase. In the mid-exponential phase, the levels of sodium and magnesium ions are elevated. During the stationary phase, protein damage builds up and can cause abnormal cytoplasmic disulfide bond formation, protein carbonylation, and direct oxidation of amino acid residues in E. coli. In log phase, the genes that encode the repair-related proteins are increased.
- Overflow metabolism is high in log phase whereby carbon flux from TCA cycle is redirected from TCA.
- During the exponential phase of bacterial growth, genes involved in glycolysis are highly active, reflecting the cell’s need for rapid energy production. In fact, ATP levels can surge up to 50 times compared to earlier stages. Cell size also increases progressively throughout the lag phase, preparing for active division. Early in the log phase, the Fis protein, which plays a crucial role in initiating DNA replication and rRNA transcription, reaches its highest concentration. At the same time, genes coding for terminal cytochrome oxidases are upregulated, indicating active aerobic respiration and high oxygen consumption.
Fig: Bacterial Growth Curve- Four Distinct Phases
Stationary phase
As nutrients begin to run low and the environment becomes less favorable, bacteria shift into the stationary phase. Division slows and eventually stabilizes — the number of new cells balances the number dying, resulting in no net population change. Entry into this phase can be triggered by nutrient depletion, oxygen limitation, accumulation of toxic byproducts, or reaching a maximum population density. Although growth halts, vital cellular functions like metabolism and biosynthesis continue. Notably, cells become less sensitive to antibiotics during this phase, reflecting significant physiological adaptation.
Molecular Events in Stationary Phase
- Metal acquisition becomes crucial. While phosphate uptake declines, levels of calcium, iron, and manganese are generally low during both the mid-exponential and stationary phases. Conversely, nickel and cobalt accumulate to their highest levels in the stationary phase.
- Ribosome numbers per cell decrease, and translation activity drops, especially from the point of inoculation.
- Genes responsible for DNA repair are downregulated, reducing the emphasis on genetic maintenance.
- Overflow metabolism—the use of alternate metabolic pathways—reaches an intermediate level.
- Genes tied to the TCA (tricarboxylic acid) cycle and the glyoxylate cycle are more active, suggesting metabolic rerouting for energy conservation and survival.
- The concentration of Fis protein declines to a moderate level.
- Genes for terminal cytochrome oxidases are downregulated, indicating a shift away from aerobic processes toward anaerobic respiration.
Death phase
Molecular Events in Death Phase
For bacteria, death is the inability to reproduce when transferred to a fresh medium in ideal condition. Depletion of nutrients, accumulation of waste products can lead to death of bacteria. In some cases, death is accompanied by cell lysing by autolysin. The number of viable cells decreases in an exponential fashion. The rate at which cells lose viability is reflected in the slope’s steepness.