Notes
Bacterial Growth & Replication
Sections
Overview
Bacterial growth refers to the increase in number of bacterial cells, which occurs via binary fission.
Ultimately, growth can produce a colony of millions of bacterial cells.
Generation time is the time it takes for takes for the cell population to double.
This time varies by species, and is moderated by environmental factors such as pH, nutrient availability, temperature, etc.
For example, the generation time for Staphylococcus aureus grown in heart infusion broth is about 30 minutes.
Bacteria are haploid
E. coli, which we use in our diagram, have chromosomal DNA organized into circular, double-stranded structures.
Pathogenicity islands refer to the distinct regions of some bacterial chromosomes that code for virulence factors; these islands are absent in non-virulent strains.
Extrachromosomal genetic elements may also be present.
For example, plasmids and bacteriophages may engage in horizontal DNA transfer.
Quorum sensing is a type of bacterial communication that arises when cell population density is high.
Quorum sensing is mediated by autoinducers, which are molecules released by bacterial cells.
As cell density increases, so does autoinducer concentration.
In different species, quorum sensing moderates virulence factor secretion, biofilm production, sporulation, and other behaviors.
For example, consider the bacteria Staphylococcus aureus, which produces autoinducer peptides (AIPs).
When the density of S. aureus increases, so does the concentration of autoinducer peptides. In turn, this induces bacterial release of virulence factors, including several toxins.
Furthermore, the cells are stimulated to release additional AIPs, thus creating a positive feedback loop.
Because quorum sensing facilitates some of the harmful effects of acute S. aureus infection, researchers are investigating treatments that prohibit or moderate this type of cellular communication.
Bacterial Growth Curve
The bacterial growth curve tracks the stages of cell population growth.
We'll plot time along the x-axis, and the log number of cells along the y-axis.
Lag stage
Bacterial cells engage in metabolic activity but not cell division; during this stage, the bacteria acclimate to the growth conditions.
Logarithmic
Aka, exponential stage.
Rapid cell division occurs.
Beta-lactam antibiotics, such as penicillin are effective during this period, because they interfere with cell wall production.
Stationary stage
The curve plateaus during the stationary phase because proliferation and cell death are in balance; this steady state is reached when nutrients are running low and/or toxin levels are elevated.
Death stage
Finally, the number of bacteria declines.
However, some bacteria may remain viable during this stage.
Important points regarding the growth cycle:
Growth requirements include carbon, nitrogen, energy sources, water, and ions; though specific requirements vary by species.
Iron is so important for growth that some bacteria can secrete siderophores that "steal" iron from the host.
Obligate anaerobes cannot grow in the presence of oxygen.
Clostridium is an example.
Obligate aerobes can only grow in the presence of oxygen.
Mycobacterium tuberculosis
Facultative anaerobes can grow with or without oxygen.
Most common; Staphylococcus, which contributes to the normal flora of the nares, is an example of this.
Obligate intracellular pathogens they can only grow within living cells because they rely on ATP derived from the host
— Chlamydia
Chromosome Replication in E. coli
Bacterial chromosome replication in E. coli occurs via binary fission - chromosome replication triggers initiation of cell division.
We illustrate this in Escherichia coli; be aware that not all bacteria replicate in this exact manner.
First drawing
Single circular chromosome with inner and outer strands.
In reality, bacterial DNA is arranged in loops. Recall that there is no distinct nucleus, as in eukaryotic cells; instead, DNA lies in the nucleoid region.
Origin of replication is marked by oriC; this is where the replication initiator proteins bind.
Terminus is located opposite oriC; this is the region where DNA replication terminates.
Second drawing
After DNA replication begins:
Outer and inner parental strands begin to separate in bidirectional replication.
The replication "bubble" is the space between the strands.
Two y-shaped replication forks form, one on each side of the bubble.
DNA helicases separate the parental DNA strands in short segments, thus creating the replication forks; if the two strands separated all at once, excessive DNA damage could occur.
Developing daughter strands: each one begins at the origin of its parental strand and grows towards its terminal end.
These complementary daughter strands are synthesized by the replisome, which comprises DNA polymerase III and other components.
Third drawing
The parental and growing daughter strands are further along in the replication process.
The parental strands have further separated from each other as the daughter strands grow towards the terminal region.
Fourth drawing
Finally, the chromosomes separate after the daughter strands are complete.
DNA replication is semiconservative:
Each new chromosome comprises one strand of DNA from the parental chromosome and one complementary daughter strand.
Full-Length Text
- Here we will learn about the basic growth and genetics of bacteria.
- To begin, start a table, and denote that growth refers to the increase in number of bacterial cells, which occurs via binary fission.
- Ultimately, growth can produce a colony of millions of bacterial cells;
- The time it takes for takes for the cell population to double is the "generation time,"
- This time varies by species, and is moderated by environmental factors such as pH, nutrient availability, temperature, etc.
- For example, the generation time for Staphylococcus aureus grown in heart infusion broth is about 30 minutes.
- Denote the following genetic information:
- Bacteria are haploid; in the example of E. coli, which we use in our diagram, chromosomal DNA is organized into circular, double-stranded structures.
- Pathogenicity islands refer to the distinct regions of some bacterial chromosomes that code for virulence factors; these islands are absent in non-virulent strains.
- Lastly, denote that extrachromosomal genetic elements may also be present.
- For example, plasmids and bacteriophages may engage in horizontal DNA transfer, which we discuss in detail, elsewhere.
Now, let's begin our diagram with a representative bacterial growth curve, which tracks the stages of cell population growth.
- We'll plot time along the x-axis, and the log number of cells along the y-axis.
- Indicate that there are 4 key stages of cell growth:
- Lag
- Logarithmic
- Stationary
- Death.
- Now, indicate that during the lag stage, bacterial cells engage in metabolic activity but not cell division; during this stage, the bacteria acclimate to the growth conditions.
- Then, during the logarithmic, aka, exponential stage, rapid cell division occurs; write that beta-lactam antibiotics, such as penicillin are effective during this period, because they interfere with cell wall production.
- The curve plateaus during the stationary phase because proliferation and cell death are in balance; this steady state is reached when nutrients are running low and/or toxin levels are elevated.
- Finally, the number of bacteria declines during the death phase; however, some bacteria may remain viable during this stage.
Let's write out some important points regarding the growth cycle.
- First, write that growth requirements include carbon, nitrogen, energy sources, water, and ions; though specific requirements vary by species, iron is so important for growth that some bacteria can secrete siderophores that "steal" iron from the host.
- Some bacteria, called obligate anaerobes, cannot grow in the presence of oxygen; Clostridium is an example of this.
- Others can only grow in the presence of oxygen; these are called obligate aerobes, and include Mycobacterium tuberculosis.
- However, most bacteria are facultative anaerobes; that is, they can grow with or without oxygen. Staphylococcus, which contributes to the normal flora of the nares, is an example of this.
- Finally, write that some bacteria are obligate intracellular pathogens; they can only grow within living cells because they rely on ATP derived from the host (Chlamydia is an example of an obligate intracellular bacterium).
Next, let's look at bacteria replication.
- Indicate that replication occurs via binary fission, and, that chromosome replication triggers initiation of cell division.
Let's illustrate chromosomal replication in Escherichia coli; be aware that not all bacteria replicate in this exact manner. Furthermore, we're only going to address the bacterial chromosomal DNA, not the extrachromosomal genetic information held in plasmids or bacteriophages.
- First, draw the single circular chromosome; label the inner and outer strands.
- We show them as two simple circles, for simplicity; in reality, bacterial DNA is arranged in loops.
- Recall that there is no distinct nucleus, as in eukaryotic cells; instead, DNA lies in the nucleoid region.
- Next, indicate that the origin of replication is marked by oriC; this is where the replication initiator proteins bind.
- Label the terminus on the opposite side; this is the region where DNA replication terminates.
Then, show the chromosome after DNA replication begins:
- Draw the outer and inner parental strands as they are separating;
- Label the replication "bubble," which is the space between the strands, and,
- To show bidirectional replication, label the two y-shaped replication forks on opposite sides of the bubble.
- Write that DNA helicases separate the parental DNA strands in short segments, thus creating the replication forks; if the two strands separated all at once, excessive DNA damage could occur.
- Then, show the developing daughter strands: each one begins at the origin of its parental strand and grows towards its terminal end.
- Write that complementary daughter strands are synthesized by the replisome, which comprises DNA polymerase III and other components.
- Next, draw the parental and growing daughter strands further along in the replication process; notice how the parental strands have further separated from each other as the daughter strands grow towards the terminal region.
- Finally, show that the chromosomes separate after the daughter strands are complete.
- Write that DNA replication is semiconservative, such that each new chromosome comprises one strand of DNA from the parental chromosome and one complementary daughter strand.
Before we conclude, let's learn about quorum sensing, which is a type of bacterial communication that arises when cell population density is high.
- Quorum sensing is mediated by autoinducers, which are molecules released by bacterial cells.
- To illustrate this relationship, draw a double-ended arrow.
- Indicate a low-density population on one end, and a high-density population on the other.
- Show that as cell density increases, so does autoinducer concentration.
- In different species, quorum sensing moderates virulence factor secretion, biofilm production, sporulation, and other behaviors.
- For example, consider the bacteria Staphylococcus aureus, which produces autoinducer peptides (AIPs).
- Show that when the S. aureus density rises, so does the concentration of autoinducer peptides.
- In turn, this induces bacterial release of virulence factors, including several toxins.
- Furthermore, the cells are stimulated to release additional AIPs, thus creating a positive feedback loop.
- Because quorum sensing facilitates some of the harmful effects of acute S. aureus infection, researchers are investigating treatments that prohibit or moderate this type of cellular communication.
References
- Murray, P. R., Rosenthal, K. S., & Pfaller, M. A. Medical microbiology. Philadelphia: Elsevier/Saunders. (2013).
- Levinson, W. E. Review of Medical Microbiology and Immunology. 14th Ed. Lange (2016)
- Bronesky, D., Wu, Z., Marzi, S., Walter, P., Geissmann, T., Moreau, K., Vandenesch, F., et al. 2016. Staphylococcus aureus RNAIII and its regulon link quorum sensing, stress responses, metabolic adaptation, and regulation of virulence gene expression. Annu. Rev. Micriobol. 70:299-316.
- O'Donnell, M., Langston, L., Stillman, B. 2013. Principles and concepts of DNA replication in Bacteria, Archaea, and Eukarya. Cold Spring Harb Perspect Biol. 5:a010108.
- Sieber, K. B., Bromley, R. E., Hotopp, J.C.D. 2017. Lateral gene transfer between prokaryotes and eukaryotes. Experimental Cell Research 358:421-426.
- Croucher, N. J., Mostowy, R., Wymant, C., Turner, P., Bentley, S. D. Fraser, C. 2016. Horizontal DNA transfer mechanisms of bacteria as weapons of intragenomic conflict. PLoS Biol 14(3): e1002394.
- Wendel, B.M., Courcelle, C.T., Courcelle, J. 2014. Completion of DNA replication in Escherichia coli. PNAS 111(46): 16454-16459.
- Yao, N. & O'Donnell, M. 2016. Bacterial and eukaryotic replisome machines. JSM Biochem Mol Biol. 3(1).