Bacterial Growth Curve

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. 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. 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 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.