Bacterial Growth & Replication (Ditki Notes)

• Bacterial growth refers to the increase in the number of bacterial cells through binary fission, potentially forming colonies of millions of cells.
• Generation time is the time it takes for a cell population to double, varying by species and environmental conditions like pH, nutrient availability, and temperature.
• Staphylococcus aureus has a generation time of about 30 minutes when grown in heart infusion broth.
• Bacteria, such as E. coli, are haploid and have chromosomal DNA in circular, double-stranded structures.
• Pathogenicity islands are regions in some bacterial chromosomes coding for virulence factors, absent in non-virulent strains.
• Plasmids and bacteriophages are extrachromosomal genetic elements that may engage in horizontal DNA transfer.
• Quorum sensing is bacterial communication when cell population density is high, mediated by autoinducers released by bacterial cells.
• Autoinducer concentration increases with cell density, moderating virulence factor secretion, biofilm production, sporulation, and other behaviors in different species.
• Staphylococcus aureus produces autoinducer peptides (AIPs), where increased S. aureus density leads to higher AIP concentration, inducing virulence factor release, including toxins, and prompting a positive feedback loop by stimulating further AIP release.
• Treatments are researched to inhibit or moderate quorum sensing to reduce harmful effects from acute S. aureus infection.

Bacterial Growth Curve

• The bacterial growth curve tracks cell population growth stages, plotting time (x-axis) against the log number of cells (y-axis).
• Lag phase: bacterial cells metabolize but do not divide, and acclimate to growth conditions.
• Logarithmic (exponential) phase: rapid cell division occurs, making beta-lactam antibiotics like penicillin effective due to their interference with cell wall production.
• Stationary phase: a plateau where proliferation and cell death are balanced, typically when nutrients are low or toxin levels are high.
• Death phase: the number of bacteria declines, though some may remain viable.

Growth Cycle Requirements

• Growth needs include carbon, nitrogen, energy sources, water, and ions, varying by species.
• Iron is vital; some bacteria secrete siderophores to "steal" it from the host.
• Obligate anaerobes, such as Clostridium, cannot grow with oxygen.
• Obligate aerobes, such as Mycobacterium tuberculosis, require oxygen to grow.
• Facultative anaerobes, like Staphylococcus (found in the normal flora of the nares), can grow with or without oxygen.
• Obligate intracellular pathogens, such as Chlamydia, can only grow inside living cells, as they depend on the host for ATP.

Chromosome Replication in E. coli

• Replication occurs via binary fission, initiating cell division.
• Chromosomal DNA, not extrachromosomal genetic material in plasmids or bacteriophages, is focused on.
• A single circular chromosome has labeled inner and outer strands shown as two simple circles.
• Bacterial DNA is arranged in loops, without a distinct nucleus. Instead the DNA lies in the nucleoid region.
• The origin of replication is marked by oriC, where replication initiator proteins bind.
• The terminus is opposite to oriC, where DNA replication terminates.
• Outer and inner parental strands separate, forming a replication "bubble".
• Two Y-shaped replication forks form on opposite sides of the bubble, showing bidirectional replication.
• DNA helicases separate the parental DNA strands in short segments, forming replication forks
• Developing daughter strands begin at the origin of their parental strand and grow towards the terminal end.
• Complementary daughter strands are synthesized by the replisome, including DNA polymerase III.
• Parental strands separate as daughter strands grow towards the terminal region, drawing the parental and growing daughter strands further along in replication
• The chromosomes separate after the daughter strands are complete.
• DNA replication is semiconservative, leading to one strand of DNA from the parental chromosome and one complementary daughter strand in each new chromosome.