Bacterial Cell Cycle and Division

Chapter 4: Archaea

• Archaea are diverse but share some common features, similar to bacteria, they share genes for metabolism, 70S ribosomes, and circular DNA chromosomes.
• Unique rRNA gene structure is a feature of archaea.
• Only archaea can produce methane (methanogenesis).
• Archaea have unique cellular structures and use unique molecules for building their structures.
• Archaea live in extreme environments: anaerobic (without oxygen), hypersaline (very salty), with extreme pH (very acidic/basic), and high temperatures (hot springs).
• Archaea have a diverse cell envelope that differs from bacteria.
• The cell envelope includes the plasma membrane and layers outside it; s-layers are often the only external layer.
• Archaea lack peptidoglycan, instead some may have pseudomurein, similar to but distinct from bacterial peptidoglycan.
• Archaea may have capsules or slime layers, rare but present in some species.
• Archaea contain L-amino acids, (instead of D-amino acids) and have B(1,3) glycosidic bonds (instead of B(1,4)).
• Archaea use N-acetyltalosaminuronic acid (instead of N-acetyl muramic acid)
• Archaea use isoprene units (branched hydrocarbons) instead of fatty acids attached to glycerol via ether linkages in their hydrocarbon chains.
• Archaea use glycerol diethers, forming bilayers (20 carbon chains), and glycerol tetraethers, forming monolayers (40 carbon chains, more rigid).
• Archaea may have phospholipids, sulfolipids, and glycolipids as additional lipids.
• Archaea membranes contain adaptations like cyclopentane rings in thermophiles (for rigidity at high temperatures).
• Archaea cell walls don't have peptidoglycan, the most common cell wall is the S-layer (a protein-based layer).
• Some archaea have pseudomurein or a protein sheath outside the S-layer.
• Some archaea have extra protein or carbohydrate layers above or below the S-layer.
• Other archaea lack cell walls entirely but may have double membranes.
• Archaea may have EUS (extracellular vesicles) – small, membrane-bound particles.
• EUS contain proteins, nucleic acids, and other cytoplasmic materials, potentially involved in gene transfer to protect DNA from high temperatures.
• Archaea may have nanotubes, array-like structures formed by the plasma membrane, which might facilitate communication or material exchange between cells.
• Archaea cytoplasmic structure has similarities to bacteria and prokaryotes.
• Both have cytoplasm with no membrane-enclosed organelles, but may contain inclusions (gas vesicles).
• Both contain ribosomes, nucleoid regions, plasmids, and cytoskeletal proteins.
• Archaea have unique versions of some structures, including tubulin and actin homologs (but lack intermediate filament-like proteins found in some eukaryotes).
• Archaea ribosomes are 70S (50S and 30S subunits) as in bacteria.
• Archaea ribosomes have more ribosomal proteins than bacteria ribosomes.
• These proteins make archaea ribosomes resistant to antibiotics that target bacterial ribosomes.
• Archaea ribosomal proteins resemble eukaryotic proteins more than bacterial ones.
• The nucleoid is the region where chromosomes are found.
• It is irregularly shaped and not membrane-bound.
• Archaea chromosomes are usually single, circular, double-stranded DNA.
• Some archaea are polyploid (containing more than one chromosome).

Chapter 7: Bacterial Growth

• Bacteria reproduce by binary fission; a single cell divides into identical daughter cells through a series of steps including chromosome replication and segregation; and cytokinesis resulting in forming a septum to split the cell into two daughter cells, each inheriting one chromosome.
• Bacteria reproduce via budding (small bud forms on a parent cell, grows, and detaches), or spore formation (filaments grow, branch out and produce aerial spores released to form new cells),
• The bacterial cell cycle can be divided into three phases; growth, chromosome replication and partitioning, and cytokinesis.
• In fast-growing cultures, bacteria may start and complete multiple chromosome replication rounds before first division, meaning daughter cells might inherit partially replicated DNA.
• The growth cycle (in E. coli is an example) begins with growth, and mass, and protein accumulation, followed by replication and partitioning of chromosomes, and the cell elongating during the process, and cytokinesis that leads to septum formation that splits the cells.
• The divisome is a protein machine that assembles at the division site, regulating and controlling the process of cytokinesis (cell division).
• The divisome controls peptidoglycan remodeling, ensuring proper division and cell-wall formation in each daughter cell.

Chapter 7: Bacterial Growth Phases

• The growth curve can have 5 distinct phases in a closed system (batch culture): lag phase, log (exponential) phase, stationary phase, death phase, and long-term stationary phase.
• Lag phase: cells adjust to the environment, synthesize new components, and replenish spent materials to adapt.
• Log phase: cells divide and grow at a maximal constant rate, with uniform characteristics.
• Stationary phase: growth ceases, and the total number of viable cells remains constant, either some cells die while others divide, and the population is heterogeneous.
• Reasons for stationary phase may include nutrient limitation, limited oxygen availability, accumulation of toxic waste, or reaching a critical population density.
• Death phase: viable cell numbers decline exponentially; cells are damaged from resource deprivation and waste buildup.
• Long-term stationary phase: some cells persist, population size remains constant, and cells evolve to adapt and survive, with successive waves of genetically distinct variants.
• Mathematical estimations for generation time (g), growth rate constant (k), and population size (Nₜ) in binary fission can be determined given initial conditions.
• Environmental factors influence microbial growth, including solutes and water activity (a water activity [aw]); pH; temperatures, and pressure.
• Solutes affect osmosis in organisms, causing their size and shape to change as water moves in and out due to concentration differences.
• Water activity measurements describe how much water is available. Most microbes need a high water activity.
• Extremophiles thrive in high salt concentrations (Osmo-tolerant, halophiles, and extreme halophiles).
• Microbes adapt to extreme hypertonic environments (either salt-out or salt-in methods).
• Psychrophiles grow optimally at low temperatures.
• Mesophiles grow optimally at moderate temperatures.
• Thermophiles thrive optimally at high temperatures.
• Hyperthermophiles grow at extremely high temperatures.
• Oxygen requirements vary among microorganisms. Aerobes require oxygen while obligate anaerobes cannot survive in the presence of oxygen. Microaerophiles need low oxygen concentrations.
• Facultative anaerobes can use or avoid oxygen.
• Protections from oxygen damage include enzymes like superoxide dismutase, catalase, and peroxidase.
• Pressure affects microbial growth. Barotolerant organisms can tolerate pressure changes while barophiles require high pressure for optimal growth.
• Microbes can be damaged by radiation, UV, or ionizing.
• Many microorganisms live in oligotrophic environments (low nutrient concentrations). Growth-arrested states may occur for microbe survival in response to stresses.

Chapter 7, Microbial Growth in Natural Environments

• Microbial environments are complex and dynamic, with changing nutrient levels and factors.
• Microbes can enter growth-arrested states in response to stresses.
• Biofilms are clusters of microbes attached to surfaces, surrounded by extracellular polymeric substances(EPS), found in water systems and natural environments.
• Biofilms are heterogeneous, with varying degrees of metabolic activity (active and inactive cells).
• Microbes can communicate in biofilms through quorum sensing.
• Quorum sensing involves the production of autoinducers (signaling molecules) that correlate to population density.
• Certain microbes form symbiotic relationships for mutual benefit, an example is of Vibrio fischeri that lives with squid.

Chapter 8: Microbial Control Methods

• Microbial control methods include sterilization, disinfection, antisepsis, and chemotherapy, all aimed at killing or inhibiting the growth of microorganisms.
• Biocides are chemical or physical agents that kill or inactivate microorganisms.
• Microbial death patterns are characterized by a steady decline in microbial numbers, not instantaneous death.
• Decimal reduction time (D-value) is the time needed to decrease a microbial population by 90%; shorter D-values indicate faster killing. A-values are used to evaluate how much change in temperature is needed to decrease the 2-value by a logarithm.
• Physical control methods include filtration, moist heat, dry heat, pasteurization, and radiation.
• Filtration removes microbes from liquids or air. Moist heat, such as steam sterilization in an autoclave, is used to kill all microbes, including spores.
• Dry heat sterilization is used for heat-resistant items like glass or metals.
• Pasteurization controls pathogens and slows spoilage in heat-sensitive foods or beverages.
• Tyndallization is intermittent sterilization, repeated cycles.
• UV radiation damages DNA and is limited to surface sterilization.
• Ionizing radiation penetrates tissues effectively killing microbes and spores.
• Chemical control methods include phenolics, alcohols, halogens (iodine and chlorine), heavy metals, quaternary ammonium compounds, and aldehydes.
• Evaluating antimicrobial agent effectiveness involves assessing population sizes, composition, concentration, contact time, temperature, and local environments.
• Phenol coefficient tests show relative strengths while use and normal-use tests measure effectiveness under realistic conditions.

Chapter 8: Biological Control Methods

• Biological methods for controlling microorganisms include organisms that predate on bacteria, viral-mediated killing of bacteria (bacteriophages), and toxins produced by bacteria that control closely related species.

Test your knowledge on the bacterial cell cycle with this quiz. Explore the essential requirements for bacterial cells before division, the phases involved, and the growth dynamics during the cell cycle. Challenge yourself with questions about bacterial reproduction and the effects of size increase on the cell.