Lecture 5 - Microbial Growth
Lecture 5: Microbial Growth
Genome Replication and Cell Growth Regulation
Genome Replication: Essential before initiating cell growth.
Cell Cycle Phases:
G1 Phase (analogous): Precedes initiation of genome replication in prokaryotes.
S Phase (analogous): DnaA binds to origin of replication (oriC), recruits machinery.
Mechanisms:
SeqA prevents genome reinitiation after synthesis initiation.
Chromosome migration occurs to opposite poles.
Formation of Z-ring signifies cell division.
Regulation of Initiation of Genome Replication
Key Components:
oriC has multiple GATC sequences with DnaA boxes bound by DnaA (needs methylation on A).
Replication Process:
DnaA binding leads to synthesis machinery recruitment.
Newly synthesized strands become unmethylated; now they are hemimethylated.
SeqA binds hemimethylated DnaA boxes, prohibiting further initiation.
Transcription Regulation:
DnaA binding permits transcription while SeqA binding inhibits it.
DnaA as ATPase
Role of DnaA:
Requires ATP binding for activity.
Replisome recruitment activates ATPase activity for DnaA.
ATP hydrolysis leads to DnaA inactivation.
Site of Action:
GATC in oriC methylated by DNA adenine methylase (Dam methylase) allowing reinitiation.
Initiation in Rapidly Dividing Cells
Replication Timing:
DNA synthesis takes approximately 40 minutes to replicate chromosome.
In nutrient-rich media, cells double in 20 minutes, quicker than replication.
Dam methylase ensures oriC reaches a fully methylated state 10 minutes post-initiation, allowing for another DNA synthesis round.
Regulation of Cell Growth
Inhibition Mechanism:
Cell growth is inhibited until genome replication is completed, ensuring daughter cells receive copies.
Min Proteins Function:
MinD forms spiral along plasma membrane, oscillates to prevent FtsZ recruitment.
MinE oscillates at the cell midpoint, pushing MinCD to poles, allowing FtsZ to bind at the midpoint and promoting growth.
Cell Growth and DNA Synthesis
MinE's Role:
Acts as an inhibitor of MinCD, promoting cell growth by facilitating FtsZ recruitment to cell midpoint.
Leads to the recruitment of enzymes for peptidoglycan and lipid biosynthesis.
Ensures production of all components necessary for cell wall growth and overall cellular growth.
Peptidoglycan Synthesis and Cell Growth
Process Initiation:
Anchoring of ZipA in membrane recruits FtsA, an ATPase liberating energy for further recruitment of divisome machinery.
Continued ATP hydrolysis by FtsA is crucial for recruiting FtsI and FtsK for peptidoglycan incorporation.
Chromosome Partitioning and Separation
Necessary Separation:
Each daughter cell must receive a chromosome copy.
FtsK facilitates chromosome migration to the cell poles.
Completion of migration allows for daughter cell separation, driven by FtsZ GTP hydrolysis promoting depolymerization.
Partitioning of Chromosomes to Daughter Cells
Mechanism:
Asymmetric partitioning in Caulobacter during budding.
ParB and parS sequence interaction ensures one chromosome copy is tethered to the stalked pole and the other is moved to the opposite pole.
Peptidoglycan Synthesis and Cell Division
Chemical Processes:
Begins with NAG-NAM precursors which cannot cross the plasma membrane without modification.
Complexed to bactoprenol and facilitated by flippase, leading to incorporation into existing layers.
Laboratory Media for Microbial Growth
Medium Types:
Chemically Defined: Precise known composition.
Chemically Complex: Rich media, often with extracts, unknown composition.
Selective Media: Promotes growth of desired organisms, suppressing undesirables.
Differential Media: Most organisms grow, unique phenotypes for desired organisms.
Total Count vs. Viable Count
Count Definitions:
Total count may not represent the number of viable cells; varies by species.
Viable count shows growth potential under specific conditions.
Measurement Techniques:
Hemacytometer for direct measurements, counting in a grid.
Serial Dilution for Viable Count
Process:
Create dilutions (e.g., 1:10), then plate via spread or pour plate methods, counting colonies formed (30-300 ideal).
Indirect Counts
Turbidity Measurements:
Assess bacterial numbers through light absorption.
Phases of Growth
Lag Phase:
No division, no change in viable cell count, varies based on inoculation and media specifics.
Exponential Phase:
Cell count doubles, growth dependent on nutrient availability.
Stationary Phase:
Nutrient limitation and toxic metabolite accumulation inhibit further growth.
Death Phase:
Viable count decreases from nutrient depletion and toxic accumulation.
Factors Affecting Microbial Growth - Temperature
Growth Rate: Influenced by temperature ranges.
Psychrophile: Example - Polaromonas vacuolata (optimally grows at 4°C).
Mesophile: Example - Escherichia coli (optimal growth at around 37°C).
Thermophile: Example - Geobacillus stearothermophilus (ties to higher temperatures).
Factors Affecting Microbial Growth - Temperature Effects
Membrane and Enzyme Kinetics:
Temperature affects membrane fluidity and enzyme function, leading to stability or denaturation.
Low-temperature changes increase membrane fluidity with unsaturated fatty acids, while high temperatures favor saturated fatty acids for membrane integrity.
Factors Affecting Microbial Growth - pH
Optimal pH:
Bacteria thrive at neutral pH (6.5-7.5), fungi prefer mildly acidic conditions (pH 5-6).
Acidophiles and alkaliphiles have unique adaptations to maintain stability in extreme environments.
Factors Affecting Microbial Growth - Osmotic Pressure
Concentration Effects:
Importance of isotonic, hypotonic, hypertonic environments; osmotic pressure impacts cell integrity and survival.
Water Activity (aw): The interrelation between solute concentration and water availability affects growth capabilities.
Factors Affecting Microbial Growth - Oxygen Requirements
Classes of Organisms:
Obligate aerobes and anaerobes differ in vital oxygen needs, affecting metabolic pathways and survival.
Problems with Oxygen Based Metabolism
Toxic Byproducts:
Oxygen metabolism generates superoxide anion (O2-) and hydrogen peroxide (H2O2), both toxic and necessitating enzyme detoxification.
Detoxification Mechanisms:
Superoxide dismutase and catalase work to convert harmful byproducts into less toxic forms.