Microbial Growth, Metabolism, Infectious Disease, and Host-Microbe Interactions: Study Guide (MMBS111 Unit 3)

Chapter 7: Fundamentals of Microbial Growth

Microbial Growth and Reproduction

Microbial growth refers to the increase in the number of cells, not cell size. Most bacteria reproduce by binary fission, a process resulting in two identical daughter cells.

• Binary Fission: Asexual process where a single cell divides into two identical cells.

• Generation Time: The time required for a cell to divide and its population to double.

• Growth Curve: Describes the pattern of growth in a closed batch system, typically with four phases.

Four Stages of Bacterial Growth

Stage	Description	Cell Activity
Lag phase	Cells adjust to new environment; no division, but metabolic activity is high.	High metabolic activity, protein synthesis
Log phase	Cells divide at a constant rate; exponential population increase.	Active DNA replication, transcription, translation
Stationary phase	Growth rate slows; nutrients deplete, waste accumulates; cell death equals cell division.	Stress responses, sporulation (in some species)
Death phase	Cells die at an exponential rate due to lack of nutrients and toxic waste buildup.	Cell lysis, decreased metabolic activity

Physical and Chemical Factors Affecting Microbial Growth

• Temperature: Microbes are classified by their optimal temperature ranges:

  • Psychrophiles: Thrive at 0–20°C (cold-loving).

  • Mesophiles: Grow best at moderate temperatures (20–45°C); most human pathogens.

  • Thermophiles: Prefer 45–70°C (heat-loving).

  • Hyperthermophiles: Grow above 70°C.

• pH: Most bacteria grow best at neutral pH (6.5–7.5). Acidophiles and alkaliphiles thrive in acidic and basic environments, respectively.

• Osmotic Pressure: High salt or sugar concentrations inhibit most microbes (plasmolysis), but halophiles tolerate or require high salt.

• Oxygen Requirements:

  • Obligate aerobes: Require oxygen.

  • Facultative anaerobes: Grow with or without oxygen.

  • Obligate anaerobes: Oxygen is toxic.

  • Microaerophiles: Require low oxygen levels.

  • Aerotolerant anaerobes: Tolerate oxygen but do not use it.

Microbial Control: Sterilization, Disinfection, and Antisepsis

Microbial control methods are used to reduce or eliminate microorganisms from surfaces, equipment, and tissues.

• Sterilization: Complete destruction of all microbial life, including spores.

• Disinfection: Destruction of vegetative pathogens on inanimate objects.

• Antisepsis: Destruction of pathogens on living tissue.

• Sanitization: Lowering microbial counts to safe public health levels.

Methods of Microbial Control

• Physical Methods: Heat (moist and dry), filtration, radiation.

• Chemical Methods: Alcohols, phenolics, halogens, heavy metals, aldehydes, peroxygens.

Levels of Disinfectants

Level	Examples	Applications
High-level	Aldehydes, peroxygens	Sterilize medical equipment
Intermediate-level	Alcohols, phenolics	Disinfect surfaces, equipment
Low-level	Soaps, detergents	General sanitation

Chapter 8: Microbial Metabolism

Metabolism: Catabolism and Anabolism

Metabolism is the sum of all chemical reactions in a cell. It includes catabolism (breaking down molecules to release energy) and anabolism (building complex molecules from simpler ones).

• Catabolism: Degradative reactions; release energy (exergonic).

• Anabolism: Biosynthetic reactions; require energy (endergonic).

ATP: The Energy Currency

• Structure: Adenosine triphosphate (ATP) contains adenine, ribose, and three phosphate groups.

• Energy Release: Energy is released when ATP is hydrolyzed to ADP + Pi: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• ATP Generation: Substrate-level phosphorylation, oxidative phosphorylation, and photophosphorylation.

Enzymes and Enzyme Regulation

• Enzymes: Biological catalysts that speed up reactions by lowering activation energy.

• Factors Affecting Activity: Temperature, pH, substrate concentration, inhibitors.

• Inhibition:

  • Competitive: Inhibitor binds active site.

  • Noncompetitive: Inhibitor binds allosteric site.

  • Feedback Inhibition: End product inhibits an earlier step in the pathway.

Cellular Respiration and Fermentation

• Glycolysis: Glucose is broken down to pyruvate, producing ATP and NADH.

• Krebs Cycle: Pyruvate is further oxidized, generating CO2, ATP, NADH, and FADH2.

• Electron Transport Chain: NADH and FADH2 donate electrons, generating ATP via oxidative phosphorylation.

• Fermentation: Anaerobic process; regenerates NAD+ and produces organic end products (e.g., lactic acid, ethanol).

Chapter 9: Principles of Infectious Disease and Epidemiology

Pathogens and Disease

• Pathogen: Microorganism capable of causing disease.

• Types of Pathogens:

  • True Pathogen: Causes disease in healthy hosts.

  • Opportunistic Pathogen: Causes disease only under certain conditions (e.g., immunocompromised host).

Classification of Infectious Agents

Type	Cell Type	Examples
Bacteria	Prokaryotic	Escherichia coli, Staphylococcus aureus
Archaea	Prokaryotic	Halophiles, thermophiles
Protists	Eukaryotic	Plasmodium, Giardia
Fungi	Eukaryotic	Yeasts, molds
Helminths	Eukaryotic	Tapeworms, roundworms
Viruses	Acellular	Influenza virus, HIV
Prions	Protein only	Creutzfeldt-Jakob disease agent

Stages of Infectious Disease

• Incubation Period: Time between infection and appearance of symptoms.

• Prodromal Period: Early, mild symptoms.

• Acute Period: Disease is most severe.

• Period of Decline: Symptoms subside.

• Convalescent Period: Recovery and return to health.

Transmission of Infectious Diseases

• Direct Contact: Physical interaction (e.g., touching, sexual contact).

• Indirect Contact: Via fomites (inanimate objects), vectors (insects), or airborne droplets.

• Common HAIs (Healthcare-Associated Infections): C. difficile, urinary tract infections, surgical site infections, MRSA.

Chapter 10: Host-Microbe Interactions and Pathogenesis

Normal Microbiota

• Definition: Microorganisms that colonize the body without causing disease; provide protection and aid digestion.

• Differences: Resident microbiota are permanent; transient microbiota are temporary.

Virulence Factors and Pathogenicity

• Virulence Factors: Traits that enhance a pathogen's ability to cause disease (e.g., toxins, capsules, enzymes).

• Pathogenicity: The ability of a microbe to cause disease.

Exotoxins vs. Endotoxins

Property	Endotoxin	Exotoxin
Made of	Lipid (LPS)	Protein
Produced by	Gram-negative bacteria	Gram-positive & some Gram-negative bacteria
Heat stability	Stable	Unstable
Toxicity	Low (high LD50)	High (low LD50)

Immune Evasion Strategies

• Antigenic Variation: Changing surface proteins to evade immune detection.

• Capsule Formation: Prevents phagocytosis.

• Enzyme Production: Destroys host defenses (e.g., IgA protease).

Prevention of Disease Transmission

• Hand washing

• Disinfection of surfaces and equipment

• Isolation of infected patients

• Use of personal protective equipment (PPE)

Additional info: These notes cover core concepts from Chapters 7–10, including microbial growth, metabolism, infectious disease, and host-microbe interactions, with tables and examples for exam preparation.