Microbial Metabolism, Infectious Disease, and Control of Microorganisms: Study Guide

Microbial Metabolism

Metabolism: Definition and Overview

Metabolism refers to the sum of all chemical reactions occurring within a cell. These reactions are essential for maintaining life, enabling cells to grow, reproduce, and respond to their environment.

• Catabolic reactions: Break down complex molecules into simpler ones, releasing energy.

• Anabolic reactions: Build complex molecules from simpler ones, requiring energy input.

Catabolic vs. Anabolic Reactions

• Catabolic Reactions

  • Break down molecules (e.g., glucose breakdown)

  • Hydrolytic (use water to break bonds)

  • Exergonic (release energy)

  • Produce ATP

  • Example: Cellular respiration

• Anabolic Reactions

  • Build molecules (e.g., protein synthesis)

  • Use dehydration synthesis (remove water to form bonds)

  • Endergonic (require energy)

  • Consume ATP

  • Example: Protein synthesis

Amphibolic Reactions

Amphibolic reactions are metabolic pathways that function in both catabolism and anabolism. They provide intermediates for both energy production and biosynthesis.

• Example: Krebs cycle (citric acid cycle)

Adenosine Triphosphate (ATP)

• Definition: ATP is the cell’s primary energy currency, storing energy in high-energy phosphate bonds.

• Structure: Composed of adenine, ribose sugar, and three phosphate groups.

• ATP-ADP Cycle: Energy is stored when ADP is phosphorylated to ATP and released when ATP is hydrolyzed to ADP and inorganic phosphate (Pi).

Key Equations:

• $\mathrm{ADP} + \mathrm{P_i} + \text{energy} \rightarrow \mathrm{ATP}$

• $\mathrm{ATP} \rightarrow \mathrm{ADP} + \mathrm{P_i} + \text{energy}$

Role in Metabolism:

• Catabolic reactions generate ATP.

• Anabolic reactions consume ATP.

Enzymes and Their Function

• Definition: Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

• Enzymes are not consumed in the reaction and can be reused.

Collision Theory: For a reaction to occur, molecules must collide with proper orientation and sufficient energy. Enzymes increase the frequency of successful collisions by bringing substrates together and stabilizing the transition state.

Enzyme-Substrate Interaction Mechanism:

1. Substrate binds to the enzyme’s active site.

2. Enzyme-substrate complex forms.

3. Chemical reaction occurs.

4. Products are released.

5. Enzyme is available for reuse.

Cellular Respiration and Fermentation

Steps of Cellular Respiration

1. Glycolysis

   • Location: Cytoplasm

   • Glucose is converted to 2 pyruvate molecules

   • Inputs: Glucose, 2 ATP

   • Outputs: 2 Pyruvate, 2 NADH, net gain of 2 ATP

2. Intermediate Step (Pyruvate Oxidation)

   • Location: Mitochondrial matrix (eukaryotes), cytoplasm (prokaryotes)

   • Pyruvate is converted to Acetyl-CoA

   • Outputs: Acetyl-CoA, NADH, CO2

3. Krebs Cycle (Citric Acid Cycle)

   • Location: Mitochondrial matrix

   • Inputs: Acetyl-CoA

   • Outputs: CO2, NADH, FADH2, 2 ATP

4. Electron Transport Chain (ETC)

   • Location: Inner mitochondrial membrane

   • Uses O2 as final electron acceptor

   • Inputs: NADH, FADH2, O2

   • Outputs: Water, 32–34 ATP

Fermentation

• Occurs in the absence of oxygen (anaerobic)

• Regenerates NAD+ to allow glycolysis to continue

• Types: Lactic acid fermentation, alcohol fermentation

• ATP yield: 2 ATP per glucose (much less than aerobic respiration)

Principles of Infectious Disease and Epidemiology

Disease Terminology

• Emerging disease: Newly identified or rapidly increasing in incidence.

• Reemerging disease: Previously controlled, now returning.

• Sporadic: Occurs occasionally and irregularly.

• Endemic: Constantly present in a specific region.

• Epidemic: Sudden increase in cases in a region.

• Pandemic: Worldwide epidemic.

• Zoonotic: Transmitted from animals to humans.

• Communicable: Spread from person to person.

• Noncommunicable: Not spread person to person.

• Contagious: Easily spread between individuals.

• Symptomatic: Shows symptoms.

• Asymptomatic: No symptoms present.

• Signs: Objective evidence (e.g., fever, rash).

• Symptoms: Subjective feelings (e.g., pain, nausea).

• Acute: Short duration.

• Chronic: Long-lasting.

• Infectivity: Ability to infect a host.

• Virulence: Severity of disease caused.

• Pathogenicity: Ability to cause disease.

ID50 and Infectivity

• ID50 (Infectious Dose 50): Number of organisms required to cause infection in 50% of hosts.

• Lower ID50 = Higher infectivity

• Higher ID50 = Lower infectivity

Comparison of Toxins

Feature	Endotoxins	Exotoxins
Chemical Nature	Lipid A (part of LPS)	Protein
Source	Gram-negative bacteria	Gram-positive & Gram-negative bacteria
Release	When cell dies/divides	Secreted by living bacteria
Toxicity	Lower	Highly toxic
Fever	Common	Usually absent
Toxoid Vaccine	No	Yes
Neutralized by Antibodies	No	Yes

Five Steps of Infection

1. Portal of entry

2. Adhesion to host tissues

3. Invasion of host tissues

4. Infection/disease development

5. Portal of exit

Pathogen Nutrient Acquisition Tools

• Transferrins: Host proteins that bind and transport iron.

• Siderophores: Bacterial molecules that scavenge iron from the host.

• Extracellular enzymes: Break down host tissues to release nutrients.

Immune Evasion Mechanisms

• Antigen masking: Pathogen covers itself with host molecules.

• Antigen mimicry: Pathogen mimics host cell surface molecules.

• Antigen variation: Pathogen changes its surface antigens to avoid detection.

Phagocytosis Avoidance Strategies

• Production of a capsule

• Formation of biofilms

• Survival inside phagocytes

• Prevention of phagosome-lysosome fusion

• Production of toxins that kill phagocytes

Control of Microorganisms

Germicidal Agents: Levels and Examples

Level	Microbial Targets	Notes
Low-level	Most bacteria, some fungi, some viruses	Not effective against endospores
Intermediate-level	Mycobacteria, most bacteria, most fungi, most viruses	Not reliable against endospores
High-level	Nearly all microorganisms, including endospores (with sufficient exposure)	Used for sterilization

Factors in Selecting Germicides

• Type of microbe present

• Number of microbes

• Presence of organic matter

• Contact time

• Temperature

• Surface/material compatibility

• Safety for users and environment

Chemical Methods of Decontamination

• Detergents: Disrupt cell membranes, remove microbes from surfaces.

• Alcohols: Denature proteins, dissolve membranes.

• Phenols: Damage proteins and membranes.

• Aldehydes: Cross-link proteins and DNA, inactivating them.

• Halogens: Oxidize proteins (e.g., chlorine, iodine).

• Peroxygens: Produce free radicals that damage cellular components.

• Ethylene oxide: Sterilizing gas that damages DNA and proteins.

Examples of Microbial Control

Microorganism	Control Method
Mycobacteria	Intermediate/high-level disinfectants
Endospores	Autoclaving, ethylene oxide
Enveloped viruses	Alcohols, detergents
Naked viruses	Stronger disinfectants (more resistant)
Protozoa	High-level disinfectants or sterilization
Prions	Prolonged autoclaving, strong chemicals (extremely resistant)