Microbial Metabolism, Disease Terminology, Immune Evasion, and Microbial Control

Microbial Metabolism

Catabolic vs. Anabolic Reactions

Metabolism encompasses all chemical reactions within a cell, divided into catabolic and anabolic pathways.

• Catabolic Reactions: Break down complex molecules into simpler ones, releasing energy. These are typically exergonic reactions.

• Anabolic Reactions: Build complex molecules from simpler ones, requiring energy input. These are endergonic reactions.

• Hydrolysis: A catabolic process where water is used to break chemical bonds (e.g., digestion of proteins).

• Dehydration Synthesis: An anabolic process where molecules are joined together by removing water (e.g., formation of polysaccharides).

• Exergonic: Reactions that release energy (e.g., cellular respiration).

• Endergonic: Reactions that require energy input (e.g., protein synthesis).

• Example of Catabolism: Breakdown of glucose during glycolysis.

• Example of Anabolism: Synthesis of DNA from nucleotides.

ATP: Structure and Function

Adenosine triphosphate (ATP) is the primary energy carrier in cells.

• Structure: Composed of adenine (a nitrogenous base), ribose (a five-carbon sugar), and three phosphate groups.

• ADP/ATP Cycle: ATP is hydrolyzed to ADP (adenosine diphosphate) and inorganic phosphate, releasing energy. ADP can be rephosphorylated to ATP using energy from catabolic reactions.

• Energy Release: The hydrolysis of the terminal phosphate bond of ATP releases energy for cellular work.

Equation:

$\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

Cellular Respiration

Cellular respiration is the process by which cells extract energy from organic molecules.

• Glycolysis: Occurs in the cytoplasm; glucose is split into two pyruvate molecules, producing a net gain of 2 ATP and 2 NADH.

• Pyruvate Oxidation: Pyruvate is converted to acetyl-CoA, producing NADH and releasing CO2.

• Krebs Cycle (Citric Acid Cycle): Acetyl-CoA is oxidized, generating NADH, FADH2, ATP, and CO2.

• Electron Transport Chain (ETC): Electrons from NADH and FADH2 are transferred through protein complexes, driving ATP synthesis via oxidative phosphorylation.

• Fermentation: An anaerobic process where pyruvate is converted to various end products (e.g., lactic acid, ethanol) to regenerate NAD+.

Overall Equation for Aerobic Respiration:

$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 \rightarrow 6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + \text{energy (ATP)}$

Principles of Infectious Disease and Epidemiology

Disease Terminology

Understanding disease terminology is essential for describing patterns and characteristics of infectious diseases.

• Emerging Diseases: Newly identified diseases or diseases increasing in incidence (e.g., COVID-19).

• Reemerging Diseases: Previously controlled diseases that are again becoming significant (e.g., tuberculosis).

• Sporadic: Diseases that occur occasionally and at irregular intervals (e.g., tetanus).

• Endemic: Diseases constantly present in a population (e.g., malaria in certain regions).

• Epidemic: A sudden increase in disease cases above normal expectations in a region (e.g., influenza outbreaks).

• Pandemic: An epidemic that spreads across countries or continents (e.g., HIV/AIDS, COVID-19).

• Zoonotic: Diseases transmitted from animals to humans (e.g., rabies).

• Communicable: Diseases that can be transmitted from one host to another (e.g., measles).

• Noncommunicable: Diseases not spread from host to host (e.g., tetanus from soil).

• Contagious: Easily spread communicable diseases (e.g., chickenpox).

• Symptomatic: Infections with observable symptoms.

• Asymptomatic: Infections without noticeable symptoms.

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

• Symptoms: Subjective feelings reported by the patient (e.g., pain, fatigue).

• Acute: Diseases with rapid onset and short duration (e.g., influenza).

• Chronic: Diseases with slow onset and long duration (e.g., hepatitis B).

• Infectivity: Ability of a pathogen to establish an infection.

• Virulence: Degree of pathogenicity or severity of disease caused by a microbe.

• Pathogenicity: Ability of a microbe to cause disease.

Host-Microbe Interactions and Pathogenesis

Immune Evasion Mechanisms

Pathogens have evolved strategies to evade the host immune system, enhancing their survival and virulence.

• Antigen Masking: Pathogens cover their antigens with host molecules to avoid immune detection.

• Antigen Mimicry: Pathogens produce molecules similar to host antigens, reducing immune recognition.

• Antigen Variation: Pathogens alter their surface antigens to evade immune responses (e.g., Trypanosoma brucei).

• Avoiding Phagocytosis: Mechanisms include producing capsules, inhibiting phagosome-lysosome fusion, or surviving inside phagocytes.

Microbial Control

Chemical Control Methods

Chemical agents are used to control microbial growth on surfaces and in environments.

• Detergents: Disrupt cell membranes; effective against many bacteria and enveloped viruses.

• Alcohols: Denature proteins and disrupt membranes; effective against bacteria, fungi, and enveloped viruses.

• Phenols: Disrupt cell walls and membranes; used in disinfectants.

• Aldehydes: Inactivate proteins and nucleic acids; used for sterilization (e.g., formaldehyde).

• Halogens: Oxidize cellular components; include chlorine and iodine.

• Peroxygens: Produce reactive oxygen species; include hydrogen peroxide.

• Ethylene Oxide: Alkylates proteins and DNA; used for sterilizing heat-sensitive materials.

Microbial Resistance to Control Methods

Different microbes vary in their resistance to chemical and physical control methods.

Microbe Type	Relative Resistance	Notes
Mycobacteria	High	Waxy cell wall resists many chemicals
Endospores	Very High	Highly resistant to heat, chemicals, and radiation
Enveloped Viruses	Low	Lipid envelope is easily disrupted
Naked Viruses	Moderate	Lack envelope; more resistant than enveloped viruses
Protozoa	Variable	Cysts are more resistant than trophozoites
Prions	Extremely High	Resistant to standard sterilization methods

Example: Endospores of Bacillus and Clostridium species are highly resistant to boiling and many disinfectants.

Additional info: Prions require special decontamination procedures, such as autoclaving at higher temperatures and using strong chemicals.