28. Immunity, Vaccines, and Antimicrobial Drugs: Microbiology Study Notes

Vaccination and Immunity

Vaccination (Immunization)

Vaccination is a cornerstone of public health and immunology, involving the induction of an artificial active immune response through exposure to antigens.

• Vaccination (immunization): The process of generating an artificial active immune response by exposure to an antigen or antigen mixture.

• Vaccine: The agent used to induce artificial active immunity.

• Artificial active immunity: Immunity acquired by deliberate exposure to an antigen via vaccination.

Innate and Adaptive Immunity

The immune system is divided into innate and adaptive branches, each with distinct roles in host defense.

• Innate immunity: Provides immediate, non-specific defense against pathogens. Primary effector cells include dendritic cells, neutrophils, and macrophages.

• Adaptive immunity: Develops more slowly and is highly specific. Primary effector cells are B and T lymphocytes.

• Innate responses occur within hours; adaptive responses require several days.

Adaptive Immunity

Mechanisms and Responses

Adaptive immunity is characterized by specificity and memory, allowing for a tailored and enhanced response upon re-exposure to antigens.

• Primary immune response: Occurs after first exposure to an antigen, involving activation of antigen-reactive leukocytes.

• Specificity: Each lymphocyte produces a unique protein (antibody or T-cell receptor) that interacts with a single antigen.

• Memory: Subsequent exposures result in a faster and stronger secondary immune response.

Active and Passive Immunity

Immunity can be acquired actively or passively, and by natural or artificial means.

Type	Natural	Artificial
Active Immunity	Infection (e.g., measles)	Vaccination
Passive Immunity	Antibodies from mother (placenta, breast milk)	Antibody injection (e.g., antivenom)

The Nature of Vaccines

Types and Effectiveness

Vaccines may contain inactivated, killed, or attenuated pathogens, and their effectiveness varies by type.

• Many vaccines use pathogens or their products inactivated or killed by heat or chemicals.

• Immunization with attenuated live cells or viruses is usually more effective than with dead or inactivated material.

• Booster immunizations are often required to produce a secondary response and higher antibody titers.

Immunization Recommendations

Immunizations are essential for controlling infectious diseases, with guidelines provided by health authorities.

• The CDC provides specific immunization schedules for children in the United States.

• Routine immunizations have greatly reduced the incidence of many infectious diseases.

Synthetic & Genetically Engineered Vaccines

Modern Vaccine Technologies

Advances in biotechnology have enabled the development of synthetic and genetically engineered vaccines.

• Synthetic vaccines: Genetically engineered antigenic components stimulate the immune response.

• Conjugate vaccines: Couple a weakly immunogenic antigen to a protein carrier (e.g., Streptococcus pneumoniae polysaccharide linked to diphtheria toxoid; Haemophilus influenzae type B polysaccharide coupled to tetanus toxoid).

• Recombinant-vector vaccines: Use viral or bacterial vectors to deliver antigens (e.g., rabies vaccine in animals).

• Recombinant-antigen vaccines: Directly use recombinant antigens (e.g., hepatitis B virus vaccine, HPV vaccine).

DNA Vaccines

DNA vaccines utilize genetic engineering to induce immunity.

• Genes for target proteins are cloned into plasmid vectors and injected intramuscularly.

• Host cells take up the DNA and express the protein, generating an immune response.

mRNA Vaccines

mRNA vaccines represent a new class of vaccines, exemplified by COVID-19 vaccines.

• mRNA encoding the antigen (e.g., spike protein) is delivered into host cells.

• Host cells produce the antigen, which stimulates an immune response.

• Immunity is achieved by the production of antibodies and activation of T cells.

Influenza Vaccines

Influenza vaccines are updated annually to match circulating strains.

• 2025-26 flu vaccines protect against three influenza viruses (two A strains, one B strain).

• Vaccine options include inactivated, live attenuated, and recombinant influenza vaccines.

Antimicrobial Drugs

Principles of Antibacterial Therapy

Antimicrobial drugs are designed to selectively target microbial pathogens without harming the host.

• Selective toxicity: The ability of a drug to target pathogens while minimizing damage to host cells.

Classification of Antimicrobial Drugs

Antimicrobial drugs are classified by their targets and spectrum of activity.

Target	Examples
Cell wall synthesis	Penicillins, Cephalosporins, Vancomycin
Protein synthesis	Aminoglycosides, Tetracyclines, Macrolides
Nucleic acid synthesis	Quinolones, Rifampin
Membrane function	Daptomycin, Polymyxins
Metabolic pathways	Sulfa drugs, Isoniazid

Cell Wall as a Drug Target

β-Lactam antibiotics are among the most widely used and effective antibacterial agents.

• β-Lactam antibiotics: Include penicillins and cephalosporins; inhibit cell wall synthesis.

• Over half of all antibiotics used worldwide are β-lactams.

Penicillins

• Discovered by Alexander Fleming.

• Primarily effective against gram-positive bacteria; some synthetic forms target gram-negative bacteria.

• Target cell wall synthesis by inhibiting transpeptidase enzymes.

Cephalosporins

• Produced by the fungus Cephalosporium.

• Same mode of action as penicillins.

• Commonly used to treat gonorrhea.

Protein Synthesis as a Drug Target

Antibiotics targeting protein synthesis bind to bacterial ribosomes, inhibiting translation.

• Examples: Aminoglycosides, tetracyclines, macrolides.

• These drugs exploit differences between prokaryotic and eukaryotic ribosomes.

Nucleic Acid Synthesis as a Drug Target

Quinolones interfere with bacterial DNA replication.

• Quinolones: Inhibit DNA gyrase (e.g., ciprofloxacin).

• Effective against both gram-positive and gram-negative bacteria.

• Fluoroquinolones are commonly used to treat urinary tract infections.

Other Antibacterial Drug Targets

• Growth factor analogs: Structurally similar to growth factors but nonfunctional in the cell.

• Sulfa drugs: Inhibit folic acid synthesis (e.g., sulfanilamide).

• Isoniazid: Effective against Mycobacterium; interferes with mycolic acid synthesis.

• Daptomycin: Forms pores in the cytoplasmic membrane; treats gram-positive infections.

• Platensimycin: Inhibits fatty acid and lipid biosynthesis; effective against MRSA and vancomycin-resistant enterococci.

Antiviral Drugs

Mechanisms of Action

Antiviral drugs target various stages of the viral life cycle.

• Nucleoside analogs: Block production of viral nucleic acid.

• Nonnucleoside reverse transcriptase inhibitors (NNRTIs): Inhibit reverse transcription in retroviruses.

• Protease inhibitors: Prevent processing of viral proteins.

• Fusion inhibitors: Prevent viral entry into host cells.

• Neuraminidase inhibitors: Limit influenza infection by inhibiting virus release (e.g., Tamiflu).

• Interferons: Stimulate antiviral protein production in uninfected cells.

Antifungal Drugs

Targets and Examples

Antifungal drugs exploit differences between fungal and human cells.

• Target membrane functions (e.g., polyenes), nucleic acid synthesis (e.g., 5-fluorocytosine), and cell wall synthesis (e.g., echinocandins).

• Fungi are eukaryotes, so fewer unique drug targets exist compared to bacteria.

Antimicrobial Drug Resistance

Development and Spread

Antibiotic use selects for resistant bacteria, leading to increased prevalence of resistance.

• Overuse and misuse of antibiotics accelerate resistance development.

• Resistance can be tracked over time and varies by drug class.

New Antimicrobial Drugs

Strategies for Discovery

New drugs are developed by modifying existing compounds, screening natural products, and using computational design.

• Modification of current compounds (e.g., vancomycin derivatives).

• Computational design of molecules to interact with microbial structures (e.g., saquinavir).

• Screening natural products led to discovery of platensimycin.

• Drug combinations with enzyme inhibitors (e.g., clavulanic acid with amoxicillin).

• Bacteriophage therapy: Use of viruses that infect bacteria as therapeutic agents.

Additional info: These notes cover topics from chapters on immunity, vaccines, antimicrobial drugs, and drug resistance, relevant to college-level microbiology.