Section 1.3: Medical Microbiology & Immunology

Medical Microbiology & Immunology

Pharmacists must possess a strong foundation in medical microbiology and clinical immunology. This knowledge is essential for selecting appropriate antimicrobial therapies, understanding the mechanisms of host defenses, interpreting vaccine immunology, and managing drug allergies and hypersensitivity reactions.

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1. Clinically Relevant Bacterial Pathogens & Resistance

Bacteria are classified based on their Gram-staining characteristics, morphology, and oxygen requirements.

Gram-Positive Pathogens

• Staphylococcus aureus: A major cause of skin/soft tissue infections, bacteremia, osteomyelitis, and endocarditis.
  • Resistance: Methicillin-resistant S. aureus (MRSA) is mediated by the mecA gene, which encodes an altered penicillin-binding protein (PBP2a) with low affinity for beta-lactams. Vancomycin is the drug of choice, though vancomycin-intermediate (VISA) and resistant (VRSA) strains exist.
• Streptococcus pneumoniae: Gram-positive diplococci; the leading cause of community-acquired pneumonia, meningitis, and otitis media.
• Enterococcus faecalis & Enterococcus faecium: Common causes of nosocomial urinary tract infections and endocarditis.
  • Resistance: Vancomycin-resistant Enterococcus (VRE) occurs via plasmid-mediated alteration of the peptidoglycan precursor from D-Ala-D-Ala to D-Ala-D-Lac or D-Ala-D-Ser, preventing vancomycin binding.

Gram-Negative Pathogens

• Enterobacteriaceae (e.g., Escherichia coli, Klebsiella pneumoniae): Primary causes of UTIs, intra-abdominal infections, and bacteremia.
  • Resistance: Extended-Spectrum Beta-Lactamases (ESBLs) hydrolyze penicillins, cephalosporins, and monobactams. Carbapenems (e.g., meropenem) are the treatment of choice. Carbapenem-resistant Enterobacteriaceae (CRE) produce carbapenemases (e.g., KPC, NDM-1) and require advanced drugs like ceftazidime-avibactam.
• Pseudomonas aeruginosa: An opportunistic, non-fermenting rod associated with ventilator-associated pneumonia, neutropenic fever, and burn wound infections.
  • Resistance: Exhibits multiple intrinsic and acquired resistance mechanisms, including efflux pumps, porin channel loss (OprD), and inducible AmpC beta-lactamases.

Atypical Pathogens

• Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella pneumophila: Lack a peptidoglycan cell wall (Mycoplasma) or are obligate intracellular organisms. They do not gram stain well and are intrinsically resistant to cell-wall active agents (beta-lactams). Macrolides, tetracyclines, or fluoroquinolones are required.

Acid-Fast Bacilli

• Mycobacterium tuberculosis: Has a unique, waxy cell wall rich in mycolic acids, requiring special acid-fast staining (Ziehl-Neelsen) and multi-drug regimens (isoniazid, rifampin, pyrazinamide, ethambutol) to prevent resistance.

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2. Viral and Fungal Pathogens

Viral Pathogens

Viruses are obligate intracellular parasites containing DNA or RNA genomes.

• DNA Viruses:
  • Herpesviridae: Includes Herpes Simplex Virus (HSV-1, HSV-2), Varicella-Zoster Virus (VZV), Cytomegalovirus (CMV), and Epstein-Barr Virus (EBV). Antivirals (e.g., acyclovir, ganciclovir) act as guanosine analogs to inhibit viral DNA polymerase.
  • Hepatitis B Virus (HBV): A partially double-stranded DNA virus.
• RNA Viruses:
  • Influenza Virus: Contains hemagglutinin (for viral entry) and neuraminidase (for release of new virions). Neuraminidase inhibitors (e.g., oseltamivir) prevent viral replication.
  • Human Immunodeficiency Virus (HIV): A retrovirus utilizing reverse transcriptase, integrase, and protease. Pharmacotherapy targets these enzymes (ART).
  • Hepatitis C Virus (HCV): Single-stranded RNA virus; now curable with direct-acting antivirals (DAAs).

Fungal Pathogens

Fungi are eukaryotic organisms with cell walls containing chitin and glucans, and cell membranes containing ergosterol.

• Opportunistic Fungi:
  • Candida albicans: Causes mucosal candidiasis (thrush, vulvovaginitis) and invasive candidiasis.
  • Aspergillus fumigatus: Mold causing invasive pulmonary aspergillosis, diagnosed via galactomannan antigen.
  • Cryptococcus neoformans: Encapsulated yeast causing meningitis in immunocompromised patients, diagnosed with India ink stain.
• Antifungal Mechanisms: Azoles inhibit ergosterol synthesis (via CYP51); polyenes (amphotericin B, nystatin) bind ergosterol to create membrane pores; echinocandins (caspofungin) inhibit glucan synthesis.

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3. Immunology Foundations: Cells and Antibodies

The immune system is divided into innate (non-specific, rapid) and adaptive (specific, memory-generating) immunity.

Key Immune Cells

• Innate Immunity:
  • Neutrophils: Primary responders to bacterial infections; engage in phagocytosis and oxidative burst.
  • Macrophages & Dendritic Cells: Professional antigen-presenting cells (APCs) that present antigens to T-cells via MHC molecules.
  • Natural Killer (NK) Cells: Destroy virus-infected and tumor cells.
• Adaptive Immunity:
  • T-Lymphocytes: Mature in the thymus. CD4+ Helper T-cells recognize antigens presented on MHC Class II and orchestrate the immune response. CD8+ Cytotoxic T-cells recognize antigens on MHC Class I and destroy infected cells.
  • B-Lymphocytes: Mature in the bone marrow. Differentiate into plasma cells to secrete antibodies or memory B-cells to provide long-term immunity.

Immunoglobulin Classes (Antibodies)

Antibodies consist of two heavy chains and two light chains, featuring antigen-binding (Fab) and constant (Fc) regions.

• IgG: The most abundant class in serum. It is the only antibody that crosses the placenta (providing passive immunity to the neonate) and mediates secondary (anamnestic) immune responses.
• IgM: A pentamer in circulation. It is the first antibody produced during a primary immune response and is highly efficient at complement activation.
• IgA: Exists as a dimer in mucosal secretions (tears, saliva, colostrum). Prevents pathogen colonization on mucosal surfaces.
• IgE: Binds to Fc receptors on mast cells and basophils. Mediates Type I hypersensitivity (allergic/anaphylactic) reactions and response to helminth infections.
• IgD: Found on the surface of mature B-cells; functions in B-cell activation.

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4. Vaccine Responses and Hypersensitivity Reactions

Vaccine Immunology

• Active Immunity: Induced by exposure to an antigen (infection or vaccination), triggering the host to produce antibodies and memory cells.
• Passive Immunity: Immediate but temporary protection via transfer of preformed antibodies (e.g., immunoglobulin administration, maternal IgG transfer).
• Live Attenuated Vaccines (e.g., MMR, Varicella, Yellow Fever): Contain live, weakened pathogens. They generate strong, long-lasting cellular and humoral responses, but are contraindicated in pregnancy and severely immunocompromised patients due to the risk of uncontrolled pathogen replication.
• Inactivated / Subunit / Conjugate Vaccines (e.g., Pneumococcal, Influenza): Cannot replicate. Conjugate vaccines (like PCV13) couple polysaccharide antigens to a carrier protein, converting a T-cell independent response into a T-cell dependent response, which successfully induces immunological memory in infants.

Hypersensitivity Reactions (Gell and Coombs Classification)