Pathogen Study Guide

📖 Core Concepts Pathogen – any organism/agent that can cause disease (virus, bacteria, fungi, protozoa, prions, viroids). Pathogenicity – combines infectivity (ability to enter a host) + virulence (severity of disease). Koch’s Postulates – 4 criteria to prove a micro‑organism causes a specific disease. Basic reproduction number (R₀) – average number of new cases generated by one infected individual; >1 means outbreak potential. Optimal virulence theory – pathogens balance host damage with transmission; too lethal = fewer hosts, too mild = poor spread. Virulence mechanisms – nutrient extraction, immune evasion, toxin production, immunosuppression. Baltimore classification – 7 virus classes based on how they produce mRNA. Antigenic escape – alteration of surface proteins to avoid host immunity. 📌 Must Remember Infectivity vs. Virulence – infectivity = “can get in”; virulence = “how bad it is”. Koch’s Postulates: (1) isolate microbe, (2) grow in pure culture, (3) reproduce disease in healthy host, (4) re‑isolate same microbe. R₀ > 1 → epidemic; R₀ < 1 → dies out. Baltimore Classes: I‑dsDNA, II‑ssDNA, III‑dsRNA, IV‑+ssRNA, V‑‑ssRNA, VI‑RT‑RNA→DNA, VII‑DNA‑RT. Major bacterial pneumonia agents: S. pneumoniae, S. aureus, K. pneumoniae, H. influenzae. Key bacterial food‑borne pathogens: Campylobacter, C. perfringens, E. coli, Listeria, Salmonella. Antibiotic‑resistant hallmark: MRSA = resistant to β‑lactams. No curative therapy for prion diseases; management is supportive only. Vaccines – effective for influenza, MMR, anthrax, pneumococcus; none for HIV, dengue, chikungunya. 🔄 Key Processes Infection Cycle (generic) Exposure → Entry (direct contact, droplets, vector) → Replication → Damage (virulence) → Transmission. Viral Lytic vs. Lysogenic Cycle Lytic: attach → inject genome → produce progeny → cell lysis → release. Lysogenic: integrate genome into host DNA → replicate silently → may later enter lytic. Antibiotic Action (e.g., doxycycline) Binds 30S ribosomal subunit → blocks tRNA entry → inhibits protein synthesis (both Gram‑±). Genetic Transformation in Bacteria Uptake of free DNA → recombination into chromosome → new traits (e.g., antibiotic resistance). Meiotic Sexual Reproduction in Eukaryotic Pathogens Gamete formation → fertilization → diploid stage → meiosis → genetic diversity (e.g., Plasmodium). 🔍 Key Comparisons Bacteria vs. Viruses Bacteria: cellular, can replicate independently, treat with antibiotics. Viruses: acellular, need host machinery, treat with antivirals or vaccines. Lytic vs. Lysogenic Cycle Lytic = rapid replication + cell death. Lysogenic = genome integration, silent, can reactivate. Gram‑positive vs. Gram‑negative (antibiotic susceptibility) Gram‑positive: thick peptidoglycan, often β‑lactam‑sensitive. Gram‑negative: outer membrane, many β‑lactam‑resistant (e.g., MRSA is Gram‑positive but β‑lactam‑resistant). Prion vs. Viral Infection Prion: misfolded protein, no nucleic acid, no immune response, untreatable. Virus: nucleic acid genome, can be targeted by immune system/vaccines. ⚠️ Common Misunderstandings “All viruses have vaccines.” – HIV, dengue, chikungunya lack approved vaccines. “Antibiotics kill viruses.” – they target bacterial processes only; misuse fuels resistance. “High virulence is always better for a pathogen.” – optimal virulence balances host survival and transmission. “All parasites are helminths.” – protozoa are single‑celled parasites; helminths are macro‑parasites. 🧠 Mental Models / Intuition “Infectivity = Door, Virulence = Damage” – visualize a pathogen first finding an open door (entry) then deciding how much to break the house (damage). “R₀ as a fire‑starter” – if each spark lights >1 new fire, the blaze spreads; if ≤1, it fizzles. “Baltimore as a production line” – each class shows the raw material (DNA/RNA) and the conversion step to mRNA, like a factory recipe. 🚩 Exceptions & Edge Cases Prions – defy classic pathogen rules (no nucleic acid, no immune detection). Retroviruses (Class VI) – RNA → DNA intermediate; integrate into host genome (e.g., HIV). Hepatitis B (Class VII) – DNA virus that uses an RNA intermediate, opposite of most DNA viruses. Antibiotic resistance can arise without exposure – spontaneous mutations can pre‑exist and be selected later. 📍 When to Use Which Choose vaccine vs. antiviral – if a safe, effective vaccine exists (influenza, MMR, pneumococcus) → vaccinate; otherwise treat symptomatically or use antivirals (e.g., HAART for HIV). Select antibiotic – match drug class to organism’s cell wall type; use doxycycline for broad Gram‑± coverage; avoid β‑lactams for MRSA. Apply Koch’s postulates – when you need to prove causation for a newly identified microbe. Use R₀ – during outbreak modeling to decide on control measures (vaccination, isolation). 👀 Patterns to Recognize Transmission route → control strategy Airborne droplet → mask, ventilation. Vector‑borne → insect control, repellents. Virulence factor clusters – capsules + siderophores often indicate invasive bacterial pathogens. Antigenic escape – recurring mutations in surface proteins (e.g., influenza HA) → vaccine strain updates needed. Foodborne outbreak clues – rapid onset GI symptoms + Campylobacter or Salmonella → suspect undercooked poultry/meat. 🗂️ Exam Traps “All viruses are RNA viruses.” – Classes I, II, III, VII are DNA‑based; only IV, V, VI are RNA‑based. “MRSA is Gram‑negative.” – MRSA is a Gram‑positive Staphylococcus aureus strain resistant to β‑lactams. “Prion diseases can be cured with antivirals.” – Prions have no nucleic acid; antivirals are useless. “High R₀ always means high virulence.” – R₀ reflects transmission efficiency, not disease severity; e.g., measles (high R₀, moderate virulence). “All parasites are treated with antibiotics.” – Protozoa and helminths require antiparasitic drugs (e.g., antimalarials, anthelmintics), not antibiotics.