Viruses
Genome: DNA or RNA
- DNA = Linear or Circular, Double or Single Stranded
- RNA = Linear or Segmented, Double or Single Stranded, Positive/Negative/Ambi- sense
Naked and Enveloped viruses
Virion (Viral Particle) Composition
Structures present in SOME viruses:
- Envelopes
- Peplomers (Glycoprotein spikes): provide target for neutralizing Antibodies
- Packaged enzymes
Viral Proteins
- Structural Proteins (part of virion)
- Non-structural (enzymes; not always packaged in virion)
Classification Properties of Viruses
Genome
- DNA or RNA
Strand type and Polarity
- ss/ds stranded, positive/negative sense
Enveloped vs. Naked
Virion Morphology
- Shape, size, symmetry
Replication Mechanism
Host Range
Type of Disease/Symptoms caused
General Progression of Viral Infection
Acquisition
- Portal of Entry (respiration, wound, STD, fecal-oral)
Primary Site Replication
- initiation of infection, primary source
Incubation Period
- no symptoms/ prodrome (early symptom)
Dissemination
- via blood, phagocytic cells, lymphatic system, neurons
- primary viremia: virus now in blood
Secondary Site Replication
- secondary viremia: replication in macrophages or vascular endothelial cells
Target Tissue, Symptoms develop
Viral Infection Types
Abortive: no progeny produced (nonpermissive infection or defective virus)
Productive (non-lytic): host cell altered but not lysed (permissive and productive)
- enveloped virus
Productive (lytic): host cell lysed and viral progeny released (permissive and productive
- naked viruses
Latent: persistence with no progeny; viral genome may integrate into host genome
Note: Tropism (recognition of target cell) of viruses determine tissue/cells targeted
Viral Replication Pathways
Naked (Lytic)/Enveloped (Budding) - Adherence - Penetration - Replication - Assembly - Release Note: Mechanism the same; progeny released differently (lytic vs. budding)
Transforming
- Adherence
- Penetration
- Integration
- Host Cell Transformed (Tumor/Cancer)
Viral Entry Mechanisms
All involve Adherence and Penetration
Fusion - enveloped virus fuses with host membranes
- leads to formation of syncytia due to Peplomers remaining in the host membrane
Receptor-mediated endocytosis (fusion with/lysis of endosome)
- think of as pH-dependent; will not fuse at neutral pH, so cannot form Syncytia
- enveloped viruses will send their genetic material to nucleus after fusing; Naked viruses will send nucleocapsid to nuclear pores, which then inject nucleic acid into nucleus
Pore Formation
Lysosomal Uncoating (capsid digested/opened up to release nucleic acid)
Viral Synthesis/Replication and Assembly/Release
- Transcription of Immediate Early/Early mRNA (non-structural regulatory proteins
- Replication of Genome
- Transcription of Late mRNA (structural protein synthesis)
Assembly/Release
- Inclusion bodies may form (insoluble protein deposits) wherever viral gene is active
- Enveloped viruses assemble and ‘bud off’ to obtain envelope
General Virus Replication Pathway
Parental DNA
Early mRNA transcribed
Early proteins translated from early mRNA
Early proteins (and cell enzymes) assist in replication of Parental DNA
Progeny DNA
Late mRNA transcribed
Late proteins (structural) translated from late mRNA
Assemble structural proteins into nucleocapsid around progeny DNA
Basis of Viral Classification
Mechanism of genomic replication differs based on genome type, strand type, and sense type.
- all viruses must generate Positive sense mRNA; this determines what a virus must carry to survive
**Be sure to understand each pathway and the differences between classes
Source of Polymerase
DNA Viruses
- dsDNA = Host polymerase used
- ssDNA = Host polymerase used
- dsDNA w/ RNA intermediate = Viral polymerase used
–Notable Exception: Pox Virus (dsDNA) uses Viral polymerase
RNA Viruses
- dsRNA = Viral polymerase used
- ssRNA (+ sense) = Host polymerase used
- ssRNA (- sense) = Viral Polymerase used
- ssRNA w/ DNA intermediate = Viral polymerase used
Replication Sites
DNA = Nucleus
- EXCEPT: Pox viruses
RNA = Cytoplasm
- EXCEPT: Orthomyxo viruses
Genotypic Mixing
2 identical(?) viruses with segmented genome infect the same cell
- Progeny viruses may be Reassortant, meaning they have mixed genome from both viruses.
- useful for viral attenuation (virulent strain + attenuated strain = attenuated virulent strain)
Phenotypic Mixing
Host infected with 2 non-identical viruses
- Genome of progeny doesn’t change, but peplomers may change, which can alter host range or resistance to antibody neutralization
Hemagglutination vs. HAI Assays
Hemagglutanin = Peplomer
HA Assay used to detect how much virus is found in a sample
- Flat disc formation due to the ‘sticking’ mechanism conferred to RBCs from viruses with Hemagglutanin
HAI Assays are used to detect how much antibody there may be in a sample
- ‘button’ formation due to neutralization of any cells bearing HA peplomer.
Fungal Cell Wall PAMPs
Mannan: component of cell wall, more abundant in Yeast
Chitin and Glucans: compound that gives rigidity to fungal cell wall
Fungal Growth Methods
Yeasts
- Unicellular
- Asexual reproduction via Budding
Molds/Filamentous fungi
- Multi-cellular
- Long chains called Hyphae; septate vs. aseptate/coenocytic (multinucleated hyphae)
- Asexual mitosis via spores call conidia (Anamorphic) OR sexual spores by meiosis (Teleomorphic)
Fungal Virulence Factors
Note: Normally do not cause serious disease in healthy individuals; typically are opportunistic infection
Virulence Factors
- Capsules
- Hydrolytic Enzymes
- AFLATOXINS (THESE SUCKERS ARE CRAZY CARCINOGENIC; THE MOST CANCEROUS TOXIN WE KNOW)
- Melanin (free radical scavenger; counters phagocytic activity that uses free radicals)
- PGE
Mycotic Diseases
Hypersensitivity Diseases
- Airborne spores, filaments, particulates; independent of growth/viability of fungus itself
- allergy and infection potentially simultaneous
Mycotoxicoses/Mycetismus
- Due to production of Secondary Metabolites, which can disrupt/alter physiological functions
Mycoses (infections)
- Superficial/Cutaneous
- Subcutaneous
- Systemic
Protozoans
Single Cells; Extra/Intracellular
4 Types:
Amoebas
- move via pseudopods
Flagellates
- move via flagella
Ciliates
- move via cilia
Apicomplexa
- non motile
Pathogenesis/Damage of Parasitic Disease
Transmission
- Fecal Oral
- Direct Penetration
- Arthropod Transmission
- Transplacental (Toxoplasmosis)
Adherence/Replication
- May be specific (Plasmodium binds to Duggy antigen) or nonspecific (hookworm’s hook)
Cell/Tissue Damage
- Hydrolytic Enzymes and toxins
- Mechanical damage
- immunopathology
- burden of organism
