Characterizing Viruses

General Properties of Viruses

Viruses are acellular infectious agents that require a host cell for replication. They are distinct from cellular life forms due to their unique structure and replication strategies.

• Obligate intracellular parasites: Viruses cannot reproduce outside a host cell and do not grow on standard laboratory media.

• Size: Too small to be seen with a light microscope; require electron microscopy for visualization.

• Replication: Multiply by directing the host cell to synthesize viral components.

• Host range: Determined by specific interactions between viral surface proteins and host cell receptors.

• Classification: Based on type of nucleic acid (DNA or RNA), presence or absence of an envelope, and other structural features.

Viral Structure

Viruses consist of a nucleic acid core (either DNA or RNA) surrounded by a protein coat called a capsid, which is made up of subunits called capsomeres. Some viruses possess an additional lipid envelope derived from the host cell membrane, often with glycoprotein spikes that facilitate host cell recognition and entry.

• Naked viruses: Lack an envelope.

• Enveloped viruses: Envelope acquired from host cell membranes during viral release.

• Spikes: Glycoproteins on the envelope that determine host specificity.

Viral Replication Cycles

Bacteriophage Replication

Bacteriophages are viruses that infect bacteria. Their replication involves several distinct steps:

1. Attachment (Adsorption): Phage binds to specific receptors on the bacterial surface.

2. Entry (Penetration): Injection of phage nucleic acid into the host cell.

3. Synthesis (Replication): Host machinery synthesizes viral components.

4. Assembly & Maturation: New virions are assembled.

5. Release: Host cell lyses, releasing new phages.

Lysogenic cycle: Some phages integrate their DNA into the host genome (prophage), leading to lysogeny and possible lysogenic conversion (alteration of host phenotype).

Animal Virus Replication

Animal viruses follow a similar but more complex replication cycle, with variations depending on the virus type.

1. Attachment: Viral proteins bind to specific host cell receptors (e.g., HIV gp120 binds CD4 and CXCR4; SARS-CoV-2 spike binds ACE2).

2. Entry: Can occur via direct penetration (naked viruses), endocytosis (enveloped viruses), or membrane fusion.

3. Uncoating: Removal of the capsid to release viral genome into the host cell.

4. Synthesis: Dependent on the nature of the viral genome (DNA, +ssRNA, -ssRNA, or retrovirus).

5. Maturation: Assembly of viral components into new virions.

6. Release: By budding (enveloped viruses) or cell lysis (naked viruses).

• Persistent infection: Virus remains in the host for extended periods, sometimes with ongoing replication.

• Latency: Viral genome persists in host cells without active replication, can reactivate later.

Viruses and Cancer (Oncoviruses)

Oncogenic Viruses

Certain viruses are linked to cancer development by interfering with normal cell cycle regulation. Examples include:

• Human papillomavirus (HPV): Cervical and other anogenital cancers.

• Hepatitis B Virus (HBV): Hepatocellular carcinoma.

• Hepatitis C Virus (HCV): Hepatocellular carcinoma.

• Epstein-Barr Virus (EBV): Burkitt lymphoma, nasopharyngeal carcinoma.

Oncogenic mechanisms often involve viral proteins (e.g., HPV E6 and E7) that inactivate tumor suppressors such as p53.

Diagnostic Virology

Laboratory Techniques

• Tissue or Cell Culture: Growth of viruses in cell monolayers; cytopathic effect (CPE) is observed microscopically.

• Chicken Eggs: Used for vaccine production (e.g., influenza).

• ELISA: Detects viral antigens in patient samples; rapid tests for SARS-CoV-2, Rotavirus, Influenza, RSV.

• PCR/DNA Probes: Amplifies and detects viral nucleic acids; highly sensitive and specific.

• Serological Testing: Detects patient antibodies against viruses (e.g., HBV, HCV, EBV, CMV, HIV).

Prions

Characteristics of Prions

Prions are infectious proteins that lack nucleic acids. They cause disease by inducing abnormal folding of normal cellular prion proteins (PrP), especially in neural tissue.

• Transmissible spongiform encephalopathies: vCJD in humans, BSE in cattle, CWD in deer.

• Prion PrP: Pathogenic form that induces misfolding of normal PrP.

Pathogenic DNA Viruses

Poxviridae Family (Orthopoxvirus)

Smallpox: Eradicated in 1980; last natural case in 1977. Concerns remain about accidental or intentional release from laboratory stocks.

• Eradication factors: No animal reservoir, effective vaccine, obvious symptoms, short infectivity period, global cooperation.

Herpesviridae Family

• General characteristics: dsDNA, enveloped, latency, most prevalent DNA viruses.

• HHV-1 (HSV-1): Oral herpes; remains dormant in trigeminal ganglia.

• HHV-2 (HSV-2): Genital herpes; remains dormant in sacral ganglia.

• HHV-3 (Varicella Zoster Virus): Chickenpox and shingles; dormant in dorsal root ganglia.

• HHV-4 (Epstein-Barr Virus): Infectious mononucleosis; linked to certain cancers.

• HHV-5 (Cytomegalovirus): Congenital infections; severe in immunocompromised patients.

Human Papillomavirus (HPV)

• Structure: ssDNA, naked.

• Oncogenic strains: Types 16 and 18 cause most cervical cancers.

• Diagnosis: Pap test, HPV DNA/mRNA testing.

• Prevention: Gardasil-9 vaccine protects against multiple oncogenic and non-oncogenic strains.

Hepatitis B Virus (HBV)

• Genome: Partly dsDNA, partly ssDNA; enveloped.

• Transmission: Blood, sexual contact, perinatal.

• Symptoms: Jaundice, fatigue, liver inflammation.

• Diagnosis: Blood tests for antigens and antibodies.

• Prevention: Recombinant vaccine.

Pathogenic RNA Viruses

Picornaviridae Family

• Rhinovirus: Common cold; transmitted via droplets and fomites.

• Enteroviruses: Polio (paralytic disease, nearly eradicated), Coxsackie A/B, Echovirus.

• Hepatitis A Virus (HAV): Fecal-oral transmission; vaccine-preventable.

Arboviruses

• West Nile Virus: Birds are natural hosts; mosquito vector; most infections asymptomatic, some severe.

• Dengue Fever: Mosquito vector; "breakbone fever"; prevention focuses on vector control.

• Zika Virus: Mosquito vector; associated with congenital abnormalities.

Coronaviruses

• Structure: +ssRNA, enveloped; four main proteins: spike (S), envelope (E), membrane (M), nucleocapsid (N).

• Entry: S protein binds ACE2 receptor on host cells.

• Diseases: SARS, MERS, COVID-19.

• Symptoms: Respiratory and systemic; severe cases may lead to ARDS, organ failure.

Retroviridae Family (HIV)

• Genome: Two copies of ss(+)RNA; enveloped.

• Enzymes: Reverse transcriptase, protease, integrase.

• Target cells: CD4+ T cells.

• Stages of infection: Acute (flu-like symptoms), clinical latency, AIDS (opportunistic infections, CD4 < 200/mm3).

• Diagnosis: Viral load, CD4 count, antibody testing.

• Treatment: Antiretroviral therapy (ART) combines drugs from multiple classes.

Parasitic Protozoa, Helminths, and Arthropod Vectors

Protozoan Life Stages

• Cyst: Dormant, environmentally resistant, infective stage.

• Trophozoite: Active, feeding, and reproductive stage within the host.

Transmission and Life Cycles

• Entamoeba histolytica: Fecal-oral transmission; causes amebic dysentery.

• Acanthamoeba: Causes keratitis and encephalitis; enters through skin or mucosa.

• Naegleria: Causes primary amebic meningoencephalitis; enters via nasal passages.

Flagellates

• Trypanosoma cruzi: Chagas' disease; vector is the triatomine bug.

• Leishmania: Leishmaniasis; vector is the sand fly. Three forms: cutaneous, mucocutaneous, visceral.

• Giardia intestinalis: Giardiasis (Beaver Fever); fecal-oral transmission; diagnosis by cysts/trophs in stool or ELISA.

• Trichomonas vaginalis: Vaginosis; diagnosis by PCR or observation of motile trophs.

Apicomplexans

• Plasmodium: Malaria; vector is the Anopheles mosquito; diagnosis by blood smear.

• Toxoplasma gondii: Toxoplasmosis; transmission via undercooked meat or cat feces; diagnosis by serology.

• Cryptosporidium parvum: Causes diarrhea; diagnosis by acid-fast stain or ELISA of stool.

Helminths

• Ascaris lumbricoides: Ascariasis; diagnosis by eggs in stool or adult worm identification.

• Enterobius vermicularis: Pinworm; diagnosis by perianal tape test or adult worm identification.

Additional info: These notes integrate and expand upon the provided guided notes, filling in academic context and explanations for key microbiology concepts relevant to viruses, prions, and eukaryotic parasites.