Viruses, Viroids, and Prions Overview

Study Notes

Viruses are not living organisms, they lack cellular structure, metabolism, and cannot reproduce independently. They rely on host cells.
Viruses were initially identified as infectious agents smaller than bacteria.
Virus characteristics include a host range specific to the organisms, tissues, or cells a virus infects, size varying from 20-300 nm, and different shapes like helical or polyhedral.
A virion is a complete virus particle, it has a protein coat called a capsid, and sometimes an envelope derived from the host cell membrane.
Viral genomes can be DNA or RNA, single- or double-stranded.
Viral life cycles include the lytic cycle (adsorption, penetration, biosynthesis, assembly, and release) and the lysogenic cycle where viral DNA integrates into the host genome (prophage), reproducing with the host cell, with the potential for induction into the lytic cycle.
Animal viruses follow a similar life cycle involving adsorption, penetration, uncoating, replication, assembly, and release.
Viruses are cultured using tissue cultures where cells are grown in vitro to support viral replication.
Virus classification is based on genome type (DNA or RNA) and replication strategies.
Prions are infectious proteins that can cause neurodegenerative diseases by converting normal cellular prion proteins into their infectious form.

Pathogens are disease-causing agents.
Infection is the colonization of the host by microbes.
Disease results from damage to the host's tissues.
Microbiota are the microbes residing on and within the human body, influenced by factors such as birth, diet, environment, and health.
Prevalence is the total number of cases in a population.
Incidence is the number of new cases in a period of time.
Disease distribution includes endemic (constant presence), epidemic (localized outbreak), and pandemic (global outbreak).
Disease reservoirs include humans, animals, or the environment.
Transmission of disease can occur directly, indirectly, through droplets, or vector-borne methods.
Emerging diseases result from environmental changes, human behavior, or microbial evolution.
Opportunistic pathogens are normal microbiota that cause disease under certain conditions.

Pathogenicity is the ability of a microbe to cause disease.
Virulence is the degree of pathogenicity.
Virulence factors include adherence (adhesins), enzymes (coagulase, hyaluronidase), antigenic variation, and biofilms.
Hosts interact with pathogens via ID50 (infectious dose required to infect 50% of a population).
Biofilms are protective communities of microbes.
Toxins include exotoxins (protein toxins) and endotoxins (lipopolysaccharides from Gram-negative bacteria), which trigger inflammation.

Antibiotics and antimicrobials can be natural, semi-synthetic or entirely synthetic.
Selective toxicity is targeting microbial processes without harming the host.
Antibiotic spectrum is categorized into broad or narrow-spectrum.
Common targets of antimicrobial drugs include cell wall synthesis, protein synthesis, DNA/RNA synthesis, and metabolic pathways.
Antibiotics like beta-lactams inhibit peptidoglycan synthesis.
Antibiotic testing (e.g., Kirby-Bauer method) measures antibiotic susceptibility.
Antimicrobial resistance mechanisms include efflux pumps, enzyme production (e.g., beta-lactamases), and target modification.