Lecture 1.2 -Introduction to microbes and infection

Bacterial Infections

• Treatment of bacterial infections involves antibiotics, which interfere with:
  • Cell wall synthesis
  • Protein synthesis
  • Nucleic acid synthesis
  • Cell membrane function

Viruses

• Viruses are intracellular obligate parasites with:
  • Genetic material that is RNA or DNA
  • Enveloped or non-enveloped structure
  • Capsid symmetry that can be helical or icosahedral, giving viruses their shape
• Envelope: a lipid bilayer membrane acquired from the host cell cytoplasmic membrane
• Capsid: a protein coat for the viral genome and proteins

Properties of Enveloped and Non-Enveloped Viruses

• Enveloped viruses are more sensitive to harsh environments (e.g. heat, dryness) and are generally transmitted by:
  • Respiratory routes
  • Parenteral routes
  • Sexual routes
• Non-enveloped viruses are more stable in adverse environments and are generally transmitted by:
  • Faecal-oral route

Specific Viruses

• Coronaviruses (CoV):
  • Enveloped
  • Positive sense
  • Single stranded RNA
  • Infect humans, other mammals, and avian species
• Monkey pox virus:
  • Same family as variola virus (small pox)
  • Same symptoms as small pox but milder and not fatal
  • Spread through direct contact with monkey pox rash and scabs, as well as contact with saliva, upper respiratory secretions, and areas around the anus, rectum, or vagina
  • Detected by taking a swab from lesions and PCR

Virus Growth

• Viruses need living cells to replicate (obligate intracellular parasites)
• They use the living cells to synthesize all the constituents of the virus
• Different viruses grow in different types of cells
• In the laboratory, viruses can be grown in:
  • Vero cells
  • HeLa cells
  • Baby hamster kidney cells (BHK)

Steps in Viral Replication

• Viruses undergo a similar sequence of events:
  • Attachment to the appropriate cells
  • Penetration of the virus into the cell (endocytosis or fusion of the envelope with the host cell membrane)
  • Unloading of the viral capsid, releasing the genome and other materials into the host cell
  • Replication: initiation of transcription or translation of the viral genome, resulting in the manufacture of virus components and genome
  • Assembly of components into new viruses
  • Release of the viruses by lysis or budding from the target cells

DNA Virus Replication

• Transcription and replication occur in the nucleus of the infected cells
• Most DNA viruses assemble in the nucleus
• Early transcription (translation of proteins for DNA replication)
• Late transcription (translation of structural proteins)
• Assembly and release

RNA Virus Replication

• RNA viruses normally undergo transcription, translation, and replication in the cytoplasm
• Positive sense single-stranded RNA can function as mRNA and get translated into proteins by the host ribosomes
• Negative sense RNA has to be changed to positive mRNA using the enzyme RNA-dependent RNA polymerase to make a positive strand copy, which can be read by the ribosomes and result in the manufacture of proteins

Latency

• Latent viral infection: the virus can remain dormant within cells and does not cause symptoms until it is activated by some factors

Protozoa and Helminths

• Protozoa: unicellular eukaryotic organisms
  • Examples: Giardia lamblia, Cryptosporidium parvum, Plasmodium falciparum, Trypanosoma brucei
• Helminths: multi-cellular organisms divided into three groups:
  • Cestodes: ribbon-like, segmented intestinal parasites (e.g. Tapeworms)
  • Nematodes: long, non-segmented worms (e.g. Roundworms)
  • Trematodes: small, flat, leaf-like worms that can infect various organs (e.g. Flukes)

Prions

• Unconventional infectious agents with no nucleic acid
• Caused by infective proteins that accumulate in the grey matter and in extracellular amyloid plaques in the brain
• Examples: Scrapie in sheep, Bovine Spongiform Encephalopathy (BSE) in cattle, Creutzfeldt-Jakob disease (CJD)