Bacterial Anatomy and Antibiotics

• Abacterium is a prokaryotic microorganism with various shapes (rod, spherical, curved, square, or star-shaped) and arrangements (chains, clusters, or pairs).
• The bacterial cell membrane encloses the cell's contents, acting as a barrier to retain nutrients, proteins, and cytoplasm components.
• Bacteria, as prokaryotes, lack membrane-bound organelles, a true nucleus, mitochondria, and chloroplasts.
• Some bacteria move using flagella, bacterial gliding, twitching motility, or by changing buoyancy.
• Some bacteria have two movement modes: forward swimming and tumbling.
• Gram-positive bacteria have thick peptidoglycan layers in their cell walls, staining purple; Gram-negative bacteria have thin cell walls, staining pink.
• Mycobacteria or Nocardia are identified using acid-fast stains like Ziehl-Neelsen.
• Antibiotics treat bacterial infections, classified as bacteriocidal (killing bacteria) or bacteriostatic (preventing growth).

Virus Components and Structure:

• A virus is smaller than bacteria and can infect bacteria and other microorganisms.
• Viruses consist of genetic material (DNA or RNA), a protein coat (capsid), and sometimes a lipid envelope.
• A complete virus particle (virion) has nucleic acid surrounded by a protective protein coat (capsid) made of capsomeres.
• Capsid shape determines morphological distinction and is made from proteins encoded by the viral genome.
• Viruses can have a lipid envelope derived from the host cell membrane.

Viral Morphology

• Helical viruses consist of capsomeres stacked around a central axis, forming a helical structure (e.g., tobacco mosaic virus).
• Icosahedral viruses (e.g., rotavirus) have more than twelve capsomeres and appear spherical, with pentons at the apices.
• Spherical viruses (enveloped viruses) have an icosahedral or helical structure surrounded by a lipid bilayer membrane (e.g., influenza, Hepatitis C, HIV).
• Complex viruses have a combination of icosahedral and helical shapes and may have a complex outer wall or head-tail morphology.

Complex Viruses

• Head-tail morphology is unique to bacteriophages, with an icosahedral head and helical tail.
• Bacteriophages attach to bacteria using the tail, create a hole in the cell wall, and insert DNA into the cell.
• Poxviruses are large viruses with a complex structure and unique outer wall and capsid (e.g., variola virus causing smallpox).
• Viruses undergo frequent mutations through antigenic drift, where individual bases in DNA or RNA mutate.
• Mutations can lead to antiviral drug resistance.

Virus Replication

• Viruses lack cellular components and cannot reproduce through cell division; they use the host cell's machinery.
• Attachment occurs via specific binding between viral capsid proteins and host cell receptors, determining the virus's host range.
• Penetration involves virions entering the host cell through receptor-mediated endocytosis or membrane fusion, followed by uncoating.
• Replication primarily involves multiplication of the viral genome.
• Release happens when viruses are released from the host cell by lysis, which kills the cell by bursting its membrane.

Parasites

• Parasites have a non-mutual symbiotic relationship with their host, typically not killing it, being smaller, and living in or on it for an extended period.
• The three main classes of parasites causing human diseases are protozoa, helminths, and ectoparasites.

Parasites - Protozoa

• Protozoa are microscopic, one-celled organisms that can infect and multiply in humans and are abundant in aqueous environments and soil.
• Some protozoa move using pseudopodia (amoebae), while others are sessile; infectious protozoa are classified into groups based on movement: Sarcodina, Mastigophora, Ciliophora, and Sporozoa.

Parasites - Helminths

• Helminths are large, multicellular organisms generally visible to the naked eye in their adult stages.
• Helminths can be free-living or parasitic; adult forms cannot multiply in humans.
• The three main groups of helminths that are human parasites are flatworms (platyhelminthes), thorny-headed worms (acanthocephalans), and roundworms (nematodes).
• Adult worms can reside in the gastrointestinal tract, blood, lymphatic system, or subcutaneous tissues.

Parasites - Ectoparasites

• Ectoparasites are organisms like ticks, fleas, lice, and mites that attach or burrow into the skin for extended periods.