Microbiology: Bacteria, Viruses, and Eukaryotes

Posted on Nov 29, 2024 in Biology

Item 21

1. Concepts and Types of Organisms

Organisms, often only visible with a microscope, are classified as prokaryotic or eukaryotic, and unicellular or multicellular. Depending on their nutrition type, they are divided into prokaryotes (archaebacteria, eubacteria—Kingdom Monera) and eukaryotes (algae, protozoa, fungi—Kingdom Protista). Viruses are not considered living organisms.

2. Viruses

Viruses are simple, microscopic particles formed by DNA or RNA and surrounding proteins. Free viruses are called extracellular virions, while those within a cell are obligate parasites of a host. The protein coat surrounding the genetic material is called a capsid, made of globular capsomers. Capsid shapes can be icosahedral, helical, or complex. Viruses have linear or circular, single-stranded or double-stranded DNA or RNA. Some have an outer lipid bilayer envelope derived from the host cell, containing glycoproteins synthesized using viral information. These proteins aid in host cell recognition and penetration.

Virus Life Cycle (Bacteriophage Lytic Cycle)

The T4 bacteriophage infects bacteria, leading to lysis:

  1. Fixation/Adsorption: Contact occurs due to receptor molecules on the bacterial wall binding with the bacteriophage’s tail fibers. Basal spines attach, and enzymes from the basal plate puncture the wall.
  2. Penetration: The tail contracts, injecting viral DNA into the bacterium.
  3. Eclipse Phase: Viral DNA utilizes bacterial resources for RNA transcription and protein synthesis. New capsids and lysozymes are formed. Lysozymes disrupt bacterial DNA activity and facilitate viral DNA replication.
  4. Assembly Phase: Capsomers assemble around replicated viral DNA.
  5. Lysis: Lysozymes break the bacterial membrane/wall, releasing new virions.

Lysogenic Cycle

In the lysogenic cycle, the virus doesn’t kill the bacterium. Viral DNA integrates into the bacterial genome, remaining unexpressed and replicating with the host for generations. The bacterium is then called a lysogenic bacterium. A stimulus can trigger the viral DNA to separate and initiate the lytic cycle.

3. Eubacteria

In 1665, Hooke discovered the cell. Later, Van Leeuwenhoek’s 200x microscope revealed red blood cells and bacteria. Bacteria belonging to Kingdom Monera are divided into eubacteria and archaebacteria.

Bacterial Structure

  1. Capsule: Present in some, especially pathogens, composed of carbohydrates, protein, and water. It provides protection from antibodies, promotes colony formation, and regulates exchange and moisture.
  2. Cell Wall: Less rigid than plant cell walls but formed by highly organized murein. Gram-positive bacteria have a single murein layer with proteins, polysaccharides, and teichoic acid. Gram-negative bacteria have a murein bilayer with an outer lipid bilayer, controlling osmotic pressure and providing antibiotic resistance.
  3. Plasma Membrane: Has inward extensions called mesosomes, rich in enzymes for DNA replication, respiration, photosynthesis, and pigment synthesis. These pigments help in processes like nitrogen fixation.
  4. Flagella: Composed of a basal body, stem, and hook. Some have discoidal structures at the base. Bacteria are classified by flagella number and arrangement: monotrichous, lophotrichous, amphitrichous, and peritrichous. Flagella enable locomotion.
  5. Pili: Hollow, tubular structures serving as communication channels. Fimbriae are short, numerous pili used for substrate attachment. Bacteria shapes include cocci, bacilli, vibrios, and spirilla. Colonies can be staphylococci, streptococci, or sarcinae.

Bacterial Physiology

  • Nutrition: Photoautotrophic (green and purple bacteria), photoheterotrophic (purple bacteria), chemoautotrophic (colorless sulfur bacteria), and chemoheterotrophic (most bacteria).
  • Response: Movement via flagella, contractions, or crawling. Some form endospores—protective layers around DNA—in adverse conditions. They respond to light, chemical, and temperature stimuli.
  • Reproduction: Asexual reproduction by binary fission, linked to mesosome activity. Some bacteria undergo parasexual processes (transformation, transduction, conjugation) involving genetic exchange without reproduction.

Types of Eubacteria

  1. Green and Purple Bacteria: Photosynthetic, containing bacteriochlorophyll and carotenoids. Some sulfurous bacteria use H2S instead of water for protons.
  2. Cyanobacteria: Contain phycocyanin, also called blue-green algae. Aerobic, oxygenic photosynthetic. Some fix atmospheric nitrogen and form symbiotic relationships with fungi (lichens).
  3. Prochlorophytes: Considered precursors to algae, containing chlorophyll.
  4. Nitrifying Bacteria: Oxidize nitrogen compounds in soil. Chemosynthetic. Nitrosomonas oxidizes ammonia to nitrite, and Nitrobacter oxidizes nitrite to nitrate.
  5. Nitrogen-Fixing Bacteria: Fix atmospheric nitrogen in soil or legume roots (e.g., Rhizobium).
  6. Spirochetes: Helical bacteria found in water, some causing diseases.
  7. Fermenting Bacteria: Produce lactic acid.
  8. Mycoplasmas: Small, lack a cell wall, and have unique membrane composition. Some are pathogenic, causing pneumonia.

4. Archaebacteria

Ancient bacteria, some with lipid bilayer or monolayer membranes, others with non-lipid hydrocarbon membranes. Live in extreme environments (anaerobic, high temperature, hypersaline, polar).

5. Eukaryotic Microorganisms

A) Algae

Belong to Kingdom Protista. Photosynthetic, containing chlorophyll. Unicellular or multicellular. Part of plankton. Classified by pigments: chlorophylls, phaeophytes, rhodophytes.

B) Protozoa

Belong to Kingdom Protista. Unicellular heterotrophs. Aquatic, mostly mobile. Classified by locomotion: flagellates, ciliates, sporozoans, rhizopods. Asexual reproduction by binary fission, some by conjugation.

C) Fungi

Unicellular or multicellular heterotrophs. Some have external digestion. Non-motile. Chitin cell wall. Mostly soil-dwelling decomposers, some parasitic. Asexual reproduction by spores, some by budding (e.g., yeast). Examples: Penicillium (mold, reproduces by spores), Saccharomyces (yeast, reproduces by budding).