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PBBSC FY MICROBIOLOGY UNIT1

  • Synopsis on Structure and Classification of Microbes in Microbiology

Structure of Microbes

  1. Bacteria:
    • Shape: Cocci (spherical), Bacilli (rod-shaped), Spirilla (spiral-shaped), Vibrios (comma-shaped).
    • Cell Wall: Composed of peptidoglycan (Gram-positive: thick layer; Gram-negative: thin layer with outer membrane).
    • Cell Membrane: Phospholipid bilayer with embedded proteins.
    • Cytoplasm: Contains ribosomes, enzymes, and genetic material.
    • Nucleoid: Single circular DNA without a nuclear membrane.
    • Flagella: Used for motility.
    • Pili/Fimbriae: Involved in adhesion and conjugation.
    • Spores: Dormant structures in harsh conditions.
  2. Viruses:
    • Core: DNA or RNA (single or double-stranded).
    • Capsid: Protein coat made of capsomeres.
    • Envelope: Lipid membrane (in some viruses).
    • Spikes: Glycoproteins for attachment to host cells.
  3. Fungi:
    • Structure: Composed of hyphae forming mycelium.
    • Cell Wall: Made of chitin.
    • Reproduction: Sexual and asexual spores.
  4. Protozoa:
    • Shape: Variable, some have flagella, cilia, or pseudopodia.
    • Nucleus: Eukaryotic with a defined nuclear membrane.
    • Reproduction: Binary fission or multiple fission.
    • Cyst Formation: Protection in harsh environments.
  5. Algae:
    • Chloroplasts: Contain pigments for photosynthesis.
    • Cell Wall: Made of cellulose.
    • Reproduction: Both sexual and asexual.
  6. Prions:
    • Structure: Misfolded proteins without nucleic acids.

Classification of Microbes

  1. Based on Morphology:
    • Cocci, Bacilli, Spirilla, Vibrios.
  2. Based on Gram Staining:
    • Gram-positive: Thick peptidoglycan layer.
    • Gram-negative: Thin peptidoglycan with an outer lipid membrane.
  3. Based on Oxygen Requirements:
    • Aerobes: Require oxygen.
    • Anaerobes: Survive without oxygen.
    • Facultative Anaerobes: Adapt to both conditions.
  4. Based on Temperature:
    • Psychrophiles: Thrive in cold temperatures.
    • Mesophiles: Thrive in moderate temperatures.
    • Thermophiles: Thrive in high temperatures.
  5. Based on Nutrition:
    • Autotrophs: Synthesize their own food (e.g., cyanobacteria).
    • Heterotrophs: Depend on others for nutrition.
  6. Based on Habitat:
    • Soil, water, human body, extreme environments.
  7. Taxonomical Classification:
    • Kingdom Monera: Bacteria.
    • Kingdom Protista: Protozoa and Algae.
    • Kingdom Fungi: Yeasts, molds, and mushrooms.
    • Kingdom Animalia: Parasites like helminths.
  8. Classification of Viruses:
    • Based on nucleic acid type (DNA or RNA).
    • Based on host (bacteria, plants, animals).
    • Based on symmetry (icosahedral, helical).
  • Morphological types

Morphological Types in Microbiology

Microorganisms are classified based on their morphology into different types. These morphological distinctions include shape, size, arrangement, and structural features. Below is a detailed explanation of the morphological types:

1. Morphological Types of Bacteria

Based on Shape:

  • Cocci (Spherical):
    • Examples: Staphylococcus aureus, Streptococcus pyogenes.
    • Arrangements:
      • Diplococci: Pairs of cocci (Neisseria gonorrhoeae).
      • Streptococci: Chains of cocci (Streptococcus pneumoniae).
      • Staphylococci: Grape-like clusters (Staphylococcus aureus).
      • Tetrads: Groups of four cocci (Micrococcus).
      • Sarcinae: Cuboidal arrangement of eight cocci (Sarcina).
  • Bacilli (Rod-shaped):
    • Examples: Escherichia coli, Bacillus anthracis.
    • Arrangements:
      • Diplobacilli: Pairs of bacilli.
      • Streptobacilli: Chains of bacilli.
      • Coccobacilli: Oval-shaped rods resembling cocci (Haemophilus influenzae).
  • Spirilla (Spiral-shaped):
    • Examples: Spirillum volutans.
    • Features: Rigid bodies with external flagella.
  • Vibrios (Comma-shaped):
    • Examples: Vibrio cholerae.
  • Spirochetes (Corkscrew-shaped):
    • Examples: Treponema pallidum, Borrelia burgdorferi.
    • Features: Flexible bodies with axial filaments for movement.

2. Morphological Types of Viruses

  • Icosahedral Viruses: Symmetrical, 20-sided structure (e.g., Adenovirus).
  • Helical Viruses: Rod-like with a spiral arrangement of nucleocapsid (e.g., Tobacco mosaic virus).
  • Complex Viruses: Complex structure, such as bacteriophages with a head and tail.
  • Enveloped Viruses: Enclosed in a lipid envelope (e.g., Influenza virus).
  • Non-enveloped Viruses: Lacking an envelope (e.g., Poliovirus).

3. Morphological Types of Fungi

  • Yeasts: Unicellular, oval or spherical (e.g., Candida albicans).
  • Molds: Multicellular, filamentous hyphae forming a mycelium (e.g., Aspergillus).
  • Dimorphic Fungi: Exhibit both yeast and mold forms depending on the environment (e.g., Histoplasma capsulatum).

4. Morphological Types of Protozoa

  • Amoeboid: Irregular shapes with pseudopodia (e.g., Entamoeba histolytica).
  • Flagellates: Possess one or more flagella (e.g., Giardia lamblia).
  • Ciliates: Covered with cilia for movement (e.g., Paramecium).
  • Sporozoans: Non-motile, with complex life cycles (e.g., Plasmodium).

5. Morphological Types of Algae

  • Unicellular Algae: Microscopic, single-celled (e.g., Chlamydomonas).
  • Filamentous Algae: Thread-like structures (e.g., Spirogyra).
  • Colonial Algae: Grouped cells forming colonies (e.g., Volvox).

6. Morphological Types of Helminths

  • Roundworms (Nematodes): Cylindrical, elongated bodies (e.g., Ascaris lumbricoides).
  • Flatworms (Platyhelminthes):
    • Trematodes (Flukes): Leaf-like shape (e.g., Schistosoma).
    • Cestodes (Tapeworms): Long, segmented bodies (e.g., Taenia solium).

7. Morphological Types of Prions

  • Misfolded Proteins: Lacking cellular structure, capable of inducing abnormal folding in other proteins (e.g., cause of Creutzfeldt-Jakob disease).
  • Size and Form of Bacteria

Size of Bacteria

Bacteria exhibit a wide range of sizes, typically measured in micrometers (µm).

  • Average Size:
    • Length: 1–10 µm
    • Width: 0.2–2 µm
  • Smallest Bacteria:
    • Mycoplasma species (e.g., Mycoplasma genitalium): 0.1–0.3 µm.
    • These bacteria lack a cell wall and are among the smallest known free-living organisms.
  • Largest Bacteria:
    • Epulopiscium fishelsoni: Up to 750 µm in length.
    • Thiomargarita namibiensis: Diameter up to 750 µm (largest known bacterium).

Form of Bacteria

Bacteria are classified based on their morphological forms, primarily shape and arrangement:

1. Shape

  • Cocci (Spherical):
    • Round and spherical in appearance.
    • Examples: Staphylococcus aureus, Streptococcus pneumoniae.
  • Bacilli (Rod-shaped):
    • Cylindrical, elongated shapes.
    • Examples: Escherichia coli, Bacillus anthracis.
  • Spirilla (Spiral-shaped):
    • Rigid, spiral forms with external flagella.
    • Examples: Spirillum volutans.
  • Vibrios (Comma-shaped):
    • Curved rods resembling a comma.
    • Examples: Vibrio cholerae.
  • Spirochetes (Corkscrew-shaped):
    • Flexible, helical forms with axial filaments for motility.
    • Examples: Treponema pallidum, Borrelia burgdorferi.
  • Coccobacilli:
    • Oval, intermediate between cocci and bacilli.
    • Examples: Haemophilus influenzae.
  • Filamentous Bacteria:
    • Long, thread-like forms.
    • Examples: Streptomyces.
  • Pleomorphic Bacteria:
    • Vary in shape due to the absence of a rigid cell wall.
    • Examples: Mycoplasma, Corynebacterium.

2. Arrangement

Cocci Arrangements:

  • Diplococci: Pairs of cocci (e.g., Neisseria gonorrhoeae).
  • Streptococci: Chains of cocci (e.g., Streptococcus pyogenes).
  • Staphylococci: Grape-like clusters (e.g., Staphylococcus aureus).
  • Tetrads: Groups of four cocci (e.g., Micrococcus luteus).
  • Sarcinae: Cuboidal arrangement of eight cocci (e.g., Sarcina).

Bacilli Arrangements:

  • Single Bacillus: Single rods (e.g., Escherichia coli).
  • Diplobacilli: Pairs of bacilli (e.g., Klebsiella pneumoniae).
  • Streptobacilli: Chains of bacilli (e.g., Bacillus subtilis).
  • Palisade: Side-by-side arrangement resembling a fence (e.g., Corynebacterium diphtheriae).

Spiral Bacteria:

  • Rarely form distinct arrangements.

Significance of Size and Form

  1. Size:
    • Determines nutrient uptake and waste elimination.
    • Smaller bacteria have a larger surface-to-volume ratio, enhancing metabolic efficiency.
  2. Shape:
    • Influences motility, surface adhesion, and pathogenicity.
    • For instance, spirochetes’ corkscrew shape aids in movement through viscous environments like tissues.
  3. Arrangement:
    • Impacts identification in microbiological analysis (Gram staining, culture, and microscopy).
    • Certain arrangements are characteristic of specific species or genera.
  • Motility.

Motility in Microbiology

Definition

Motility refers to the ability of microorganisms to move independently, often in response to environmental stimuli (e.g., nutrients, temperature, or light). It is an essential characteristic of many microorganisms and contributes to their survival, colonization, and pathogenicity.

Mechanisms of Motility in Microorganisms

1. Flagellar Motility

  • Description: Movement facilitated by whip-like structures called flagella.
  • Flagellar Arrangements:
    • Monotrichous: Single flagellum at one end (e.g., Vibrio cholerae).
    • Lophotrichous: Multiple flagella at one or both ends (e.g., Pseudomonas).
    • Amphitrichous: A single flagellum at both ends (e.g., Spirillum).
    • Peritrichous: Flagella distributed all over the surface (e.g., Escherichia coli).
  • Example: Salmonella typhimurium moves using peritrichous flagella.
  • Mechanism:
    • Powered by proton motive force.
    • Flagella rotate clockwise or counterclockwise to propel the bacterium forward or backward.

2. Gliding Motility

  • Description: A smooth and slow movement over solid surfaces without the use of flagella.
  • Mechanism:
    • Involves secretion of polysaccharide slime or surface proteins.
  • Example: Myxococcus xanthus.

3. Twitching Motility

  • Description: Intermittent jerky movements on solid surfaces facilitated by type IV pili.
  • Mechanism:
    • Extension and retraction of pili generate movement.
  • Example: Pseudomonas aeruginosa.

4. Swarming Motility

  • Description: Coordinated movement of bacterial populations on moist surfaces.
  • Mechanism:
    • Involves flagella and surface proteins for group movement.
  • Example: Proteus mirabilis.

5. Corkscrew Motility

  • Description: Spiral movement exhibited by spirochetes.
  • Mechanism:
    • Axial filaments (endoflagella) are located in the periplasmic space, enabling rotation and corkscrew-like motion.
  • Example: Treponema pallidum.

6. Amoeboid Motility

  • Description: Movement by extending pseudopodia (false feet).
  • Mechanism:
    • Cytoplasmic streaming moves the cell forward.
  • Example: Amoeba proteus.

7. Passive Motility

  • Description: Movement through external forces such as water or air currents, rather than active mechanisms.
  • Example: Spores of fungi.

Motility in Viruses

Viruses are non-motile and rely on passive mechanisms such as:

  • Airborne transmission.
  • Host cell-mediated processes (e.g., endocytosis or exocytosis).

Techniques to Test Microbial Motility

  • Hanging Drop Method:
    • Uses a depression slide to observe live bacteria under a microscope.
  • Motility Agar Test:
    • Semi-solid agar is inoculated, and diffusion away from the stab line indicates motility.
  • Flagella Staining:
    • Visualizes bacterial flagella under a microscope.

Significance of Motility

  1. Survival:
    • Enables microorganisms to move toward favorable environments and escape hostile conditions.
  2. Pathogenicity:
    • Enhances colonization, invasion, and dissemination within the host (e.g., Helicobacter pylori uses flagella to colonize the stomach lining).
  3. Biofilm Formation:
    • Motility facilitates the initial stages of biofilm formation on surfaces.
  4. Identification:
    • Motility patterns help in the identification of microorganisms in laboratory diagnostics.
  • Classification of Microorganisms.

Classification of Microorganisms

Microorganisms are diverse and classified based on their morphology, structure, function, genetics, and evolutionary relationships. The classification system provides a framework for organizing microbial life into categories that reflect their similarities and differences.

1. Domains of Life

Microorganisms are broadly classified into three domains based on molecular and genetic analysis:

  1. Bacteria (Prokaryotes):
    • Unicellular organisms with no membrane-bound nucleus.
    • Cell wall contains peptidoglycan.
    • Example: Escherichia coli.
  2. Archaea (Prokaryotes):
    • Similar to bacteria but with distinct genetic and metabolic features.
    • Cell wall lacks peptidoglycan.
    • Often found in extreme environments (e.g., hot springs, salty lakes).
    • Example: Methanogens.
  3. Eukarya (Eukaryotes):
    • Organisms with membrane-bound organelles and a defined nucleus.
    • Includes fungi, protozoa, algae, and helminths.
    • Example: Saccharomyces cerevisiae (yeast).

2. Kingdom Classification

Under the Five Kingdom System, microorganisms are grouped into:

  1. Monera:
    • Includes prokaryotes such as bacteria and cyanobacteria.
  2. Protista:
    • Includes unicellular eukaryotes such as protozoa and algae.
  3. Fungi:
    • Includes yeasts, molds, and mushrooms.
  4. Plantae:
    • Includes photosynthetic algae.
  5. Animalia:
    • Includes parasitic worms (helminths).

3. Classification Based on Cell Type

  1. Prokaryotes:
    • Simple cells without a nucleus.
    • Examples: Bacteria, Archaea.
  2. Eukaryotes:
    • Complex cells with a nucleus and organelles.
    • Examples: Fungi, Protozoa, Algae.
  3. Acellular Microorganisms:
    • Non-living infectious agents.
    • Examples: Viruses, Prions.

4. Classification Based on Morphology

  1. Bacteria:
    • Shapes: Cocci (spherical), Bacilli (rod-shaped), Spirilla (spiral).
    • Arrangements: Chains (strepto), Clusters (staphylo), Pairs (diplo).
  2. Fungi:
    • Unicellular (yeasts) or multicellular (molds).
  3. Protozoa:
    • Amoeboid, flagellated, or ciliated forms.
  4. Algae:
    • Unicellular or multicellular.
  5. Viruses:
    • Helical, icosahedral, or complex shapes.

5. Classification Based on Nutrition

  1. Autotrophs:
    • Synthesize their own food.
    • Examples: Cyanobacteria, Algae.
  2. Heterotrophs:
    • Depend on external organic sources.
    • Examples: Fungi, Protozoa.
  3. Saprophytes:
    • Feed on dead and decaying matter.
    • Examples: Decomposing fungi.

6. Classification Based on Habitat

  1. Aquatic Microorganisms:
    • Found in water environments.
    • Examples: Algae, Vibrio cholerae.
  2. Terrestrial Microorganisms:
    • Found in soil.
    • Examples: Streptomyces, Clostridium.
  3. Host-associated Microorganisms:
    • Found in or on living hosts.
    • Examples: Normal flora, pathogens.

7. Classification of Viruses

  1. Based on Host:
    • Bacteriophages (infect bacteria).
    • Animal viruses (infect animals).
    • Plant viruses (infect plants).
  2. Based on Nucleic Acid:
    • DNA viruses.
    • RNA viruses.
  3. Based on Structure:
    • Enveloped or non-enveloped.
    • Helical or icosahedral.

8. Classification Based on Metabolism

  1. Aerobes:
    • Require oxygen for growth.
    • Example: Pseudomonas aeruginosa.
  2. Anaerobes:
    • Grow in the absence of oxygen.
    • Example: Clostridium tetani.
  3. Facultative Anaerobes:
    • Can survive with or without oxygen.
    • Example: Escherichia coli.

9. Taxonomic Hierarchy

  • Domain
  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species
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