Introduction to Microbiology

Introduction to Microbiology

• Microbiology is the study of microscopic organisms, organisms with a diameter of 1 mm or less, which require a microscope to see.
• Microscopes were invented in the 17th century.
• There are two types of microscopes: Simple microscope which uses a single lens and compound microscope which uses two lenses.

Branches of Microbiology

• Bacteriologists study bacteria.
• Protozoologists study small, animal-like, single-celled organisms like amoeba.
• Parasitologists mainly study small animals that cause disease.
• Mycologists study fungi.
• Phycologists study algae.
• Immunology is often studied in microbiology.

Main Characteristics of Microorganisms

• Microorganisms are small and require microscopes to be seen.
• They are unicellular, meaning one cell performs all functions.
• Microorganisms can be found everywhere and are present in the bodies of animals, plants, soil, water, air, and dust.

Historical Developments in Microbiology

• Robert Hooke made one of the first microscopes and used it to view plant cells and microbes.

• Antony van Leeuwenhoek used his rudimentary microscope to see the first microscopic cells.

• The debate regarding spontaneous generation vs. biogenesis:

  • Spontaneous generation theory suggests the creation of living organisms from non-living matter.
  • Biogenesis theory suggests that living organisms are formed from pre-existing organisms.

• Louis Pasteur is crucial to our understanding of microbiology.

  • He proved that air contains microscopic organisms.

  • He studied lactic acid fermentation and alcoholic fermentation, discovering the role of microorganisms in these processes.

  • He disproved spontaneous generation.

  • He introduced the terms aerobic and anaerobic.

  • He is credited with the germ theory of disease.

  • He discovered the process of pasteurization.

  • He developed vaccines for anthrax and rabies.

• Robert Koch continued Pasteur’s work and provided the foundations for bacteriology:

  • He confirmed Pasteur’s germ theory of disease with four postulates, now known as Koch's Postulates:
    • The microbe must be present in every case of the disease.
    • The microbe must be isolated from the diseased host and grown in pure culture.
    • The specific disease must be reproduced when a pure culture of the microbe is injected into a healthy susceptible host.
    • The microbe must be recovered once again from the experimentally inoculated host.

Cell Theory

• There are two types of cells: prokaryotic and eukaryotic.
  • Prokaryotes include bacteria and archaea.
  • Eukaryotes include fungi, plants, and animals.

Eukaryotic Cells

• Animal, plant, and fungal cells are eukaryotic cells.
• Eukaryotic cells have membrane-bound organelles, such as nuclei and mitochondria.
• Eukaryotic cells are larger than prokaryotic cells.
• Examples of eukaryotic cell structures include the cytoplasm, nucleus, cell membrane, and mitochondria.

Prokaryotic Cells

• Bacteria are prokaryotic cells.
• Bacterial cells are smaller than eukaryotic cells and do not have membrane-bound organelles like a nucleus or mitochondria.

Essential Structures of Prokaryotic Cells

• Capsule (polysaccharides) - Protects against phagocytosis.
• Pili (glycoproteins)- Enables attachment to cell surface.
• Flagellum (proteins)- Provides motility.
• Spore (keratin-like coat and dipicolinic acid)- - Provides resistance to dehydration, heat, and chemicals.
• Plasmid (DNA) - Contains genes for antibiotic resistance, enzymes, and toxins.
• Granules (Glycogen, lipids, and polyphosphates) - Function as storage sites for food.

Bacterial Cell Wall Structures

• Bacterial cell walls are rigid and multi-layered, made up of peptidoglycan.
• Peptidoglycan is unique to bacteria and essential for their survival.
• Penicillin inhibits the formation of interpeptide bonds in peptidoglycan, stopping its synthesis.
• Lysozyme breaks the glycosidic bond between NAM and NAG in peptidoglycan, also inhibiting its synthesis.

Gram Positive and Gram Negative Bacteria

• Gram staining is a technique used to differentiate between bacteria.
• Gram positive bacteria have a thick peptidoglycan layer, staining purple during gram staining and are susceptible to antibiotics.
• Gram negative bacteria have a thin peptidoglycan layer and an outer membrane, staining pink during gram staining, and tend to be less susceptible to antibiotics.
• Teichoic acids are found in the thick peptidoglycan layer of gram-positive bacteria. They are absent in gram negative bacteria.
• E. coli is a gram-negative example, commonly used as a model organism in bacterial studies.
• Gram negative bacteria are generally more pathogenic due to their resistance to antibiotics.

Functions of the Cell Wall

• The cell wall provides morphology (cocci, bacilli, spiral, filamentous).
• It protects bacteria from high internal osmotic pressure.
• It is involved in cell division.
• Safeguards bacteria from external stress.
• It plays a role in inflammation and chemotaxis.
• Provides passage of metabolites (essential elements).
• It is responsible for the antigenic determinants on the cell surface.
• It is important for resistance to antibiotics.
• It is the target site for some antibiotics (e.g., penicillin).

Specialized Structures Outside the Cell Wall

• Capsule:

  • Composed of polysaccharides in most bacteria and D-glutamic acid in anthrax bacillus.
  • Contributes to bacterial virulence by resisting phagocytosis and attachment to host tissues.

• Pili (Fimbria):

  • Short, hair-like extensions found in gram-negative bacteria.
  • Important for attachment.

• Flagella:

  • Function in motility.
  • Composed of the protein flagellin.
  • H antigen: Flagella are antigenic.

• Spores:

  • Allow bacteria to survive harsh conditions of desiccation, nutrient depletion, and waste buildup.
  • Have low water content and incorporate Ca-dipicolinic acid.
  • Resistant to disinfectants, antibiotics, radiations, and boiling.

• Exosporium - The outermost layer of the spore, composed primarily of protein. It is also composed of lipids and carbohydrates. The exact function is unknown, but may play a role in pathogenicity.

• Spore coat - The second layer of the spore, composed of protein. The spore coat is a critical resistance mechanism against chemicals, particularly oxidizing agents like hydrogen peroxide, ozone, and hypochlorite.

Bacterial Genetics

• Bacterial genome: Bacterial DNA is typically organized into a single, circular chromosome.
• Genes are carried on chromosomes and plasmids.