History of Microbiology

History of Microbiology

• Girolamo Fracastoro (1483-1553) proposed the "germ theory" of disease, stating that diseases spread from person to person via invisible viruses.
• Antonie Van Leeuwenhoek (1632-1723) developed simple microscopes, magnifying 50-300x, and described bacteria and protozoa in dirty water.
• Francesco Redi (Italian doctor, 1665) demonstrated that "worms" in rotting meat are fly larvae.
• Ignaz Semmelweisz (Hungarian gynaecologist, 1865) recommended handwashing and adding slaked lime to water to prevent the spread of disease.

Louis Pasteur and Robert Koch

• Louis Pasteur (French chemist, 1822-1895) studied yeasts that convert grape juice into wine, leading to the discovery of pasteurization (wet heat under 100°C).
• Pasteur developed vaccines against anthrax and rabies, laying the foundation for immunology and biochemical/industrial microbiology.
• Robert Koch (German researcher, 1843-1910) emphasized the importance of isolating, culturing, and characterizing microorganisms that cause diseases in humans.
• Koch's postulates must be met to establish the relationship between a microorganism and a specific disease: • The microorganism must be present in every case of the disease. • The microorganism must be isolated from the diseased host and grown in pure culture. • The specific disease must be reproduced when a pure culture of the microorganism is inoculated into a healthy but susceptible host. • The microorganism must be re-isolated from the host that was experimentally infected.

Other Important Discoveries

• Joseph Lister (Scottish surgeon) advocated for the use of phenol as a disinfectant in 1860, leading to the modern era of aseptic surgery.
• Elie Metchnikoff (Russian zoologist) worked at the Pasteur Institute and was awarded the Nobel Prize in 1908 for his work on cellular immunity.
• Selman Waksman discovered streptomycin in 1944, leading to the era of antibiotics.
• Max Theiler developed a vaccine against yellow fever in 1950 using an attenuation technique.
• Jonas Salk developed a vaccine against polio in 1951.

The Greatest Biological Discovery Ever

• James Watson and Francis Crick (Nobel Prize winners, 1959) elucidated the mechanism of DNA replication, gene function, and cell division.

Taxonomy

• Taxonomy is the science of biological classification, involving the arrangement of organisms according to specific laws.
• Classification involves arranging organisms into groups based on similarities, such as size, structure, and method of reproduction.
• The most general sequence of classification is: Species, Genus, Family, Order, Class, Phyla or Divisions, and Kingdom.
• Microbiologists use the binomial system to name microorganisms, consisting of a genus and species.

Morphology of Bacteria

• Bacteria come in various shapes and arrangements, including coccus (sphere), bacillus (cylinder/rod), spiral shaped spirillum (rigid), and spirochaete (flexible).

Bacterial Structure

• The cell wall of bacteria is essential for protecting the protoplast from osmotic lysis and is composed of peptidoglycan.
• Gram-positive bacteria have a thick peptidoglycan layer, while Gram-negative bacteria have a thin layer of peptidoglycan surrounded by an outer membrane.
• The outer membrane contains lipopolysaccharide (LPS or endotoxin), which is toxic to animals.

Cell Wall and Membranes

• The cell wall is essential for bacterial viability and is a target for antibiotic treatment.
• The Gram staining procedure distinguishes between Gram-positive and Gram-negative bacteria.
• Gram-positive bacteria have a thick peptidoglycan layer, while Gram-negative bacteria have a thin layer of peptidoglycan surrounded by an outer membrane.
• The outer membrane contains proteins and lipopolysaccharide (LPS or endotoxin).

Cytoplasm and Periplasmic Space

• The cytoplasm is the portion of the cell that lies within the cytoplasmic membrane.
• The cytoplasm contains proteins, vitamins, ions, nucleic acids, and other cellular constituents.
• The periplasmic space is the area between the peptidoglycan and the outer membrane, containing hydrolytic enzymes, binding proteins, and chemoreceptors.

Flagella, Fimbriae, and Pili

• Flagella are organs of motility, allowing bacteria to move to areas with favorable conditions.
• Fimbriae and pili are fine hairlike projections from the cell envelope, found in many Gram-negative bacteria.
• Fimbriae are involved in attachment and are important virulence factors.
• Pili are involved in the transfer of genetic material between bacteria during conjugation.

Capsule and Biofilms

• A capsule is a discrete layer of polysaccharides deposited outside the cell wall, protecting bacteria from phagocytosis and desiccation.
• Biofilms are multilayer bacterial populations embedded in a polysaccharide matrix, attached to a surface, and can act as a source of infection.

Endospores

• Endospores are produced within cells and are resistant to heat, radiation, chemicals, and desiccation.
• Endospores are characteristic of organisms that live in soil and need to withstand adverse conditions.
• Bacillus and Clostridium are spore producers, and their spores can survive for a long time and then re-germinate.

Bacterial Structures

• Flagella: made from globular protein (flagellin) to form a hollow cylindrical structure, allowing bacteria to move to areas with favorable conditions through random movement.
• Flagella also act as a virulence factor for some organisms, transporting them to attachment sites for colonization.

Fimbriae and Pili

• Fimbriae: fine hair-like projections from the cell envelope, found in many Gram-negative bacteria, shorter and finer than flagella.
• Fimbriae: also called common pili, important virulence factor, acting as adhesins for attachment, essential for pathogenesis.
• Pili: also referred to as sex pili, few per cell, mediate transfer of genetic material between bacteria during conjugation.

Capsule

• Capsule: a discrete, detectable layer of polysaccharides deposited outside the cell wall, exhibiting marked serological specificity in many bacteria.
• Functions: protects bacterial cells from engulfment by protozoa or white blood cells, protects against drying or desiccation, reserves carbohydrates for subsequent metabolism, mediates adherence to surfaces.

Biofilms

• Biofilms: multilayer bacterial populations embedded in a polysaccharide matrix, attached to a surface, acting as a source of organisms.
• Functions: play a role in formation of plaque, cause colonization of plastic catheters and implants, protect bacteria against phagocytes, inhibit penetration of antibodies and antimicrobial drugs, resistant to disinfections.

Endospores

• Endospores: produced within cells, cell wall consists of a polymer of disaccharides cross-linked by short chains of amino acids (peptides), a type of peptidoglycan called murein.
• Functions: essential structure for viability, important target for antibiotic treatment, important adherence and receptor sites, can cause symptoms of disease, provide immunological distinction and variability between bacterial strains.

Gram-Positive and Gram-Negative Cell Walls

• Gram-positive cell wall: thick layer of peptidoglycan, stains purple with crystal violet dye during Gram-staining.
• Gram-negative cell wall: thin layer of peptidoglycan, surrounded by a membranous structure called the outer membrane, stains red, contains lipopolysaccharide (LPS or endotoxin), which is toxic to animals.

Gram-Positive Cell Wall

• Thick peptidoglycan layer, traversed by teichoic acids, plus carbohydrates and proteins depending on the species.

Gram-Negative Cell Wall

• Two separate areas with an additional membrane: periplasmic space, thin layer of peptidoglycan, and outer membrane.

Outer Membrane Proteins

• Outer membrane proteins: contribute a significant portion of the outer membrane, with major outer membrane proteins (OMPs) present in the greatest amount.
• Porins or matrix proteins: form channels in the outer membrane, allowing passive transport of aqueous materials into the periplasm.

Functions of the Outer Membrane

• Confers negative charge to the bacterial cell.
• Selective barrier, with pores for entrance of hydrophilic molecules (antibiotics).
• Phage receptors.
• Pathogenic properties.
• Stabilizes mating cells and keeps enzymes in the periplasm.

Periplasmic Space and Cytoplasmic (Cell) Membrane

• Periplasmic space: area between the peptidoglycan and the outer membrane, contains hydrolytic enzymes, binding proteins, chemoreceptors, and detoxifying enzymes.
• Cytoplasmic membrane: composed of a hydrophobic phospholipid bilayer and proteins, retains the cytoplasm, acts as a selective barrier, and facilitates transport.

Cytoplasm

• Cytoplasm: portion of the cell that lies within the cytoplasmic membrane, contains proteins, vitamins, ions, nucleic acids, amino acids, sugars, carbohydrates, and fatty acids.
• Functions: holds many cellular constituents, within the cytoplasm that many of the functions for cell growth, metabolism, and replication are carried out.