Bacteria: General Account and Economic Importance

Introduction to Bacteria and Microbiology

• Evolutionary Origin: Bacteria are recognized as the simplest organisms living on Earth. Fossilized forms have been found in geological strata dating back approximately $3.5 \times 10^9$ years ago, making them almost three times as old as any other known organism on the planet.
• Diversity and Distribution: Approximately $5000$ species of bacteria are currently recognized. They are ubiquitous, inhabiting every possible environment, including plants, animals, soil, water, ice caps, hot springs, and hypersaline environments.
• Extreme Habitats: They thrive in atmospheres rich in toxic gases like methane ($CH_4$) or hydrogen sulfide ($H_2S$) that would be lethal to most other life forms.
• Ecological Significance: Life on Earth cannot exist without bacteria. They perform essential functions such as:     * Fixing nitrogen from the atmosphere.     * Decomposition of organic matter in aquatic and terrestrial communities.     * Cycling substances essential to all life-forms and maintaining productivity.
• Basic Organization: Bacteria are the only organisms characterized by prokaryotic cellular organization. They are typically too small to be seen with the unaided eye and are the most abundant organisms on Earth.

Historical Timeline and Milestones in Microbiology (1676–2004)

• The Golden Age of Microbiology (1857–1910): This period established microbiology as a formal science through rapid discovery.
• Chronological Milestones:     * 1676: Anton van Leeuwenhoek — First observation of "very little animalcules," now recognized as protozoa.     * 1684: Anton van Leeuwenhoek — Discovery of bacteria.     * 1798: Edward Jenner — Vaccination with cowpox to prevent smallpox.     * 1857: Louis Pasteur — Discovery of lactic acid fermentation.     * 1860: Louis Pasteur — Identification of yeast's role in alcoholic fermentation.     * 1861: Louis Pasteur — Proof of biogenesis.     * 1864: Louis Pasteur — Development of Pasteurization.     * 1867: Joseph Lister — Introduction of antiseptic principles in surgery.     * 1876: Robert Koch — Identified Bacillus anthracis as the causative agent of anthrax.     * 1881: Robert Koch — Developed methods for studying bacteria in pure culture.     * 1882: Robert Koch — Discovered Mycobacterium tuberculosis (Nobel Prize awarded).     * 1884: Robert Koch — Publication of Koch’s postulates.     * 1884: Elie Metchnikoff — Description of phagocytosis (Nobel Prize awarded).     * 1884: Hans Christian Gram — Developed the Gram staining technique.     * 1885: Louis Pasteur — Developed vaccination against rabies in humans.     * 1887: Richard J. Petri — Developed the Petri dish (plate).     * 1889: Sergei Winogradsky — Discovered nitrification and chemolithotrophy.     * 1889: Martinus W. Beijerinck — Crystallized virus and isolated Rhizobium.     * 1890: Emil von Behring — Developed antitoxins for diphtheria (Nobel Prize awarded).     * 1892: Dmitri Ivanovsky — Identified the virus as the causative agent of tobacco mosaic disease.     * 1901: Martinus W. Beijerinck — Developed the enrichment culture method and coined the term "Virus."     * 1910: Paul Ehrlich — Developed chemotherapeutic agents (Salvarsan) for syphilis (Nobel Prize awarded).     * 1928: Frederick Griffith — Discovered transformation in bacteria.     * 1929: Alexander Fleming, Ernst Chain, Howard W. Florey — Discovered the "miracle drug" penicillin from Penicillium notatum (Nobel Prize awarded).     * 1935: Wendell M. Stanley, Northrop, and Sumner — Crystallized tobacco mosaic virus (Nobel Prize awarded).     * 1941: George Beadle and Edward Tatum — Proposed the one gene-one enzyme hypothesis (Nobel Prize awarded).     * 1944: Oswald Avery, Colin McLeod, Maclyn McCarty — Proved DNA is the genetic material.     * 1944: Selman Waksman — Discovered streptomycin.     * 1953: James Watson and Francis Crick — Discovered the molecular structure of DNA (Nobel Prize awarded).     * 1961: Francois Jacob and Jacques Monod — Developed the concept of an Operon (Nobel Prize awarded).     * 1970: Daniel Nathans, Hamilton O. Smith, Werner Arber — Used restriction enzymes for genetic engineering (Nobel Prize awarded).     * 1975: Georges Kohler and Cesar Milstein — Developed monoclonal antibodies.     * 1977: Fred Sanger, Steven Niklen, Alan Coulson — Developed methods for sequencing DNA.     * 1978: Peter Mitchell — Discovered chemiosmotic mechanisms (Nobel Prize awarded).     * 1982: Aaron Klug — Determined the structure of tobacco mosaic virus (Nobel Prize awarded).     * 1983: Luc Montagnier — Discovered HIV, the cause of AIDS.     * 1986: Benjamin Hall and Gustav Ammerer — Produced Hepatitis B vaccine via genetic engineering.     * 1994: R. J. Cano — Cultured $4.0 \times 10^7$ year old bacteria.     * 1996: Andre Goffeau et al. — Sequenced the yeast genome.     * 1998: James B. Kaper et al. — Discovered two circular chromosomes in Vibrio cholerae.     * 1999: Frederick R. Blattner et al. — Sequenced the Escherichia coli genome.     * 2000: Edward Delong — Discovered marine Archaea.     * 2002: Jeronimo Cello et al. — Synthesized infectious poliovirus from basic chemical building blocks.     * 2004: J. Craig Venter et al. — Performed the first large-scale environmental genome shotgun sequencing of the Sargasso Sea.

Key Figures in Pre-Twentieth Century Microbiology

• Early Philosophical Suggestions: The Roman philosopher Lucretius ($98-55\,BC$) and physician Girolamo Fracastoro ($1478-1553$) proposed that invisible living creatures caused disease.
• Robert Hooke: Built a compound microscope in $1664$. He observed "cells" in cork slices and reported seeing the fruiting structures of fungi.
• Anton van Leeuwenhoek (1632–1723):     * Known as the "Father of Microbiology."     * A Dutch merchant without university education who constructed over $250$ simple microscopes using double convex lenses capable of magnifying $50$ to $300$ times.     * First to view bacteria and protozoa, referring to them as "animalcules."     * Examined river water, saliva, feces, and pepper infusions.
• Louis Pasteur (1822–1895):     * Disproved the Theory of Spontaneous Generation (Abiogenesis) using the "Pasteur flask" (swan-necked flask).     * Biogenesis Theory: Proved that living cells arise only from pre-existing living cells.     * Fermentation: Demonstrated that yeast produced alcohol (good wine) while rod-shaped bacteria produced lactic acid (sour wine/vinegar).     * Pasteurization: Recommended heating beverages to moderate temperatures to control contamination.     * Germ Theory of Disease: Initiated the concept that microorganisms cause infectious diseases.     * Vaccines: Developed vaccines for Anthrax (using cultures treated with Potassium Dichromate at $40^{\circ}C$ to $43^{\circ}C$) and Rabies (using dried spinal cords and brains of infected rabbits).
• Robert Koch (1843–1910):     * Proved the role of bacteria in disease using Bacillus anthracis.     * Koch’s Postulates (Verbatim Requirements):         1. A specific microorganism is present in every diseased animal but absent from a healthy one.         2. The suspected microorganism can be isolated from the host and grown in a pure culture.         3. The same disease must result when the isolated microorganism is inoculated into a healthy but susceptible host.         4. The same microorganism must be isolated from the diseased host and be identical to the original microorganism.     * Laboratory Innovations:         * Used agar as a solidifying agent (suggested by Fannien Hesse) because it remains solid up to $100^{\circ}C$ and solidifies at $40^{\circ}C$, unlike gelatin which melts at $28^{\circ}C$.         * Isolated Mycobacterium tuberculosis in $1882$, utilizing staining (methylene blue and Bismarck brown).
• Martinus Beijerinck (1851–1931):     * Studied Nitrogen fixation (isolated Azotobacter and Rhizobium).     * Developed the "Enrichment Culture Technique" to isolate specific microbes from mixed populations.     * Characterized viruses as specific disease-causing molecules that hijack host metabolic machinery.
• Sergei Winogradsky (1856–1953): Along with Beijerinck, established the biogeochemical role of microbes in carbon, nitrogen, and sulfur cycles.
• Elie Metchnikoff (1845–1916): Discovered "phagocytes" (cell-eating cells) in the $1880s$, establishing the cellular basis of immunity.

Transition into 20th Century Microbiology

• Joseph Lister (1827–1912):     * Introduced the first aseptic techniques in surgery to prevent sepsis.     * Used carbolic acid (phenol) to soak bandages and as an air spray during operations.
• Paul Ehrlich (1854–1915):     * Sought a "Magic Bullet" — a chemical that kills pathogens without harming host cells.     * Synthesized the $606\text{th}$ compound "Salvarsan" and the $914\text{th}$ compound "Neosalvarsan" for syphilis.     * Coined the term "Chemotherapy."
• Alexander Fleming (1881–1955): Observed that the mould Penicillium notatum produced a substance (penicillin) that inhibited Staphylococcus aureus.
• Selman Waksman:     * Soil microbiologist who coined the term "Antibiotic" in $1941$.     * Discovered Streptomycin from Streptomyces griseus in $1943$, a broad-spectrum drug used to treat tuberculosis.

Prokaryotic vs. Eukaryotic Cell Comparison

• Phylogenetic Domains: Cellular evolution moved along three paths: Bacteria, Archaea, and Eukarya, all originating from a universal ancestor (progenitor).

General Characteristics of Bacteria: Size, Shape, and Arrangement

• Size Measurement Units:     * $1\text{ micron (}\mu m\text{)} = 10^{-6}\text{ meters} = 10^{-3}\text{ millimeters}.$     * $1\text{ millimicron (}nm\text{)} = 10^{-3}\text{ microns } = 10^{-6}\text{ millimeters}.$     * $1\text{ Angstrom (}\AA\text{)} = 0.1\,nm.$
• Specific Size Ranges:     * Cocci: $0.5$ to $3.0\,\mu m.$     * Bacilli: $0.2$ to $2.0\,\mu m$ diameter; $0.5$ to $20.0\,\mu m$ length.     * Vibrios/Spirilla: $0.2$ to $2.0\,\mu m$ diameter; $0.5$ to $100\,\mu m$ length.     * Spirochaetes: $0.1$ to $3.0\,\mu m$ diameter; $0.5$ to $250\,\mu m$ length.
• Morphological Shapes:     * Coccus (pl. Cocci): Spherical or nearly spherical.     * Bacillus (pl. Bacilli): Rod-shaped or cylindrical. Short rods similar to spheres are "coccobacilli."     * Vibrios: Comma-shaped rods with vibratory motility.     * Spirillum (pl. Spirilla): Rigid spiral/helical forms.     * Spirochetes: Flexuous (flexible) spiral forms.     * Mycoplasma: Cell wall-deficient, lacking stable morphology; occur as oval bodies or filaments.
• Cellular Arrangements (Groupings):     * Diplococci: Pairs.     * Chains: Genus Streptococcus, Enterococcus, Lactococcus.     * Clusters (Grape-like): Genus Staphylococci (random division planes).     * Tetrads: Groups of four (genus Micrococcus).     * Sarcina: Cubical packets of eight (division in three planes).     * Bacilli arrangements: Chains (Streptobacilli), or cuneiform "Chinese letter" arrangements (characteristic of Corynebacterium diphtheriae).

The Bacterial Cell Wall and Surface Layers

• Cell Wall Composition:     * Made of Mucopeptide (Peptidoglycan), a polymer of alternating units of NAG ($N\text{-acetyl glucosamine}$) and NAM ($N\text{-acetyl muramic acid}$) joined by $\beta, 1\text{-}4$ linkages.     * Lateral cross-linking occurs via short amino acid chains (e.g., lysine) originating from the carboxyl group of muramic acid.
• Comparison of Gram-Positive vs. Gram-Negative:     * Gram-Positive: Homogeneous, thick layer ($85\%\text{ or more}$ mucopeptide). Contains Teichoic Acids (polymers of ribitol/glycerol phosphates) which serve as antigens.     * Gram-Negative: Tripartite structure (inner membrane, periplasmic space with mucopeptide, outer membrane). Mucopeptide is only $3\text{ to }12\%$. High lipid content (lipoprotein/lipopolysaccharide complex). Contains Porins (aqueous channels) in the outer membrane.
• Functions of the Cell Wall:     * Imparts shape and rigidity.     * Supports against internal osmotic pressure ($5\text{ to }25\,atm$) to prevent osmolysis.     * Provides receptors for viral attachment.
• Surface Adherents:     * Glycocalyx: A "sweet coat" of polysaccharides/proteins. If thick and covalently bound, it is a Capsule; if thin and loose, it is a Slime Layer.     * Dextran: Polysaccharide in the slime layer of Streptococcus mutans (agent of tooth decay).

Bacterial Locomotion and Attachment

• Flagella:     * Atrichous: No flagella.     * Polar: Attached at one or both ends (e.g., Vibrio).     * Peritrichous: Distributed over the entire surface (e.g., E. coli).     * Structure: Helical filament (made of protein flagellin), a hook, and a basal body (rotary engine). Gram-negative bacteria have four rings in the basal body; Gram-positive have two.     * Movement: Driven by proton diffusion ($H^+$). Anticlockwise rotation produces a "run"; clockwise rotation produces a "tumble."     * Chemotaxis: Running and tumbling to move toward nutrients (attractants).     * Endoflagella: Found in spirochetes within the periplasm, causing corkscrew motion.
• Pili (Fimbriae):     * Rigid fibers composed of the protein pilin with adhesins at the tip.     * Attachment Pili: Stick bacteria to surfaces or each other.     * F Pilus (Fertility Pilus): Involved in Conjugation (DNA transfer).     * Motility: "Twitching" or "gliding" motility via retraction.

Protoplasm and Internal Components

• Cell Membrane: Approximately $7\,nm$ thick; composed of $32\text{-}40\%\text{ phospholipid}$ and $\%60\%\text{ protein}$. Site of energy production (respiration/photosynthesis).
• Genome (Nucleoid): No nuclear membrane. A single circular double-stranded DNA molecule. While the cell is $2\,\mu m$, the DNA can be $1200\,\mu m$ long.
• Plasmids: Extra-chromosomal circular DNA loops ($5\text{ to }100\text{ genes}$). Code for antibiotic resistance or toxins. Used as vectors in genetic engineering.
• Ribosomes ($70S$): Composed of $30S$ and $50S$ subunits. Free ribosomes make internal proteins; membrane-associated ones make secreted proteins.
• Magnetosomes: Membrane-bound vesicles containing $Fe_3O_4$ (magnetite) in magnetotactic bacteria, used to navigate using Earth's magnetic field.
• Gas Vesicles: Provide buoyancy for aquatic species.

Economic Importance of Bacteria

• Agriculture and Environment:     * Nitrogen Fixation: Rhizobium, Azotobacter, and Azospirillum fix atmospheric nitrogen in root nodules.     * Bioremediation: Degradation of petroleum oil spills and industrial toxic waste.     * Biological Control: Bacillus thuringiensis ($BT$) acts as a specific insecticide for Lepidoptera.     * Biomining: Thiobacillus ferrooxidans solubilizes metals (e.g., copper, gold) from ores.
• Food Industry:     * Fermentation: Saccharomyces cerevisiae (yeast) starts fermentation, but Acetobacter converts ethanol to acetic acid (vinegar).     * Wine: Secondary malolactic fermentation (converting sour malic acid to lactic acid) uses Leuconostoc oenos.     * Dairy: Lactococcus lactis and Lactobacillus bulgaricus are used for cheese and yogurt.     * Preserved Foods: Sauerkraut (cabbage), Kimchi, and pickles utilize lactic acid bacteria like Pediococcus cerevisiae.
• Pharmaceutical and Medicine:     * Antibiotics: Streptomycin from Streptomyces griseus.     * Hormones: Recombinant DNA technology allows bacteria to produce Insulin and Growth Factors.     * Vaccines: Formulations for Cholera, Tuberculosis (BCG), and Pneumococcus.
• Industrial Applications:     * Textiles: Retting of hemp and jute using Clostridium.     * Leather/Tobacco: Used for tanning leather and drying/curing tobacco leaves.

Pathogenic Bacteria: Selected Diseases

• Human Diseases:     * Anthrax: Bacillus anthracis     * Cholera: Vibrio cholerae     * Tuberculosis: Mycobacterium tuberculosis     * Tetanus: Clostridium tetani     * Peptic Ulcers: Helicobacter pylori     * Plague: Yersinia pestis     * Syphilis: Treponema pallidum
• Plant Diseases:     * Citrus Canker: Xanthomonas axonopodis pv. citri     * Crown Gall: Rhizobium tumefaciens     * Fire Blight (Apple/Pear): Erwinia amylovora     * Soft Rot (Carrot): Pectobacterium carotovora

Questions & Discussion

• Q: What is pasteurization?     * A: It is the process of disinfecting liquids, especially dairy products, by heating them at mild temperatures to reduce the number of microbes without necessarily killing all organisms, preventing deterioration.
• Q: How did Joseph Meister survive a rabid dog bite?     * A: He was inoculated $13$ times with the attenuated rabies virus. This induced an immune response, allowing his body to produce antibodies to neutralize the infection.
• Q: Match Robert Koch to his contribution.     * A: Pure culture.
• Q: Match Fannie Hesse to her contribution.     * A: Agar (solidifying agent).
• Q: What is the main constituent of the bacterial cell wall?     * A: Mucopeptide (Peptidoglycan).
• Q: Are bacteria found on Mars?     * A: Magnetosomes associated with bacillus-shaped structures have been found in meteorites from Mars, leading to beliefs of potential past bacterial life there.