Microbiology

Lecture 1

Metabolic Potential of Microbes

Metabolism can come in many flavors! Microbes demonstrate an astonishing range of metabolic diversity, utilizing various energy sources (light, organic compounds, inorganic chemicals) and diverse metabolic pathways to thrive in nearly every environment on Earth.

What is Microbiology?

Definition

• "micro" == small

• "bio" = life

• "-logy" (logos) = discourse, written or spoken debate

Microbiology is defined as the scientific study of all living organisms that are too small to be visible with the naked eye. This diverse group includes:

• Bacteria

• Archaea

• Viruses (non-cellular biological entities)

• Fungi

• Prions (infectious proteins)

• Protozoa

• Algae

These organisms are collectively known as 'microbes' or 'microorganisms,' forming the foundation of many ecosystems.

Importance of Microbes

• Key Roles: Microbes play vital and multifaceted roles in global processes:

  • Nutrient cycling: Essential for recycling elements like carbon, nitrogen, and sulfur through ecosystems.

  • Biodegradation/Biodeterioration: Decomposing pollutants and organic matter, as well as causing material spoilage.

  • Climate change: Influencing atmospheric gas composition (e.g., methane production, CO$\text{2}$ fixation).

  • Food spoilage: Causing degradation of food products, but also used in food production (e.g., fermentation).

  • The cause and control of disease: Pathogenic microbes cause infectious diseases, while others are critical for developing treatments (e.g., antibiotics).

  • Biotechnology: Utilized in industrial processes, genetic engineering, and pharmaceutical production.

• Research Questions: Microbiology research continuously seeks to answer significant fundamental and applied questions such as:

  • "How diverse is life on Earth?" - Exploring the vast, uncharacterized microbial world.

  • "Does life exist elsewhere in the Universe?" - Investigating extremophiles and astrobiological potential.

History of Microbial Life

• Oldest Bacterial Fossils: Stromatolites, layered sedimentary structures formed by the growth of microbial mats, are dated at 3.5 billion years (Gyr) old. While they are the oldest known bacterial fossils, they do not necessarily represent the first microbes. Life on Earth was predominantly microbial for the first 2/3 to 3/4 of Earth's living history, highlighting their foundational role in early planetary evolution.

Development of Life and Oxygen

• The evolution of life on Earth is a grand timeline, marked by significant microbial contributions:

  1. Origin of Earth (4.6 billion years ago): Formation of our planet.

  2. Anoxic Earth with an origin of cellular life and anoxygenic phototrophic bacteria (4 Gyr): Early life forms developed in an oxygen-free environment, with bacteria capable of photosynthesis without producing oxygen.

  3. Origin of cyanobacteria leading to the gradual oxygenation of Earth (2-3 Gyr): The evolution of cyanobacteria, which perform oxygenic photosynthesis, led to the Great Oxidation Event, dramatically changing Earth's atmosphere.

  4. Emergence of modern eukaryotes: Cells with a nucleus and other membrane-bound organelles evolved, incorporating features possibly through endosymbiosis with prokaryotes.

  5. Current biodiversity leads to mammals and humans: The long evolutionary path culminating in the complex life forms seen today, all underpinned by microbial processes.

Cell Types

Two Fundamental Kinds of Cellular Life

• Prokaryotes:

  • Bacteria

  • Archaea

• Eukaryotes:

  • Fungi

  • Protozoa

  • Algae

Note: Viruses are important microbes but are not considered cellular life forms; they are obligate intracellular parasites.

Distinguishing Characteristics of Cellular Life

• Understanding how to differentiate between prokaryotic (Bacteria and Archaea) and eukaryotic cells, and further distinguishing Bacteria from Archaea, is essential in microbiology:

  • Prokaryotes (Bacteria & Archaea): Lack a membrane-bound nucleus and other organelles. Typically smaller (0.5-5 $\mu$m).

    • Bacteria: Contain peptidoglycan in their cell walls; have ester-linked lipids in their cell membranes; found in diverse environments including human hosts and soil.

    • Archaea: Lack peptidoglycan in cell walls; possess unique ether-linked lipids in cell membranes; often found in extreme environments (e.g., hot springs, saline lakes, anoxic sediments).

  • Eukaryotes (Fungi, Protozoa, Algae): Possess a true membrane-bound nucleus and various organelles (e.g., mitochondria, chloroplasts). Generally larger (typically >5 $\mu$m).

Diversity of Microbial Life

Major Lineages of Bacteria and Archaea

A comprehensive list of bacterial phyla includes, but is not limited to, the following examples, highlighting the vast genetic and metabolic diversity within the bacterial domain:

• Actinobacteria

• Bacteroidetes

• Cyanobacteria

• Firmicutes

• Proteobacteria (a very large and diverse phylum including Alpha, Beta, Gamma, Delta, and Epsilon groups)

• Spirochaetes

• And many others as represented in evolutionary trees and genomic databases, indicating the expansive diversity in microbial life that continues to be discovered.

Characterization of Microorganisms

1. Size and Shape

• Microorganisms vary greatly in size and shape, with examples illustrating the enormous scale differences, from viral particles in nanometers to some protists visible to the naked eye at hundreds of micrometers.

• Examples:

  • Tobacco mosaic virus: approximately 100 nm (nanometers) in length

  • Circovirus: approximately 1 $_\mu$m (micrometer) in diameter (though viral sizes vary greatly)

  • Escherichia coli: a typical bacterium, approximately 1 $_\mu$m in length

  • Paramecium: a protozoan, often >100 $_\mu$m, making it visible without a powerful microscope

Defining Characteristics of Cells

• Properties defining a cell include fundamental biological processes:

  • Metabolism

  • Growth

  • Reproduction

  • Evolution

  • Differentiation (in some cells)

  • Communication (in some cells)

  • Genetic exchange (in some cells)

  • Motility (in some cells)

Properties of All Cells

1. Metabolism: Engaging in nutrient uptake from the environment, their transformation into cellular components, and the expulsion of waste products. This encompasses:

   • Genetic processes: Replication of genetic material, transcription of genetic information into RNA, and translation of RNA into proteins.

   • Catalytic processes: Energy generation through various biochemical pathways (e.g., respiration, fermentation) and biosynthesis of complex molecules from simpler precursors.

2. Growth: The orderly increase in cell size and number. Nutritional uptake and the subsequent synthesis of new cellular materials lead to an increase in biomass, which eventually results in the formation of new cells through division.

3. Evolution: The inherent potential for all cells to evolve and develop new properties and adaptations over generations. Phylogenetic trees graphically illustrate these evolutionary relationships and shared ancestry among diverse life forms.

Properties of Some Cells

• Differentiation: The process where a cell develops into a new cell structure (e.g., bacterial cells forming resistant endospores for survival under harsh conditions).

• Communication: The ability of cells to interact with each other, often via chemical messengers or direct contact, influencing group behaviors like biofilm formation or quorum sensing.

• Genetic Exchange: Mechanisms enabling the transfer of genetic material between cells, which can lead to rapid adaptation and evolution (e.g., bacterial conjugation, transformation, transduction).

• Motility: The capability for self-propulsion through various mechanisms (e.g., flagella in bacteria, cilia in protozoa), allowing microbes to move towards nutrients or away from toxins.

Microbial Distribution on Earth

Table 1.2 Distribution of Microorganisms

• Microbial Biomass: The estimated global microbial biomass is about $\mathbf{2.5 \times 10^{30}}$ cells, collectively containing approximately $\mathbf{5 \times 10^{17}}$ grams of carbon. This vast quantity underscores their immense contribution to Earth's biogeochemical cycles.

Prokaryotes: The Unseen Majority

• Key Take Home Message: Prokaryotes (Bacteria and Archaea) account for an astonishing approximately 25% of all cellular carbon on Earth, and around 80% of the cellular nitrogen and phosphorous in the biosphere. This highlights their critical role in global nutrient cycling far beyond their microscopic size.

Historical Perspective of Microbiology

Stages of Microbiology Development

1. Ancient History (pre-1660): The Age of Sanitation, characterized by early public health practices.

2. Early History (1660-1850): The Age of Discovery, marked by the invention of the microscope and initial observations of microbes.

3. Microbiology’s Renaissance (1850-1920): The Age of Diagnoses, a period of establishing germ theory and developing methods for culturing and identifying pathogens.

4. Modern History (1920-Present): The Age of Biotechnology, driven by genetic discoveries and their applications.

Ancient History: The Age of Sanitation

• Early civilizations demonstrated remarkable foresight in public health, implementing sanitation practices as early as 2800 BC.

• Contribution of Classical Civilizations:

  • Indus Valley (2700 BC): Utilized sophisticated water pipes crafted from clay for supplying clean water and managing waste.

  • Egypt (2450 BC): Employed advanced metal pipes for water supply systems, indicating early engineering prowess.

  • Rome (315 AD): Famous for its extensive public lavatories, aqueducts providing fresh water, and the appointment of water commissioners dedicated to ensuring clean water supply for its citizens.

  • China (589 AD): Pioneered the production and usage of toilet paper, further advancing personal hygiene.

Early History: The Age of Discovery

• 1665: Robert Hooke is famously credited with observing "cells" in cork under his primitive microscope. He also identified plant and fungal cellular structures, but his microscope lacked the magnification to discern bacteria.

• 1668: Francesco Redi, through a series of elegant experiments involving maggots and rotten meat in sealed vs. unsealed jars, scientifically contested the prevailing theory of spontaneous generation, suggesting life arose from pre-existing life.

• 1676: Anton van Leeuwenhoek, a Dutch draper, made groundbreaking observations using his self-grounded single-lens microscopes. He was the first to accurately see and describe various types of bacteria and protozoa, which he termed “wee animalcules.”

  • Important point: Despite Leeuwenhoek’s monumental findings, significant technological and conceptual developments in microbiology took another 175 years to fully materialize and be appreciated.

Vaccination Development

• 1798: Edward Jenner introduces the revolutionary concept of vaccination, using cowpox (a milder infection) to prevent smallpox, a deadly and disfiguring disease. This marked a major turning point in infectious disease control.

• Variolation: This earlier, more risky technique involved intentionally exposing individuals to weak strains of smallpox to induce immunity.

• Louis Pasteur: Coined the term 'vaccination' in honor of Jenner's work ('vacca' means 'cow' in Latin) and went on to develop vaccines for diseases like rabies and anthrax.

Germ Theory and Medical Advances

• 1859: Louis Pasteur definitively resolves the long-standing spontaneous generation debate using his iconic swan-neck flasks experiments, thus providing conclusive evidence that microorganisms do not spontaneously arise from non-living matter and establishing the foundational Germ Theory of Disease.

• 1867: Joseph Lister popularizes antiseptic practices in surgery, dramatically reducing post-operative infections by using carbolic acid to sterilize wounds and surgical instruments. His work laid the groundwork for modern aseptic surgical techniques.

Koch’s Postulates (1884)

Robert Koch established a set of criteria to demonstrate that a specific microorganism causes a specific disease. These postulates remain foundational in medical microbiology:

1. The suspected pathogenic organism should be present in all cases of the disease and absent from healthy animals.

2. The suspected organism should be grown in pure culture (isolated from other microbes).

3. Cells from a pure culture of the suspected organism should cause disease in a healthy, susceptible animal.

4. The organism should be reisolated from the experimentally infected animal and shown to be identical to the original suspected pathogen.

Microbial Ecology Co-Founders

• 1876: Robert Koch establishes the direct causal relationship between specific microorganisms and infectious diseases, solidifying the Germ Theory.

• 1889-91: Sergei Winogradsky and Martinus Beijerinck, often considered the fathers of microbial ecology, make significant contributions to understanding how microbes interact with their environment and drive biogeochemical cycling (e.g., nitrogen fixation, sulfur oxidation) through selective cultures and enrichment techniques.

Modern History (1920-present)

The Age of Biotechnology

• 1929: The serendipitous discovery of penicillin by Alexander Fleming marks the beginning of the antibiotic era. Its subsequent mass-scale production revolutionized medicine by providing the first widely effective treatment for bacterial infections.

• 1953: The elucidation of the double helix structure of DNA by James Watson and Francis Crick (based on Rosalind Franklin's and Maurice Wilkins' work) provides the molecular basis for heredity and genetic engineering.

• 2000s: Significant advancements in genetic engineering, genomics, and high-throughput sequencing technologies (e.g., Next-Generation Sequencing) enable unprecedented study and manipulation of microbial genomes and communities.

CRISPR Technology

• CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats): This system, originally discovered as a bacterial immune system that confers resistance to foreign genetic elements (like viruses), has been adapted into a powerful and precise genetic editing tool. It allows scientists to make targeted changes to DNA, with applications ranging from correcting genetic defects in gene therapy to developing new disease models and engineering microbes for industrial production.

Conclusion: Long-term Implications of Microbial Research

• Microbiologists emphasize that microbes, often unseen, play an utterly crucial and indispensable role in maintaining health, driving nutrient cycles, and regulating environmental processes. Continued and deeper study into microbial genetics, ecology, and physiology is essential for advancements in biotechnology, medicine, agriculture, and our understanding of life itself.

Final Quote

• "The microbes will have the last word." - Louis Pasteur (A profound statement emphasizing the enduring power and adaptability of microbial life).

The Human Microbiome

• The human body is a complex ecosystem, hosting approximately 100 trillion microorganisms, which collectively outnumber human cells by a factor of tenfold. This massive community of microbes, known as the human microbiome, is predominantly found in the gut.

• Microbial genes integrated within the human microbiome impact various human factors, including digestion, metabolism, immune system development and function, susceptibility to disease, and even behavior. Understanding these intricate human-microbe interactions is a major frontier in modern medical research.

Lecture 2
Microbial Cell Structure and Functions

Basic Unit of Living Organisms

The cell is identified as the basic unit of living organisms and the smallest unit capable of life, enclosed by a membrane and containing genetic material. All cells share fundamental features that enable them to carry out essential life processes:

• Ribosomes: These complex molecular machines, composed of ribosomal RNA (rRNA) and proteins, are indispensable for protein synthesis. They serve as the cellular sites where messenger RNA (mRNA) sequences are translated into polypeptide chains, eventually forming functional proteins. Ribosomes can be found free in the cytoplasm or attached to the endoplasmic reticulum.

• ATP Energy: Adenosine Triphosphate (ATP) acts as the primary energy currency of the cell. It powers a vast array of metabolic processes, including active transport, muscle contraction, biosynthesis of macromolecules, and signal transduction. ATP is generated primarily through cellular respiration in most organisms and photosynthesis in autotrophs.

• Cell Membrane: This flexible, selectively permeable lipid bilayer, often described by the fluid mosaic model, forms the boundary of the cell. Composed mainly of phospholipids, proteins, and cholesterol, it protects the cell's interior, regulates the passage of substances (nutrients, waste, ions) into and out of the cell, and plays a crucial role in cell signaling and adhesion.

• External Stimuli: Cells possess the sophisticated ability to perceive and respond to changes in their environment. This involves various signaling pathways, often initiated by receptors on the cell surface or within the cytoplasm, which detect specific chemical or physical stimuli and trigger appropriate cellular responses.

• Genetic Material: Deoxyribonucleic Acid (DNA) serves as the hereditary blueprint for all cellular functions. Organized into a double helix structure composed of nucleotides, DNA contains genes that carry instructions for building and operating the cell. Ribonucleic Acid (RNA) plays a critical role in expressing this genetic information, acting as intermediaries and functional molecules in protein synthesis.

• Regulate Flow: Cells meticulously control the movement of substances across their membrane to maintain a stable internal environment (homeostasis). This regulation involves various mechanisms, including passive transport (diffusion, facilitated diffusion, osmosis) and active transport, which expends energy to move substances against their concentration gradients.

• Cytoplasm: This refers to the entire contents within the cell membrane, excluding the nucleus in eukaryotic cells. It consists of the cytosol (a gel-like substance primarily water, ions, and macromolecules) and various organelles. Most biochemical reactions and metabolic pathways, such as glycolysis, occur within the cytoplasm.

• Reproduce: Cells have the fundamental capacity to divide and reproduce, ensuring the perpetuation of life. This process can be asexual, such as binary fission in prokaryotes or mitosis in eukaryotes, leading to genetically identical daughter cells, or sexual (meiosis), leading to genetically diverse offspring.

Comparative Cell Structure of Bacteria, Archaea, and Eucarya

The distinctions between Bacteria, Archaea, and Eucarya highlight significant evolutionary divergence, particularly in cellular structure and composition. While all three domains share common cellular features, their fundamental differences define the two major cell types: prokaryotic (Bacteria and Archaea) and eukaryotic (Eucarya).

Bacterial Cell Structure

Bacterial cells are prokaryotic, meaning they lack a membrane-bound nucleus and other membrane-bound organelles. Their structure is optimized for rapid growth and adaptation:

• Flagella: These are long, whip-like appendages extending from the cell surface, constructed from a protein called flagellin. They are primarily responsible for bacterial motility, enabling movement through liquid environments. Bacteria can have various arrangements of flagella (e.g., monotrichous, lophotrichous, amphitrichous, peritrichous), influencing their characteristic "run and tumble" swimming motion.

• Pili (Fimbriae): Shorter and thinner than flagella, pili are hair-like proteinaceous structures on the surface of many bacteria. They are primarily involved in attachment to surfaces or other cells (fimbriae) and are crucial for biofilm formation and colonization. Specialized pili, known as sex pili, facilitate the transfer of genetic material (plasmids) between bacteria during conjugation.

• Key Structures in Bacterial Cells:

  • Nucleoid: This irregularly shaped region within the cytoplasm contains the cell's genetic material (a single, circular chromosome). Unlike a eukaryotic nucleus, it is not enclosed by a membrane but is densely packed with DNA.

  • Ribosomes: Similar to those in eukaryotes but smaller (70S vs. 80S), bacterial ribosomes are responsible for protein synthesis and are abundant throughout the cytoplasm.

  • Plasma Membrane: A phospholipid bilayer similar to that in eukaryotes, it surrounds the cytoplasm, regulating molecular transport, harboring enzymes for electron transport and ATP synthesis, and participating in nutrient uptake.

  • Cell Wall: Located external to the plasma membrane, the cell wall provides structural support, maintains cell shape, and protects the cell from osmotic lysis and mechanical stress. In most bacteria, it is primarily composed of peptidoglycan (murein), a complex polymer of sugars (N-acetylglucosamine and N-acetylmuramic acid) and amino acids. The thickness and composition of the peptidoglycan layer distinguish Gram-positive bacteria (thick peptidoglycan, teichoic acids) from Gram-negative bacteria (thin peptidoglycan, outer membrane containing lipopolysaccharide (LPS), and a periplasmic space).

  • Capsule: A protective outer layer, typically composed of polysaccharides, that surrounds the cell wall of some bacteria. The capsule enhances virulence by protecting bacteria from phagocytosis by host immune cells, aids in adherence to surfaces, and prevents desiccation.

Electron Micrographs of Bacteria

Electron microscopy provides highly detailed images of bacterial ultrastructure, revealing morphology, internal components, and surface features at high resolution.

• Example: Bacillus coagulans (TEM), illustrating typical rod shape morphology, internal structures, and size (
  $.5 ext{ μm}$ wide). Transmission Electron Microscopy (TEM) allows visualization of internal details.

• Example: Escherichia coli analyzed under various magnifications, revealing its characteristic rod shape, flagella, pili, and internal organization through both TEM and Scanning Electron Microscopy (SEM).

Size of Cells

Cell sizes vary tremendously, ranging from macroscopic organisms composed of billions of cells to microscopic individual cells, each adapted to its specific environment and function:

• Human Height: $ext{~}1 ext{ m}$ (representing a multicellular organism's scale)

• Length of Nerve and Muscle Cells: $ext{~}0.1 ext{ m}$ (showing specialized cells can be quite long)

• Chicken Egg: $ext{~}1 ext{ cm}$ (a large single cell, primarily yolk)

• Frog Egg: $ext{~}1 ext{ mm}$ (another example of a large, visible single cell)

• Most Plant and Animal Cells: $ext{~}10 ext{ μm} - 100 ext{ μm}$ (the typical range for eukaryotic cells)

• Most Bacteria: $ext{~}1 ext{ μm} - 10 ext{ μm}$ (the typical range for prokaryotic cells)

• Smallest Bacteria (e.g., Mycoplasma): $ext{~}0.2 ext{ μm} - 0.3 ext{ μm}$ (among the smallest self-replicating organisms)

• Viruses: $ext{~}10 ext{ nm} - 100 ext{ nm}$ in size (acellular entities, significantly smaller than even the smallest bacteria).

• Lipids and Proteins: $1 ext{ nm} - 10 ext{ nm}$ range at a molecular level (the scale of individual macromolecules).

Examples of Cell Dimensions

• Oscillatoria: $8 imes 50 ext{ μm}$, a filamentous cyanobacterium.

• Bacillus megaterium: $1.5 imes 4 ext{ μm}$, a relatively large rod-shaped bacterium.

• Escherichia coli: $1 imes 3 ext{ μm}$, a common intestinal bacterium.

• Streptococcus pneumoniae: diameter $ext{~}0.8 ext{ μm}$, a spherical (coccus) bacterium.

• Haemophilus influenzae: $0.25 imes 1.2 ext{ μm}$, a small rod-shaped or coccobacillary bacterium.

Surface Area and Volume Relationships in Bacterial Cells

The relationship between a cell's surface area and its volume is a critical factor influencing cellular efficiency, especially for nutrient uptake, waste removal, and heat exchange. As a cell grows larger, its volume increases at a faster rate than its surface area.

• For a spherical cell with a radius ($r = 1 ext{ μm}$):

  • Surface Area = $4 ext{π}r^2 = 4 imes 3.14159 imes (1 ext{ μm})^2 ag{A = } 12.6 ext{ μm}^2$

  • Volume = $rac{4}{3} ext{π}r^3 = rac{4}{3} imes 3.14159 imes (1 ext{ μm})^3 ag{V = } 4.2 ext{ μm}^3$

• As radius increases (e.g., $r = 2 ext{ μm}$):

  • Surface Area = $4 ext{π}r^2 = 4 imes 3.14159 imes (2 ext{ μm})^2 ag{A = } 50.3 ext{ μm}^2$

  • Volume = $rac{4}{3} ext{π}r^3 = rac{4}{3} imes 3.14159 imes (2 ext{ μm})^3 ag{V = } 33.5 ext{ μm}^3$

The non-proportional increase in volume relative to surface area means that larger cells have a smaller surface area to volume ratio. This impacts nutrient uptake and metabolic efficiency because the cell membrane (surface area) is the primary site for exchange with the environment. A smaller ratio can limit the rate at which nutrients can enter and waste products can leave, thus dictating the practical upper limit for cell size and explaining why most cells are microscopic.

Notable Bacterial Specimens

Several bacterial species are notable for their unusually large size or unique cellular features, pushing the boundaries of what is typically expected for prokaryotes:

• Titanospirillum velox: An exceptionally large spirillum, up to $40 ext{ μm}$ long, often found in microbial mats. It is characterized by its speedy motility and is a Gram-negative bacterium, demonstrating the diversity in bacterial morphology and lifestyle.

• Epulopiscium spp.: These are symbiotic Gram-positive bacteria found in the gut of surgeonfish, reaching astounding lengths up to $700 ext{ μm}$. They are characterized by extreme polyploidy (possessing thousands of copies of their genome), a highly invaginated cell membrane, and a unique viviparous mode of reproduction where they form intracellular offspring, which grow to near mature size within the mother cell before being released.

• Thiomargarita namibiensis: Known as the "sulfur pearl of Namibia," this is one of the largest bacteria ever discovered, visible to the naked eye, with individual cells up to $750 ext{ μm}$. A dominating feature is its massive central vacuole, which comprises up to 98 ext{%} of its total volume and is filled with nitrate. This adaptation allows the bacterium to store electron acceptors for its sulfur utilization processes, enabling it to survive in sulfide-rich environments.

Cellular Functions: The Machine and Coding Aspects

Cellular functions can broadly be categorized into coding (information processing) and machine (executing work) aspects, illustrating the intricate interplay required for life.

Coding Functions

These functions involve the storage, retrieval, and expression of genetic information, following the central dogma of molecular biology:

• DNA: The primary molecule responsible for storing the cell's genetic information. It serves as the master blueprint, guiding all cellular activities, including the synthesis of proteins and RNA molecules. DNA's stable double-helix structure protects hereditary information.

• Key processes include:

  • Replication: The precise duplication of DNA to ensure that each daughter cell receives a complete and identical copy of the genetic material during cell division. This process is semi-conservative, meaning each new DNA molecule consists of one original strand and one newly synthesized strand, facilitated by DNA polymerase.

  • Transcription: The process where specific segments of DNA (genes) are used as templates to synthesize complementary RNA molecules (mRNA, tRNA, rRNA). RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template, carrying the genetic code from the nucleus to the cytoplasm in eukaryotes.

  • Translation: The final step in gene expression, where the genetic information encoded in mRNA is used to synthesize proteins. This occurs on ribosomes, where transfer RNA (tRNA) molecules bring specific amino acids corresponding to codons (three-nucleotide sequences) on the mRNA, forming a polypeptide chain.

Machine Functions

These functions involve the actual work of the cell, driven by energy and utilizing precursor molecules to build structures and perform activities:

• Energy (ATP): As the universal energy currency, ATP is central to cellular metabolism. Its hydrolysis to ADP + Pi releases free energy that powers endergonic (energy-requiring) cellular reactions, such as active transport, protein synthesis, muscle contraction, and maintaining ion gradients. This energy coupling is critical for sustaining life.

• Precursors of Macromolecules: These are the fundamental building blocks required for synthesizing all cellular structures and components. They include sugars (for polysaccharides like peptidoglycan and glycogen), amino acids (for proteins), fatty acids and glycerol (for lipids), and nucleotides (for DNA and RNA). Cells must acquire or synthesize these precursors to grow and maintain themselves.

• Reproduction (growth): This refers to the mechanisms ensuring the continuation of cellular life through division into daughter cells. In bacteria, this typically occurs via binary fission, a rapid process of cell growth and division. In eukaryotes, it involves the complex cell cycle, including interphase (growth and DNA replication) and M phase (mitosis/meiosis and cytokinesis), to produce new cells or organisms.

Overview of Bacterial Chemical Composition

Analyzing Escherichia coli, a well-studied bacterium, during balanced growth provides a representative view of bacterial chemical composition, highlighting the abundance and roles of various molecular components, with an estimated cell weight of roughly $10^{-12} ext{ grams}$, or $1 ext{ picogram}$.:

• Water: 70 ext{%} of total cell weight. Water is the primary solvent, essential for biochemical reactions and maintaining cell structure and turgor.

• Proteins: 16 ext{%} (approximately $20 ext{ billion molecules/cell}$). Proteins are the most diverse macromolecules, serving as enzymes (catalyzing reactions), structural components (cytoskeleton), transporters (membrane channels), and regulatory molecules (transcription factors).

• RNA: 6 ext{%} (comprising ribosomal RNA (rRNA), transfer RNA (tRNA), messenger RNA (mRNA), etc.). RNA molecules are crucial for gene expression; rRNA is a major component of ribosomes, tRNA carries amino acids for protein synthesis, and mRNA carries genetic instructions from DNA to ribosomes.

• Lipids: 3 ext{%} (mainly phospholipids forming membranes). Lipids form the structural basis of cell membranes, providing a barrier and compartmentalization. They also serve as energy storage molecules.

• DNA: 1 ext{%}. The genetic material, encoding all the information required for the cell's existence and reproduction.

• Others: Remaining percentage, including metabolites (intermediates of metabolic pathways), peptidoglycan (a massive structural molecule in the cell wall), inorganic ions (essential cofactors for enzymes, maintaining osmotic balance), and polyamines (involved in DNA stabilization and cell growth).

Take Home Message

• Proteins are the predominant molecular components by weight in bacterial cells, reflecting their diverse and critical roles as enzymes, structural elements, and regulatory molecules that drive nearly all cellular processes.

• Lipids represent the most abundant type by number of molecules (especially phospholipids in the membrane), forming crucial barriers and ensuring cellular integrity and compartmentalization.

• Peptidoglycan acts as a massive structural molecule, especially in the bacterial cell wall, providing rigidity, shape, and protection against environmental stresses.

• RNA primarily consists of ribosomal RNA (rRNA), which is integral to the structure and function of ribosomes and protein synthesis pathways, while DNA functions as a remarkably vast polymer holding the complete genetic information, dictating the hereditary traits and operational functions of the cell.

Organizing Cellular Macromolecules

Cellular contents are not merely a disordered mixture but are highly and specifically organized. This organization is dynamic, adapting based on cellular conditions and metabolic states, ensuring efficient function. Within the cell, proteins, nucleic acids, polysaccharides, and lipids are strategically distributed into various compartments (in eukaryotes) or dynamic aggregates and structures (in prokaryotes) to facilitate their interactions and functions. This intricate spatial and temporal organization is fundamental to life, allowing for complex biochemical pathways and regulatory networks to operate effectively.

Classification of Microbial Cellular Features

Key invariant features common to all cells regardless of their domain of life include:

• Ribosomes: Referred to as the protein synthesis factories, they are universally present in all forms of cellular life, underscoring the fundamental nature of protein synthesis for survival.

Lecture 3

Variant Features (Not Common to All)

• Cell Wall: Essential for structural support and protection against osmotic lysis, maintaining cell shape. Compositions vary significantly between bacterial groups (e.g., peptidoglycan in bacteria, pseudopeptidoglycan in some Archaea, chitin in fungi).

• Endospores: Highly resistant, dormant structures formed by some Gram-positive bacteria (e.g., Bacillus, Clostridium) under unfavorable conditions. They resist heat, radiation, chemicals, and desiccation, enabling survival for extended periods. Contains dipicolinic acid and calcium.

• Bacterial Flagella: Helical protein filaments (composed of flagellin) extending from the cell surface, anchored in the cell envelope. They rotate for motility, allowing chemotaxis (movement towards attractants or away from repellents). Different arrangements exist (monotrichous, lophotrichous, amphitrichous, peritrichous).

• Gas Vesicles: Hollow, protein-bound structures impermeable to water but permeable to gas. They regulate buoyancy, allowing photosynthetic aquatic microorganisms to position themselves at optimal light intensities in the water column.

• Capsules/Slime Layer: Polysaccharide or polypeptide layers external to the cell wall. Capsules are well-organized and tightly associated, providing protection from phagocytosis and desiccation, and aiding in adhesion. Slime layers are less organized and loosely associated, also involved in adhesion and protection.

• Inclusion Bodies: Intracellular granules that serve as storage depots for nutrients (e.g., poly-β-hydroxybutyrate for carbon and energy, volutin/polyphosphate for phosphate), or for specific metabolic functions (e.g., magnetosomes for magnetic navigation).

• Pili (Fimbriae): Hair-like protein appendages, shorter and thinner than flagella. Fimbriae function primarily in adhesion to surfaces and host tissues, contributing to pathogenicity. Sex pili (F-pili) are involved in conjugation, facilitating the transfer of genetic material (e.g., plasmids) between bacteria.

Comparison of Major Taxonomic Groupings

• Bacteria vs. Archaea vs. Eucarya: These three domains represent fundamental evolutionary divergences.

  • Cell Structure:

  • Bacteria and Archaea: Prokaryotic (no membrane-bound nucleus or organelles).

  • Eucarya: Eukaryotic (membrane-bound nucleus and organelles).

  • Cell Wall Composition:

  • Bacteria: Peptidoglycan (murein) typically present.

  • Archaea: Varied; can be pseudopeptidoglycan, S-layers (protein/glycoprotein), or other polysaccharide compounds; never peptidoglycan.

  • Eucarya: Variable; chitin in fungi, cellulose in plants/algae; animals lack cell walls.

  • Membrane Lipids:

  • Bacteria and Eucarya: Ester-linked fatty acids.

  • Archaea: Ether-linked isoprenoids (branched hydrocarbons). This unique lipid structure confers resistance to extreme conditions.

  • Ribosomal RNA (rRNA) Sequences: Distinct sequences for 16S rRNA (prokaryotes) and 18S rRNA (eukaryotes) are used for phylogenetic classification.

  • Metabolism: Wide range in all domains, but unique metabolic pathways are found (e.g., methanogenesis in Archaea).

  • Genetic Material:

  • Bacteria and Archaea: Circular chromosomes, sometimes plasmids.

  • Eucarya: Linear chromosomes within a nucleus.

  • RNA Polymerase:

  • Bacteria: Single, relatively simple RNA polymerase.

  • Archaea and Eucarya: Multiple, more complex RNA polymerases.

Bacterial Morphologies

Common Shapes of Bacteria

• Coccus: Spherical or ovoid cells. Can occur in various arrangements: diplococci (pairs), streptococci (chains), staphylococci (clusters), tetrads, sarcinae (cuboidal packets).

• Rod (Bacillus): Cylindrical or elongated cells. Can be short rods (coccobacillus), long chains (streptobacillus), or single.

• Spirillum: Rigid, wavy, or helical cells with external flagella.

• Spirochete: Flexible, corkscrew-shaped cells with internal flagella (endoflagella/axial filaments) located in the periplasmic space, enabling characteristic twisting motility.

• Stalk: A slender, non-reproductive extension of the cell, often serving to attach cells to surfaces (e.g., Caulobacter).

• Hypha: Elongated, thread-like structures, characteristic of filamentous bacteria (e.g., Streptomyces) and fungi, often forming mycelia.

• Budding and Appendaged Bacteria: Some bacteria reproduce by budding off new cells from a parent cell. Appendages (e.g., prosthecae) increase surface area for nutrient absorption or aid in attachment.

• Filamentous: Cells that grow as long, unbranched or branched chains of cells, sometimes enclosed in a sheath.

Bacterial Cell Walls

Types of Bacterial Cell Walls

1. Gram-positive Cell Walls:

   • Characterized by an exceptionally thick (20-80 nm) peptidoglycan layer, which can constitute 90% of the cell wall material. This thick layer retains the crystal violet-iodine complex during Gram staining, causing the cells to appear purple.

   • The peptidoglycan is composed of repeating N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) units, cross-linked by short peptide chains.

   • Teichoic acids and lipoteichoic acids are significant components embedded within the peptidoglycan and extending to the cell surface. Teichoic acids are polymers of glycerol phosphate or ribitol phosphate, often carrying D-alanine or sugar residues; lipoteichoic acids are covalently linked to the cytoplasmic membrane lipids. They contribute to the negative charge of the cell surface, aid in cation binding, and play roles in cell adhesion and autolysin regulation.

2. Gram-negative Cell Walls:

   • Comprised of a relatively thin (2-7 nm) peptidoglycan layer, accounting for only 5-10% of the dry weight of the cell wall. This thin layer is located in the periplasmic space between two membranes: the inner (cytoplasmic) membrane and an outer membrane.

   • The periplasmic space is a crucial compartment containing various proteins and enzymes (e.g., hydrolytic enzymes, detoxification enzymes, nutrient-binding proteins) involved in nutrient uptake, metabolism, and defense.

   • The outer membrane is a unique feature, asymmetrical in composition. Its outer leaflet is composed mainly of lipopolysaccharides (LPS), while the inner leaflet contains phospholipids.

     • LPS consists of three parts:

     • Lipid A: The conserved lipid component embedded in the outer membrane; acts as an endotoxin, responsible for toxic properties of Gram-negative bacteria (e.g., fever, septic shock).

     • Core Polysaccharide: A short, conserved chain of unusual sugars connecting Lipid A to the O-antigen.

     • O-Antigen (O-polysaccharide): A highly variable, hydrophilic polysaccharide chain extending outwards from the cell surface; important for immune evasion and serological identification.

     • Porins: Proteins embedded in the outer membrane, forming non-specific channels that allow passive diffusion of small hydrophilic molecules (e.g., sugars, amino acids) into the periplasmic space, while excluding larger or hydrophobic molecules.

   • Due to the outer membrane, Gram-negative cells do not retain the crystal violet-iodine complex and are decolorized by alcohol, allowing them to be counterstained by safranin, appearing pink or red.

Peptidoglycan Structure

• Constant Features: The fundamental backbone consists of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) units, forming long glycan chains. These units are connected by strong $\beta-(1,4)$ glycosidic linkages, which provide considerable strength in the horizontal direction.

• Variable Features: Differences primarily lie in the peptide bridges that connect the glycan sheets vertically. These interbridge peptides vary in length and amino acid composition between different bacterial species, determining the flexibility and overall structural integrity of the cell wall. For example, in Staphylococcus aureus (a Gram-positive bacterium), a pentaglycine interbridge connects the tetrapeptide side chains whereas in many Gram-negative bacteria, direct peptide bonds link the tetrapeptides.

Peptidoglycan Composition

• N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) are the core disaccharide units that form the glycan backbone. NAM residues have a lactyl group to which a short peptide chain (tetrapeptide) is attached.

• The presence of cross-linked peptides provides the essential structural rigidity. These peptides link adjacent glycan chains, creating a strong, mesh-like network.

  • Typical cross-linking amino acids found in the tetrapeptide side chains attached to NAM include L-Alanine, D-Glutamic acid, L-Lysine (or diaminopimelic acid (DAP) in Gram-negative bacteria and some Gram-positives), and two D-Alanine residues. The D-amino acids are unique to bacterial peptidoglycan and are not found in proteins, making them targets for antibiotics.

  • Cross-linking occurs between the D-Alanine of one peptide chain and the L-Lysine (or DAP) of an adjacent peptide chain, either directly or through an interbridge peptide.

• Bonding Considerations:

  • The $\beta-(1,4)$ glycosidic linkages between NAG and NAM are susceptible to hydrolysis by lysozyme, an enzyme found in tears, saliva, and phagocytes. Lysozyme breaks these bonds, weakening the cell wall and leading to cell lysis, especially in Gram-positive bacteria.

  • Lysostaphin is an endopeptidase specifically targeting the pentaglycine interbridge in Staphylococcus aureus peptidoglycan.

  • Many antibiotics, such as penicillin (a $\beta$-lactam antibiotic), interfere with the transpeptidation (cross-linking) enzymes (penicillin-binding proteins, or PBPs) responsible for forming the peptide bonds during cell wall synthesis, thereby inhibiting the formation of a strong, intact peptidoglycan layer and ultimately leading to osmotic lysis.