Introduction to Microbiology: Microbial World, Cell Structure, and Growth

The Microbial World and You

What are Microorganisms?

Microorganisms, or microbes, are organisms too small to be seen with the unaided eye. They are found in nearly every environment on Earth, including water, soil, air, polar ice caps, hot springs, ocean depths, volcanic soil, salt flats, and even on and inside the human body.

• Definition: Microorganisms are living entities invisible to the naked eye.

• Habitats: Ubiquitous—found in extreme and common environments.

• Human association: The human body hosts vast numbers of microbes, many of which are beneficial.

Classification of Life

In 1978, Carl Woese introduced the three-domain system, revolutionizing biological classification.

• Bacteria: Prokaryotic, single-celled organisms.

• Archaea: Prokaryotic, often extremophiles, distinct from bacteria.

• Eukarya: Eukaryotic organisms, including protists, fungi, plants, and animals.

Types of Microorganisms

• Prokaryotes (no nucleus): Bacteria, Archaea

• Eukaryotes (with nucleus): Fungi, Protozoa, Algae

• Acellular: Viruses (not composed of cells)

Major Microbial Groups

• Bacteria: Prokaryotic, cell wall with peptidoglycan, reproduce by binary fission, diverse energy sources, some pathogenic.

• Archaea: Prokaryotic, lack peptidoglycan, often extremophiles, not known to cause disease.

• Fungi: Eukaryotic, cell walls with chitin, absorb organic chemicals, includes molds, mushrooms, yeasts; some pathogenic.

• Protozoa: Eukaryotic, ingest/absorb nutrients, motile (pseudopods, cilia, flagella), many are pathogens.

• Algae: Eukaryotic, cell walls with cellulose, photosynthetic, oxygen producers, not pathogenic.

• Viruses: Acellular, DNA or RNA core, protein coat, sometimes lipid envelope, obligate intracellular parasites.

Key Structures in Fungi

• Hyphae: Filamentous structures.

• Mycelium: Mass of hyphae.

Early Discoveries in Microbiology

• Robert Hooke (1665): Observed cells in cork.

• Anton van Leeuwenhoek (1673–1723): First to observe living microorganisms, called them “wee animalcules.”

Spontaneous Generation vs. Biogenesis

• Spontaneous Generation: Life arises from nonliving matter.

• Biogenesis: Life arises from pre-existing life.

Key Experiments

• Francesco Redi (1668): Showed maggots come from fly eggs, not spontaneous generation.

• John Needham (1745): Claimed spontaneous generation after observing microbial growth in boiled broth.

• Lazzaro Spallanzani (1765): Disproved spontaneous generation by boiling broth longer and sealing flasks—no growth observed.

• Louis Pasteur (1861): Swan-neck flask experiment demonstrated that microbes come from the air, supporting biogenesis.

The Golden Age of Microbiology

• Pasteurization: Developed by Louis Pasteur; involves heating beverages to high temperatures for a short time to kill harmful bacteria.

Importance and Uses of Microorganisms

• Harmful: Cause infectious diseases.

• Beneficial: Used in agriculture (nitrogen cycle, plant growth), food production (yogurt, cheese, fermentation), medicine (antibiotics), biotechnology (insulin production), environmental cleanup (bioremediation), energy (biofuels), and research (model organisms like E. coli).

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Prokaryotes vs. Eukaryotes

• Prokaryotes: One circular chromosome (not membrane-bound), no organelles, 70S ribosomes, reproduce by binary fission, cell wall contains peptidoglycan (bacteria) or pseudomurein (archaea).

• Eukaryotes: Multiple linear chromosomes (in nucleus), membrane-bound organelles, 80S ribosomes, divide by mitosis.

Bacterial Cell Size and Shape

• Average size: 0.2–1.0 µm wide, 2–8 µm long.

• Smallest likely size: ~0.15 µm diameter.

• Monomorphic: Consistent shape.

• Pleomorphic: Variable shapes.

• Bacillus: Rod-shaped

• Coccus: Spherical

• Spiral: Vibrio (curved rod), Spirillum (rigid spiral), Spirochete (flexible spiral)

• Cocci arrangements:

  • Diplo-: Pairs

  • Strepto-: Chains

  • Tetrads: Groups of four

  • Sarcina: Cubic packets

  • Staphylo-: Grapelike clusters

Plasma Membrane

• Model: Fluid mosaic model

• Amphipathic: Molecules with hydrophilic and hydrophobic regions

• Phospholipid structure: Polar head, nonpolar fatty acid tails

• Property: Selectively permeable

• Stabilization: Hopanoids (in bacteria), cholesterol (in animals)

Functions of the Plasma Membrane

• Encloses cytoplasm

• Selective permeability

• ATP production (electron transport chain)

• Houses photosynthetic pigments

• Anchors flagella and pili

Transport Across Membranes

• Passive transport: High to low concentration, no energy required

  • Simple diffusion: Molecules move until equilibrium

  • Facilitated diffusion: Uses membrane proteins

  • Osmosis: Water movement across a selectively permeable membrane

• Tonicity:

  • Isotonic: Equal solute concentration

  • Hypotonic: Water enters cell (cell swells)

  • Hypertonic: Water leaves cell (cell shrinks, plasmolysis)

• Active transport: Against concentration gradient, requires ATP

• Group translocation: Substance chemically modified during transport

Bacterial Cell Wall

• Functions: Maintains shape, prevents osmotic lysis

• Peptidoglycan: Made of NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid), linked by peptide cross-bridges

Gram-Positive vs. Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic acids	Present	Absent
Outer membrane	Absent	Present
Periplasmic space	Absent	Present
Lipopolysaccharide (LPS)	Absent	Present
Endotoxin (Lipid A)	Absent	Present

Antibiotics and Cell Wall

• Penicillin: Blocks peptidoglycan cross-bridge formation, weakening the cell wall.

• Lysozyme: Enzyme that breaks glycosidic bonds in peptidoglycan.

Mycobacteria

• Unique feature: Mycolic acids in cell wall

• Growth: Slow due to poor nutrient transport through thick lipid wall

Glycocalyx

• Location: Outside cell wall

• Capsule: Organized, protective layer

• Slime layer: Loose, unorganized glycocalyx

Flagella

• Function: Motility

• Protein: Flagellin

• Parts: Filament, hook, basal body

Chemotaxis

• Definition: Movement toward or away from chemical stimuli

• Flagellar movements: Run and tumble

Axial Filaments

• Found in: Spirochetes

• Movement: Corkscrew motion

Fimbriae and Pili

• Fimbriae: Attachment to surfaces

• Pili: DNA transfer and attachment (sex pili for conjugation)

Bacterial DNA

• Nucleoid: Region containing bacterial chromosome

• Plasmids: Extra circular DNA with accessory genes

Ribosomes

• Prokaryotic: 70S

• Eukaryotic: 80S

Inclusions

• Definition: Storage granules inside bacteria

• Examples: Sulfur granules, polyhydroxyalkanoate, phosphate reserves

Endospores

• Definition: Dormant, highly resistant cells formed during starvation

• Formed by: Bacillus and Clostridium

• Sporulation: Endospore formation

• Germination: Return to vegetative cell

Microbial Growth

Basics of Microbial Growth

• Microbial growth: Increase in cell number, not cell size

• Population: Group of cells growing together

• Colony: Visible cluster of microorganisms on a surface

Bacterial Cell Division

• Binary fission: Main method of reproduction; one cell divides into two identical daughter cells

• Steps:

  1. Cell elongates

  2. DNA replicates

  3. DNA moves to each side

  4. Septum forms

  5. Cell divides into two daughter cells

• Budding: New cell forms from a small outgrowth of parent cell

• Filamentous growth: Elongation of filaments (like fungi)

Exponential Growth

• Population doubles every generation

• Population growth equation:

$N_t = N_0 \times 2^n$

• n: Number of generations

• To calculate n:

$n = \frac{\log N_t - \log N_0}{0.301}$

• Generation time (g):

$g = \frac{t}{n}$

• Where: t = time, n = number of generations

Batch vs. Continuous Culture

• Batch culture: Closed system with fixed nutrients and volume

• Continuous culture: Open system; nutrients added, waste removed

• Chemostat: Device for continuous culture, keeps population constant

Phases of Bacterial Growth (Batch Culture)

1. Lag phase: Cells adapt, no division

2. Exponential (log) phase: Rapid division, fastest growth

3. Stationary phase: Growth rate equals death rate

4. Death phase: Cells die faster than they reproduce

Environmental Factors Affecting Growth

• Temperature

• Pressure

• pH

• Osmotic pressure

• Oxygen

Temperature

• Cardinal temperatures: Minimum, optimum, maximum

• Mesophiles: 32–37°C (most human pathogens)

• Psychrotrophs: 0–30°C (spoil refrigerated foods)

• Psychrophiles: ≤15°C

• Thermophiles: ≥55°C

• Hyperthermophiles: Optimum ~100°C

Pressure

• Barophiles (piezophiles): Grow at high pressure (deep ocean)

pH

• pH homeostasis: Maintenance of stable internal pH

• Most bacteria: Grow best at pH 6.5–7.5

• Acidophiles: Grow in acidic environments

• Alkalophiles: Grow in alkaline environments

Osmotic Pressure

• Hypertonic: Water leaves cell, causing plasmolysis

• Halophiles: Require high salt concentrations

Oxygen Requirements

• Obligate aerobes: Require oxygen

• Obligate anaerobes: Oxygen is toxic

• Facultative anaerobes: Can grow with or without oxygen

• Aerotolerant anaerobes: Tolerate but do not use oxygen

• Microaerophiles: Require low oxygen levels

• Oxygen toxicity: Due to reactive oxygen species (superoxide, peroxide)

• Detoxifying enzymes: Catalase, peroxidase, superoxide dismutase

Growth in Nature: Biofilms

• Biofilms: Microbial communities attached to surfaces, embedded in extracellular polymeric substance (EPS)

• EPS: Made of polysaccharides, proteins, DNA, lipids

• Quorum sensing: Chemical communication to detect population density

• Advantages: Protection from antibiotics, immune system, environmental stability

• Examples: Dental plaque, ear infections, cystic fibrosis lung infections

Osmosis, Tonicity, and Penicillin

Osmosis and Tonicity

• Osmosis: Movement of water across a semipermeable membrane from low to high solute concentration

• Hypotonic solution: Lower solute outside; water enters cell; cell swells, may burst (lysis)

• Hypertonic solution: Higher solute outside; water leaves cell; cell shrinks (plasmolysis)

• Isotonic solution: Equal solute; no net water movement; cell remains normal

Solution	Water Movement	Result
Hypotonic	Into cell	Cell swells
Hypertonic	Out of cell	Cell shrinks
Isotonic	No net movement	Cell stays normal

• Application: Hypertonic environments (e.g., salted meat, pickles, jam) prevent bacterial growth by drawing water out of cells.

Penicillin and the Bacterial Cell Wall

• Penicillin: Antibiotic that blocks synthesis of peptidoglycan, weakening the bacterial cell wall.

• Effect: In hypotonic environments, water enters the weakened cell, causing lysis (bursting).

• Specificity: Penicillin targets bacteria only; human cells lack peptidoglycan cell walls.

Exam tip: Penicillin kills bacteria but not human cells because only bacteria have peptidoglycan cell walls.

Key Equations for Microbial Growth

• Number of generations (n):

$n = \frac{\log N_t - \log N_0}{0.301}$

• Generation time (g):

$g = \frac{t}{n}$

• Population growth:

$N_t = N_0 \times 2^n$

• Where: Nt = final cell number, N0 = initial cell number, n = number of generations, t = time

Summary Table: Microbial Groups and Features

Group	Cell Type	Cell Wall	Energy Source	Pathogenic?
Bacteria	Prokaryotic	Peptidoglycan	Organic/inorganic/photosynthesis	Some
Archaea	Prokaryotic	No peptidoglycan	Varied	No
Fungi	Eukaryotic	Chitin	Organic chemicals	Some
Protozoa	Eukaryotic	None	Organic nutrients	Many
Algae	Eukaryotic	Cellulose	Photosynthesis	No
Viruses	Acellular	None	Host cell machinery	Many

Additional info:

• Some explanations and context were expanded for clarity and completeness, such as the details of the Gram stain, the role of hopanoids, and the significance of biofilms.

• Tables were inferred and constructed to summarize key comparisons and features.