Fundamentals of Microbiology: Microorganisms, Cell Structure, and Function

Microorganisms and Their Classification

Definition and Ubiquity of Microorganisms

Microorganisms are living organisms that are too small to be seen with the naked eye. They are found in virtually every environment on Earth, including water, soil, air, the human body, polar ice caps, hot springs, and salt flats.

• Key Point: Microorganisms include bacteria, archaea, fungi, protozoa, algae, and viruses.

• Example: Escherichia coli is a bacterium commonly found in the human gut.

Classification of Organisms

Organisms are classified based on cellular organization and evolutionary relationships. Carl Woese developed the three-domain system in 1978.

• Three Domains:

  • Bacteria

  • Archaea

  • Eukarya (includes Protists, Fungi, Plants, Animals)

Types of Microorganisms

Prokaryotes

Prokaryotes are unicellular organisms lacking a nucleus and membrane-bound organelles. They include bacteria and archaea.

• Bacteria:

  • Peptidoglycan cell walls

  • Binary fission for reproduction

  • Generate energy by oxidizing organic or inorganic materials

  • Includes pathogens

• Archaea:

  • No peptidoglycan in cell walls

  • Live in extreme environments (e.g., hot springs, salt lakes)

  • None are pathogenic

Eukaryotes

Eukaryotes have a nucleus and membrane-bound organelles. They include fungi, protozoa, and algae.

• Fungi:

  • Chitin cell walls

  • Use organic chemicals for energy

  • Yeasts are unicellular; molds are multicellular

  • Some are pathogenic

• Protozoa:

  • No cell walls

  • Absorb or ingest organic chemicals

  • May be motile via pseudopods, cilia, or flagella

  • Some are pathogenic (e.g., Plasmodium causes malaria)

• Algae:

  • Cellulose cell walls

  • Photosynthetic

  • Produce oxygen and organic compounds

  • None are pathogenic

Acellular Microbes

• Viruses:

  • Consist of DNA or RNA core surrounded by a protein coat

  • Require a host cell for replication

Cell Structure and Function

Bacterial Cell Size and Shape

Bacterial cells are typically 0.2–10 micrometers in diameter. Their shapes include bacillus (rod-shaped), coccus (spherical), spirillum (spiral), vibrio (comma-shaped), and spirochete (corkscrew-shaped).

• Key Point: Most bacteria are monomorphic (single shape), but some are pleomorphic (variable shapes).

• Example: Streptococcus pneumoniae forms chains of cocci.

Bacterial Cell Wall

The cell wall provides structural support and protection. It is composed mainly of peptidoglycan in bacteria.

• Peptidoglycan: Polymer of repeating disaccharide units (N-acetylglucosamine [NAG] and N-acetylmuramic acid [NAM]) linked by peptide bridges.

• Gram-Positive Cell Wall:

  • Thick peptidoglycan layer

  • Teichoic acids present

• Gram-Negative Cell Wall:

  • Thin peptidoglycan layer

  • Outer membrane contains lipopolysaccharides (LPS)

  • Periplasmic space between outer and plasma membranes

Comparison of Gram-Positive and Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Thickness	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
Lipopolysaccharide (LPS)	Absent	Present
Periplasmic Space	Absent	Present

Plasma Membrane Structure and Function

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins.

• Fluid Mosaic Model: Describes the dynamic nature of the membrane.

• Functions:

  • Selective permeability

  • Transport of nutrients and waste

  • Site for metabolic activities

• Permeability: Nonpolar molecules (e.g., O2, CO2) pass easily; polar molecules (e.g., H2O, ions) require transport proteins.

External Structures of Bacteria

Capsule and Slime Layer

Some bacteria possess an external capsule or slime layer composed of polysaccharides.

• Capsule: Organized, protective, aids in attachment, prevents desiccation.

• Slime Layer: Unorganized, loose, also aids in attachment.

Flagella

Flagella are long, whip-like appendages used for motility.

• Structure: Filament, hook, and basal body.

• Arrangement: Monotrichous (single), lophotrichous (tufts), amphitrichous (both ends), peritrichous (all over).

• Function: Movement toward or away from stimuli (chemotaxis).

Fimbriae and Pili

Fimbriae are short, hair-like structures for attachment; pili are longer and involved in DNA transfer (conjugation).

• Fimbriae: Aid in attachment to surfaces and host tissues.

• Pili: Facilitate transfer of genetic material between cells.

Bacterial Genome and Cell Division

Bacterial Chromosome and Plasmids

Bacteria typically have a single, circular chromosome located in the nucleoid region. Plasmids are small, extrachromosomal DNA molecules that carry additional genes, such as antibiotic resistance.

• Key Point: Plasmids can be transferred between bacteria via conjugation.

Bacterial Cytoskeleton

The bacterial cytoskeleton maintains cell shape and is involved in cell division.

• FtsZ Protein: Forms a ring at the site of cell division (binary fission).

• MreB Protein: Maintains rod shape.

• Crescentin: Maintains curved shape in some bacteria.

Inclusions and Storage Granules

Bacteria contain various inclusions for storage and metabolic functions.

• Phosphate Granules: Store phosphate for ATP synthesis.

• Carboxysomes: Contain enzymes for CO2 fixation.

• Gas Vesicles: Provide buoyancy in aquatic bacteria.

• Magnetosomes: Contain magnetic iron particles for orientation in magnetic fields.

Early Discoveries in Microbiology

Historical Milestones

• 1665: Robert Hooke reported that living things are composed of cells.

• 1673–1723: Anton van Leeuwenhoek described live microorganisms ("wee animalcules").

• 1861: Louis Pasteur demonstrated that microorganisms are present in the air and disproved spontaneous generation.

Microorganisms in Modern Agriculture and Biotechnology

Applications

• Bioremediation: Use of microbes to degrade environmental pollutants.

• Biofuels: Production of ethanol from biomass using bacteria.

• Cellulose Degradation: Microbes break down cellulose in plant material.

Exam Review Topics

• Bacterial cell size and shape

• Cell wall structure (gram positive, gram negative, mycobacteria)

• Capsule and external structures

• Cytoplasm and organelles

• Functions of lysozyme and penicillin

• Microbes in agriculture and biotechnology

Additional info: Some details on cell wall structure, external appendages, and historical experiments were expanded for clarity and completeness.