Microbial Size, Shape, and Diversity: Study Notes

Microbial Size, Shape, and Diversity

Introduction

Microbiology is the study of microscopic organisms, including bacteria, archaea, viruses, fungi, and protozoa. Understanding the size, shape, and diversity of microbes is fundamental to appreciating their roles in nature, disease, and biotechnology.

Limits on Microbial Size

Physical Constraints and Adaptations

The size of microorganisms is limited by physical and biological factors, which influence nutrient uptake, waste removal, and cellular organization.

• Surface Area-to-Volume Ratio: As cells increase in size, their volume grows faster than their surface area, limiting the rate of nutrient exchange and metabolic efficiency.

• Small Size Advantage: Smaller cells have a higher surface area-to-volume ratio, allowing for more efficient nutrient uptake and waste removal.

• Adaptations: Some microbes overcome size limitations by developing specialized structures (e.g., internal membranes, vacuoles) or by forming multicellular arrangements.

• Visibility: Objects must be at least 100 μm to be visible without a microscope; most bacteria and viruses are much smaller.

Example: Typical viruses measure about 100 nm, which is 10 times smaller than a typical bacterium (~1 μm).

Classification and Diversity of Microorganisms

Major Groups and Defining Characteristics

Microorganisms are classified based on cellular structure, genetic material, and metabolic capabilities. The main groups include bacteria, archaea, eukaryotes (fungi, protozoa, algae), and viruses.

• Bacteria: Prokaryotic, single-celled, peptidoglycan cell walls, diverse metabolic pathways.

• Archaea: Prokaryotic, single-celled, unique membrane lipids, pseudomurein cell walls, often extremophiles.

• Eukaryotes: True nucleus, membrane-bound organelles, includes fungi, protozoa, algae.

• Viruses: Acellular, DNA or RNA core, protein coat, require host cell for replication.

Example: Plasmodium species (protozoa) cause malaria and have a complex life cycle involving both human and mosquito hosts.

Cellular and Viral Microorganisms

Cell Structure and Function

Microbial cells exhibit diverse structures that reflect their evolutionary adaptations and ecological roles.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles; DNA is in a single circular chromosome; cell wall contains peptidoglycan (bacteria) or pseudomurein (archaea).

• Eukaryotic Cells: Possess a nucleus and membrane-bound organelles; DNA is organized in paired chromosomes; cell wall (when present) is made of polysaccharides.

• Viruses: Consist of a nucleic acid core (DNA or RNA) surrounded by a protein coat (capsid); some have an additional lipid envelope.

Feature	Prokaryote	Eukaryote
Chromosomes	Single, circular, not in nucleus	Paired, in nuclear membrane
Plasmids	Present	Absent
Organelles	Absent	Present
Ribosome	70S	80S
Cell Wall	Peptidoglycan (bacteria), pseudomurein (archaea)	Polysaccharide (when present)
Division	Binary fission	Mitosis

Cell Morphology and Arrangement

Shapes and Growth Patterns

Microbes exhibit a variety of shapes and arrangements, which can influence their growth, survival, and pathogenicity.

• Cocci: Spherical bacteria; may occur singly, in chains (streptococci), or clusters (staphylococci).

• Bacilli: Rod-shaped bacteria; may be single or in chains.

• Spirilla: Spiral-shaped bacteria.

• Other Arrangements: Filamentous, pleomorphic (variable shape), and non-walled forms.

• Variation: Some bacteria can modify their morphology in response to environmental conditions or during infection (e.g., Escherichia coli in urinary tract infections).

Pathogenic vs. Non-Pathogenic Microbes

Roles in Health and Disease

Microorganisms can be classified based on their ability to cause disease.

• Pathogenic: Cause disease in hosts.

• Non-pathogenic: Do not cause disease; may be commensal or beneficial.

• Opportunistic: Cause disease only under certain conditions (e.g., weakened immune system).

• Beneficial Roles: Production of chemicals, drugs, vitamins, acids; waste cleanup; fermentation (e.g., pickles, cheese, alcohol); photosynthesis.

Viruses and Prions

Structure and Infectious Properties

Viruses and prions are non-cellular infectious agents with unique properties.

• Viruses: Composed of a nucleic acid core and protein coat (capsid); some have a lipid envelope; require host cells for replication.

• Prions: Infectious proteins that cause neurodegenerative diseases by inducing abnormal folding of normal host proteins (e.g., Creutzfeldt-Jakob disease, mad cow disease).

Example: Prion diseases are untreatable, fatal brain diseases caused by misfolded prion proteins.

Case Study: Lyme Disease

Pathogen, Transmission, and Symptoms

Lyme disease is a tick-borne illness caused by the spirochete bacterium Borrelia burgdorferi.

• Transmission: Ixodes ticks feed on infected mice and deer, transmitting the bacterium to humans.

• Symptoms: Early symptoms include rash, fever, and joint pain; chronic forms can affect the heart and nervous system.

• Unique Features: B. burgdorferi has a unique cell wall and can acquire cholesterol from host cells, incorporating it into its outer membrane as cholesterol-glycolipids.

Summary Table: Microbial Groups and Key Features

Group	Cell Type	Genetic Material	Cell Wall	Reproduction	Example
Bacteria	Prokaryotic	DNA (circular chromosome)	Peptidoglycan	Binary fission	Escherichia coli
Archaea	Prokaryotic	DNA (circular chromosome)	Pseudomurein	Binary fission	Halobacterium
Eukaryotes	Eukaryotic	DNA (paired chromosomes)	Polysaccharide (when present)	Mitosis	Plasmodium (protozoa)
Viruses	Acellular	DNA or RNA	None (protein coat)	Requires host cell	Influenza virus
Prions	Acellular	None (protein only)	None	Protein aggregation	PrPSc (scrapie)

Key Equations

• Surface Area-to-Volume Ratio:

$\text{Surface Area-to-Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

• Volume of a Sphere (for cocci):

$V = \frac{4}{3} \pi r^3$

• Surface Area of a Sphere:

$A = 4 \pi r^2$

Conclusion

Microbial diversity encompasses a wide range of sizes, shapes, and cellular organizations. Understanding these features is essential for studying microbial ecology, pathogenesis, and biotechnology.