Microbial Size, Shape, and Diversity: Study Notes for Microbiology

Microbial Size, Shape, and Diversity

Introduction

Microorganisms exhibit remarkable diversity in their size, shape, and cellular organization. Understanding these features is fundamental to microbiology, as they influence microbial classification, physiology, and ecological roles.

Learning Outcomes

• Identify factors that limit bacterial size and describe adaptations in microorganisms.

• Explain how microorganisms are classified and distinguished as unique species.

• Describe major types of microorganisms and their defining characteristics.

• Compare cellular and viral microorganisms and infectious agents.

• Discuss similarities and differences between Archaea and Bacteria.

• Recognize basic cell morphologies and how microbes vary their morphology.

• Outline the disease progression of malaria and the life cycle of Plasmodium.

• Explain how sickle cell trait provides a heterozygote advantage.

Size Range of Various Microorganisms

Relative Sizes and Visualization

Microorganisms vary greatly in size, from nanometers to millimeters. Their size determines the type of microscope required for observation and influences their biological functions.

• Viruses: Typically measure about 100 nm, much smaller than bacteria.

• Bacteria: Range from 0.5 to 5 μm in diameter.

• Eukaryotic cells: (e.g., animal and plant cells) are usually 10–100 μm.

• Microscopy: Objects smaller than 100 μm require a microscope for visualization.

Example: The flu virus (~100 nm) is 10 times smaller than a typical bacterium (~1 μm), and at least 10 times smaller than a typical plant or animal cell (~10–100 μm).

Prokaryotic and Eukaryotic Cells: A Comparison

Key Differences

Cells are classified as either prokaryotic or eukaryotic based on their structural and genetic features.

Feature	Prokaryote	Eukaryote
Chromosomes	Single circular chromosome, not in nuclear membrane	Paired chromosomes in nuclear membrane
Plasmids	Often present	Absent
Histones	Absent	Present
Organelles	No membrane-bound organelles	Membrane-bound organelles
Ribosome Size	70S	80S
Cell Wall Composition	Peptidoglycan (Bacteria), Pseudomurein (Archaea)	Polysaccharide (when present)
Cell Division	Binary fission	Mitosis

Cell Structure

Prokaryotic vs. Eukaryotic Cell Anatomy

Prokaryotic cells (e.g., bacteria) lack a nucleus and membrane-bound organelles, while eukaryotic cells (e.g., animal and plant cells) possess these structures.

• Prokaryotic cell: Contains cell wall, cytoplasmic membrane, nucleoid, cytoplasm, plasmid, and ribosomes.

• Eukaryotic cell: Contains cell wall (in plants/fungi), cytoplasmic membrane, mitochondrion, nucleus, nuclear membrane, ribosomes, endoplasmic reticulum, Golgi complex, and cytoplasm.

Example: The presence of mitochondria and a defined nucleus distinguishes eukaryotic cells from prokaryotic cells.

Classification and Diversity of Microorganisms

Major Types and Defining Characteristics

Microorganisms are classified into several major groups based on cellular structure, metabolism, and genetic material.

• Bacteria: Prokaryotic, peptidoglycan cell walls, diverse morphologies.

• Archaea: Prokaryotic, pseudomurein cell walls, often extremophiles.

• Fungi: Eukaryotic, chitin cell walls, includes yeasts and molds.

• Protozoa: Eukaryotic, usually unicellular, lack cell walls.

• Algae: Eukaryotic, photosynthetic, cell walls of cellulose.

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

• Prions: Infectious proteins, cause neurodegenerative diseases.

Additional info: Helminths (parasitic worms) are multicellular animal parasites studied in microbiology due to their disease relevance.

Cell Morphology and Adaptations

Basic Morphologies and Variation

Bacteria exhibit several basic shapes, which can vary based on environmental conditions and genetic factors.

• Coccus: Spherical shape.

• Bacillus: Rod-shaped.

• Spirillum: Spiral-shaped.

• Vibrio: Comma-shaped.

• Arrangement: Can be single, pairs, chains, clusters.

Example: Escherichia coli can modify its shape in response to environmental stress or antibiotic exposure.

Surface Area to Volume Ratio

Importance in Microbial Physiology

The surface area to volume ratio (SA:V) is a critical factor that limits cell size and influences nutrient uptake and metabolic rates.

• Smaller cells have a higher SA:V ratio, facilitating efficient nutrient exchange.

• Larger cells may develop internal structures to overcome diffusion limitations.

Equation:

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

Example: Bacteria remain small to maximize their SA:V ratio, while eukaryotic cells use organelles to maintain efficiency.

Pathogenicity and Non-pathogenic Microbes

Roles in Health and Environment

Microbes 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 under specific conditions (e.g., weakened immunity).

• Environmental roles: Decompose waste, produce chemicals, drugs, vitamins, and food via fermentation.

Example: Lactobacillus species are used in fermentation to produce yogurt and cheese.

Viruses and Prions

Structure and Infectious Nature

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

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

• Prions: Misfolded proteins that induce abnormal folding in host proteins, leading to neurodegenerative diseases.

Example: Prion diseases include Creutzfeldt-Jakob disease and "mad cow" disease.

Case Study: Lyme Disease

Pathogen and Transmission

Lyme disease is caused by the spirochete bacterium Borrelia burgdorferi, transmitted by ticks. The disease involves complex interactions between the pathogen, vector, and host.

• Reservoirs: Mice and deer are common reservoirs for ticks.

• Transmission: Tick bite introduces the bacterium into the host.

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

• Unique features: B. burgdorferi incorporates cholesterol from host cells into its membrane.

Additional info: Detection of antibodies to B. burgdorferi peptidoglycan in joint fluid is associated with Lyme arthritis.