Bacteria and Archaea: How We Look at Them

Introduction

Bacteria and Archaea are two major groups of prokaryotic microorganisms, distinguished by their cellular structure and genetic makeup. Understanding their classification, morphology, and the methods used to observe them is fundamental in microbiology.

Types of Microbes

Classification of Microbes

• Cellular Microbes: Include Bacteria, Archaea, Fungi, Protozoa, and Helminths.

• Acellular Microbes: Include Viruses and Bacteriophages.

Prokaryotes (Bacteria and Archaea) lack a nucleus and membrane-bound organelles, while Eukaryotes (Fungi, Protozoa, Helminths) possess these structures.

Microscopy and Microbial Size

Size of Microbes

• Bacteria: typically 0.5–5 μm

• Eukaryotic cells: 10–30 μm

Microbes are observed using various types of microscopes, depending on the size and nature of the specimen.

Preparation of Specimens

• Specimens are mounted on glass slides for observation.

• Preparation depends on:

  • Condition of the specimen (living or preserved)

  • Purpose of examination (structure, motility, etc.)

  • Type of microscopy available

Relative Sizes of Biological Structures

Living Preparations

Observation of Living Microbes

To study the natural shape and motility of microbes, living preparations are used. A drop of culture is placed on a slide and covered, sometimes using a depression slide for hanging drop preparations.

• Allows observation of cell shape and motility under a light microscope.

• Preserves natural behavior and structure.

• Reduces risk of damaging or killing cells during preparation.

Hanging Drop Technique

• Specimen is suspended in a drop of fluid on a coverslip over a depression slide.

• Vaseline is used to seal the coverslip and prevent drying.

• Enables observation of motility and true cell shape.

Advantages and Limitations

• Advantages: Maintains natural cell shape and behavior; useful for motility studies.

• Limitations: Larger cells may be damaged or die; risk of contamination; observer safety must be considered.

Key Terms and Concepts

• Prokaryote: A unicellular organism lacking a nucleus and membrane-bound organelles.

• Eukaryote: An organism whose cells contain a nucleus and organelles.

• Microscopy: The use of microscopes to view small organisms and structures.

• Hanging Drop: A technique for observing living microorganisms in a suspended drop of fluid.

Example Application

Using a hanging drop preparation, a microbiologist can observe the motility of Escherichia coli and distinguish it from non-motile species.

Additional info: The notes reference textbook chapters and include diagrams from a microbiology textbook, supporting their relevance for college-level microbiology study.