Members of the Microbial World

Definition of Microbiology

Microbiology is the scientific study of microorganisms, which are organisms and acellular entities too small to be seen clearly by the unaided eye. This field encompasses the biology, ecology, physiology, and genetics of bacteria, archaea, viruses, fungi, protozoa, and algae.

• Microorganisms include both cellular (living) and acellular (non-living) entities.

• Most microbes are less than 1 mm in size, though some are macroscopic.

• Microbes are relatively simple in their construction and lack highly differentiated cells and distinct tissues.

Carl Woese and the 3-Domain System

Carl Woese revolutionized the classification of cellular life by introducing the three-domain system based on ribosomal RNA (rRNA) gene sequences. This system divides all cellular life into three domains:

• Bacteria: True bacteria, characterized by peptidoglycan cell walls.

• Archaea: Distinguished from bacteria by unique rRNA sequences, lack of peptidoglycan, unique membrane lipids, and often unusual metabolic characteristics (e.g., methanogenesis).

• Eukarya: Organisms with membrane-bound nuclei, including protists, fungi, plants, and animals.

Example: Escherichia coli is a bacterium, while methanogens belong to Archaea.

Types of Microbes

Microbes can be classified as cellular or acellular:

Relative Sizes of Microorganisms

Microorganisms vary greatly in size, which affects how they are studied:

• Bacteria (e.g., Bacillus megaterium, Escherichia coli): Typically 1-10 μm, visible with light microscopes.

• Viruses: 20-300 nm, require electron microscopes for visualization.

• Mycoplasma: Among the smallest cellular microbes.

Limit of visibility: The unaided human eye can see objects larger than about 0.2 mm (200 μm).

The Importance of Microorganisms

Ecological and Societal Roles

Microorganisms are the most populous and diverse group of organisms on Earth, found in virtually every environment.

• Recycling Essential Elements: Microbes play a major role in biogeochemical cycles, such as carbon, nitrogen, and sulfur cycles.

• Source of Nutrients: Some microbes carry out photosynthesis, producing oxygen and organic matter.

• Industrial Benefits: Microbes are used in the production of food (e.g., yogurt, cheese), beverages (e.g., beer, wine), antibiotics, and vitamins.

• Pathogenicity: Some microbes cause diseases in plants and animals, impacting agriculture and health.

Example: Phytophthora infestans (a water mold) caused the Irish potato famine.

Bacteria "R" Us: The Human Microbiome

Bacterial Populations in the Human Body

The human body hosts a vast number of bacteria, especially in the gastrointestinal (GI) tract.

• Total bacterial population in and on the human body exceeds the number of human cells by an order of magnitude (100-500 trillion cells).

• Over 200 species of microbes reside in the GI tract, with a biomass of about 1.5 kg (220 g in the colon alone).

• The collection of bacterial genes (the microbiome) outnumbers human genes by a factor of 100.

The Human Microbiome

The human microbiome refers to the collective genomes of the microorganisms living in and on the human body. These microbes are distributed across various body sites, including the skin, mouth, gut, and more.

• Functions: The microbiome aids in digestion, synthesizes vitamins, protects against pathogens, and influences immune responses.

• Health Impact: Imbalances in the microbiome are linked to diseases such as obesity, diabetes, and autoimmune disorders.

• Behavioral Influence: Recent research suggests gut microbes may impact cognitive function and behavior, potentially influencing conditions like autism.

Example: The gut microbiota produces neuroactive compounds that may regulate behavior and stress responses.

Summary Table: Divisions and Types of Microbes

Key Equations and Units

• Metric Units: Microbial sizes are measured in meters (m), micrometers (μm), and nanometers (nm).

• Conversion:

Additional info: The study notes have expanded on the brief points to provide definitions, examples, and context for each major topic, including the ecological and medical significance of microbes and the human microbiome.