Chapter 1: The Evolution of Microorganisms and Microbiology – Study Notes

1.1 Members of the Microbial World

Definition and Scope of Microbiology

Microbiology is the study of organisms and acellular entities too small to be seen clearly by the unaided eye, generally less than 1 mm in diameter. These organisms are relatively simple in construction and lack differentiated tissues.

• Microorganisms include bacteria, archaea, protists, fungi, viruses, viroids, virusoids, and prions.

• They are the most populous and diverse group of organisms, found everywhere on Earth.

• They play major roles in recycling essential elements, serve as nutrient sources, and some carry out photosynthesis.

• Microbes benefit society by producing food, beverages, antibiotics, and vitamins, but some also cause disease.

Types and Divisions of Microbes

• Cellular microbes: Fungi (yeasts, molds), protists (algae, protozoa, slime molds, water molds), bacteria, and archaea.

• Acellular microbes: Viruses (protein and nucleic acid), viroids and virusoids (RNA), prions (protein only).

Types of Microbial Cells

• Prokaryotic cells lack a true, membrane-delimited nucleus. They are generally smaller and simpler than eukaryotic cells.

• Eukaryotic cells have a membrane-enclosed nucleus, are more complex morphologically, and are usually larger.

Classification Schemes

The three-domain system, based on ribosomal RNA (rRNA) gene comparisons, divides life into:

• Bacteria (true bacteria)

• Archaea

• Eukarya (eukaryotes)

Carl Woese was instrumental in establishing this system, demonstrating that Bacteria and Archaea are fundamentally different.

Table: Comparison of Domains Bacteria and Archaea

Feature	Bacteria	Archaea
Cell Wall	Peptidoglycan present	No peptidoglycan
Membrane Lipids	Ester-linked	Ether-linked (unique)
Habitat	Ubiquitous, some extreme	Many in extreme environments
Pathogenicity	Some cause disease	Rarely pathogenic

Domain Eukarya

• Protists: Algae (photosynthetic), protozoa (motile, hunters/grazers), slime molds (two life stages), water molds (plant pathogens).

• Fungi: Yeasts (unicellular), molds and mushrooms (multicellular).

Acellular Infectious Agents

• Viruses: Smallest microbes, require host cell for replication, cause many diseases.

• Viroids: Infectious RNA agents, cause plant diseases.

• Satellites: Nucleic acid in protein shell, require helper virus for replication (e.g., Hepatitis delta virus).

• Prions: Infectious proteins causing neurodegenerative diseases (e.g., mad cow disease).

1.2 Microbes Have Evolved and Diversified for Billions of Years

Defining Life and Microbial Evolution

Life is defined by reproduction, metabolism, cellular organization, response to environment, growth, evolution, and homeostasis.

• Most scientists agree life began 3.5–3.8 billion years ago, as evidenced by fossils and molecular markers (e.g., hopanes).

• Abiogenesis: Non-cellular compounds gave rise to the first cellular life in molecular-rich oceans (~3.6 billion years ago).

Origin of Life and Early Molecules

• Hydrothermal vents supplied minerals for early molecular evolution.

• RNA world hypothesis: Early life used RNA for both genetic information and catalysis (ribozymes).

• Experiments (e.g., Jack Szostak) show RNA-containing liposomes can form and replicate, modeling early cell evolution.

Today's RNA

• Modern cells use RNA in ribosomes (rRNA, tRNA, mRNA) for protein synthesis.

• Adenosine triphosphate (ATP) is a ribonucleotide and the universal energy currency.

• RNA can regulate gene expression and may have given rise to double-stranded DNA.

Earliest Metabolism

• Early energy sources: Inorganic compounds (e.g., FeS), photosynthesis.

• Cyanobacteria evolved ~2.7 billion years ago, producing oxygen and enabling aerobic respiration.

• Stromatolites: Fossilized microbial mats, evidence of ancient microbial life.

Evolution of the Three Domains of Life

• Universal phylogenetic tree based on small subunit rRNA (SSU rRNA) sequences.

• Aligned rRNA sequences from diverse organisms are compared to determine evolutionary relationships.

• 16S rRNA gene is a key marker for prokaryotic phylogeny.

Last Universal Common Ancestor (LUCA)

• LUCA is placed on the bacterial branch, but this is debated.

• Archaea and Eukarya evolved independently from Bacteria; Eukarya may have evolved from Archaea.

Evolution of Cellular Microbes

• Genetic variation arises from mutation and horizontal gene transfer (HGT).

• HGT allows rapid acquisition of new traits and is a major driver of microbial evolution.

Endosymbiont Theory

• Some eukaryotic organelles (mitochondria, chloroplasts) originated from prokaryotic endosymbionts.

• Evidence: Organelle size, binary fission, circular DNA, 70S ribosomes.

• Hydrogenosomes may have evolved from anaerobic bacteria producing H2 and CO2.

Table: Steps in Endosymbiotic Hypothesis

Step	Description
1. Loss of cell wall	Ancestral eukaryote lost cell wall
2. Engulfment	Engulfed endosymbiont (produced ATP)
3. Genome reduction	Endosymbiont genome reduced
4. Organelle evolution	Mitochondria, hydrogenosome, chloroplasts

Primary and Secondary Endosymbiosis

• Primary endosymbiosis: Eukaryote engulfs prokaryote.

• Secondary endosymbiosis: Eukaryote engulfs another eukaryote that already contains an endosymbiont.

Microbial Species

• Eukaryotic microbes: Species defined as reproductively isolated populations.

• Bacteria and Archaea: Do not reproduce sexually; classified as strains (descendants of a single, pure culture).

• Binomial nomenclature: Genus and species (e.g., Yersinia pestis).

1.3 Microbiology Advanced as New Tools for Studying Microbes Were Developed

Origins and Tools of Microbiology

• Study of microorganisms using microscopes, culture techniques, molecular genetics, and genomics.

Discovery of Microorganisms

• Antony van Leeuwenhoek: First to observe and describe bacteria and protists using microscopes (50–300x magnification).

The Conflict Over Spontaneous Generation

• Spontaneous generation: Idea that life arises from nonliving matter.

• Francesco Redi: Disproved spontaneous generation by showing maggots on meat came from fly eggs.

• Louis Pasteur: Swan-neck flask experiments showed no microbial growth in sterilized, open flasks, disproving spontaneous generation.

Role of Microorganisms in Disease

• Early theories attributed disease to supernatural forces or imbalances of bodily humors.

• Evidence for microbe-disease connection developed with improved techniques.

Key Contributors

• Agostino Bassi: Showed fungi cause silkworm disease.

• Heinrich de Bary: Demonstrated fungal causes of crop diseases.

• Louis Pasteur: Demonstrated microbial fermentation, developed pasteurization, and vaccines.

• Joseph Lister: Developed antiseptic surgery, reducing infections.

• Robert Koch: Established Koch's postulates, linking specific microbes to specific diseases.

Koch's Postulates

1. The microorganism must be present in every case of the disease but absent from healthy organisms.

2. The suspected microorganism must be isolated and grown in pure culture.

3. The same disease must result when the isolated microorganism is inoculated into a healthy host.

4. The same microorganism must be isolated again from the diseased host.

Limitations: Some microbes cannot be cultured, and ethical issues prevent human experimentation. Molecular and genetic evidence can supplement these postulates.

1.4 Microbiology Encompasses Many Subdisciplines

Basic and Applied Aspects

• Basic microbiology: Focuses on physiology, genetics, molecular biology, taxonomy.

• Applied microbiology: Addresses practical problems in disease, water, food, and industry.

Major Fields in Microbiology

• Public health microbiology: Study of disease spread and control.

• Immunology: Study of immune system and host defense.

• Microbial ecology: Interactions of microbes with their environment.

• Food and dairy microbiology: Use of microbes in food production and safety.

• Industrial microbiology: Microbes in industrial processes.

• Microbial physiology, genetics, molecular biology, bioinformatics: Study of microbial metabolism, genetics, and information processing.

Take Home Message

• Microbiology is a dynamic field with a long history and ongoing contributions to science and society.

• It provides insights into the unseen world and will continue to drive scientific discovery.