Microbial Diversity: Classification, Structure, and Unique Groups

Microbial Diversity

Introduction

Microbial diversity encompasses the vast variety of microorganisms, including bacteria, archaea, eukaryotic microbes (algae, fungi, protozoa, helminths), and acellular entities (viruses, viroids, prions). Understanding their classification, structure, and ecological roles is foundational in microbiology.

Classification & Taxonomy

Principles of Taxonomy

• Taxonomy involves classification, nomenclature (naming), and identification of organisms.

• Organizes large amounts of information and helps predict characteristics based on relatedness.

• Modern taxonomy aims to reflect phylogenetic hierarchy—evolutionary relationships based on genetic material.

Historical Approaches

• Carolus Linnaeus (18th century): Developed a system based on common characteristics, grouping organisms into species and using binomial nomenclature.

• Initially proposed two kingdoms: Animalia and Plantae.

• Later, a five-kingdom system: Animalia, Plantae, Fungi, Protista, Prokaryotae.

• Carl Woese: Introduced three domains (Eukarya, Bacteria, Archaea) based on rRNA sequence similarities.

Prokaryotes vs. Eukaryotes

Key Differences

Feature	Prokaryotes (Bacteria & Archaea)	Eukaryotes (Plants, Animals, Fungi, Algae, Protozoa)
Cellularity	Unicellular	Multicellular or unicellular
Nucleus	No nucleus (nucleoid)	True nucleus
Chromosomes	Single, circular	Paired, linear
Organelles	Absent	Present (membrane-bound)
Cell Wall	Peptidoglycan (bacteria), pseudomurein (archaea)	Polysaccharide (fungi, plants)
Ribosomes	70S	80S
Reproduction	Binary fission (asexual)	Sexual and asexual

Eukaryotic Microbes

Algae

• Eukaryotic, unicellular or multicellular (colonial).

• Photosynthetic (photoautotrophs), mostly aquatic.

• Simple reproductive structures; important in carbon fixation and as food sources.

Classification of Algae

• Based on pigmentation and cell wall composition.

• Major groups:

  • Division Chlorophyta (green algae)

  • Kingdom Rhodophyta (red algae)

  • Phaeophyta (brown algae)

  • Chrysophyta (golden algae, diatoms, yellow-green algae)

Fungi

• Eukaryotic, chemoheterotrophic, with cell walls of chitin.

• Do not perform photosynthesis; lack chlorophyll.

• Two main morphologies: Molds (multicellular, filamentous) and Yeasts (unicellular).

Significance of Fungi

• Saprobes: Decompose dead organisms, recycling nutrients.

• Symbiotic with plants, aiding water and mineral absorption.

• Used in food, beverage production, and as research tools.

• Produce antibiotics and other drugs.

• Some are pathogens; can spoil food.

Classification of Fungi

• Zygomycota: Mostly saprobes; some insect parasites.

• Basidiomycota: Saprobes; includes mushrooms.

• Ascomycota: Largest group; includes food spoilers and pathogens.

• Deuteromycetes: Heterogeneous; most now classified as Ascomycota.

Lichens

• Symbiotic partnerships between fungi and photosynthetic microbes (algae or cyanobacteria).

• Fungus provides nutrients, water, and protection; microbe provides carbohydrates and oxygen.

• Important in soil formation and nitrogen cycling; found in diverse habitats.

Protozoa

• Unicellular eukaryotes, similar to animals in nutrient needs and structure.

• Classified by locomotion:

  • Pseudopods (amoebae): Cell extensions for movement.

  • Cilia: Short, numerous protrusions for propulsion.

  • Flagella: Longer, fewer extensions for movement.

  • Non-motile: Apicomplexans.

Parasitic Helminths

• Eukaryotic, small multicellular animals (parasitic worms).

• Major groups:

  • Nematodes (roundworms)

  • Trematodes (flukes)

  • Cestodes (tapeworms)

• Studied due to their medical and ecological importance.

Prokaryotic Microbes

Archaea

• Unicellular, lack nuclei, smaller than eukaryotes.

• Found in diverse environments, including extreme conditions (extremophiles).

• No known human pathogens.

Extremophiles

• Require extreme temperature, pH, or salinity.

• Thermophiles: Thrive above 45°C; Hyperthermophiles above 80°C (e.g., Thermococcus).

• Halophiles: Require >9% NaCl; often pigmented (e.g., Halobacterium).

Methanogens

• Largest group of archaea; produce methane from CO2, H2, and organic acids.

• Important in carbon cycling and as a source of environmental methane.

Bacteria

• Deeply Branching Bacteria: Resemble early life forms; often thermophilic.

• Phototrophic Bacteria: Use light and inorganic carbon for photosynthesis; crucial for geochemical cycles.

• Gram-negative Bacteria: Largest, most diverse group (includes Proteobacteria, Spirochetes, Rickettsias, Chlamydias).

• Gram-positive Bacteria: Classified by G+C content (Actinobacteria, Firmicutes).

Viruses, Viroids, and Prions

Viruses

• Minuscule, acellular infectious agents with DNA or RNA genome.

• Lack cytoplasmic membrane, cytosol, or organelles; cannot carry out metabolism or reproduce independently.

• Exist in extracellular (virion) and intracellular states.

• Cause diseases in humans, animals, plants, and bacteria.

Classification of Viruses

• By type of nucleic acid, presence of envelope, shape, and size.

• Viral genera are organized into families; relationships are not fully understood.

Viroids

• Extremely small, circular pieces of single-stranded RNA (ssRNA).

• Infectious and pathogenic in plants; lack capsid.

• Viroid RNA binds complementary plant RNA, leading to disease.

Prions

• Proteinaceous infectious agents; lack nucleic acids.

• Normal cellular PrP has α-helices; prion PrP has β-pleated sheets.

• Prion diseases (spongiform encephalopathies) include CJD, BSE, scrapie, kuru, CWD, and vCJD.

• Transmitted by ingestion, transplantation, or contact with infected tissues; no standard treatment.

Comparison Table: Bacteria, Viruses, Viroids, Prions

Feature	Bacteria	Viruses	Viroids	Prions
Width	200–2000 nm	~10–400 nm	~10 nm	~10 nm
Nucleic Acid	DNA and RNA	DNA or RNA	RNA	None
Protein	Present	Present (capsid)	Absent	Present (PrP)
Cellular Structure	Cellular	Acellular	Acellular	Acellular
Growth	Present	Absent	Absent	Absent
Self-Replicating	Yes	No (require host)	No (host polymerase)	No (converts PrP)
Metabolism	Present	Absent	Absent	Absent

Additional info: This guide integrates and expands upon the provided slides and notes, ensuring coverage of all major microbial groups and their classification, structure, and significance in microbiology.