2. DIFFERENT METHODS OF CLASSIFICATION OF MICROBES

Introduction

Microorganisms differ from each other in structure, size, nutrition, reproduction and genetic makeup. Because of these differences, scientists developed several methods to classify microbes. Understanding classification helps in identification, diagnosis of diseases, and selection of appropriate antimicrobial treatment.

Different Methods of Classification of Microbes

1. Classification Based on Shape and Structure

This is one of the oldest methods used for identifying microbes, mainly bacteria.

  • Cocci: Spherical bacteria
  • Bacilli: Rod-shaped bacteria
  • Spirilla & Spirochetes: Helically curved bacteria
  • Pleomorphic: Variable shapes depending on environment

Cocci Arrangements

  • Monococci (single cocci)
  • Diplococci (pairs)
  • Streptococci (chains)
  • Tetracocci (groups of four)
  • Staphylococci (clusters)
  • Tetrads (square arrangement)
  • Sarcina (cubes of eight)

Bacilli Arrangements

  • Monobacilli (single rods)
  • Diplobacilli (pairs)
  • Streptobacilli (chains)
  • Coccobacilli (short rods)
  • Palisade bacilli (angular patterns)
  • Mycelial/filamentous forms (actinomycetes)

2. Classification Based on Flagella Arrangement

Flagella help in bacterial movement, and their arrangement is a useful classification tool.

  • Monotrichous: One flagellum at one end
  • Lophotrichous: Cluster of flagella at one end
  • Amphitrichous: Flagella at both ends
  • Peritrichous: Flagella all over the surface

3. Classification Based on Cell Wall Composition (Gram Staining)

  • Gram-positive bacteria: Thick peptidoglycan layer, fewer lipids, contain teichoic acid
  • Gram-negative bacteria: Thin peptidoglycan layer, outer lipid membrane, periplasmic space

4. Classification Based on Nutrition

  • Autotrophs: Prepare their own food (photosynthetic or chemosynthetic)
  • Heterotrophs: Depend on external organic matter

5. Classification Based on Oxygen Requirement

  • Aerobic
  • Anaerobic
  • Facultative anaerobes
  • Microaerophilic

6. Classification Based on Genetic Relatedness

Modern classification uses molecular techniques such as rRNA sequencing to determine phylogenetic relationships.

Study of Bacteria

Bacteria are microscopic, unicellular prokaryotes ranging from 1–3 µm. Their cell structure includes external and internal components.

External Structures

  • Flagella: For movement; composed of filament, hook and basal body.
  • Pili/Fimbriae: Hair-like structures for attachment and gene transfer.
  • Capsule: Gel-like covering that protects from dehydration and helps in attachment.
  • Sheath, prostheca and stalk: Structures formed under stress conditions.

Internal Structures

  • Cell wall: Contains peptidoglycan.
  • Cytoplasmic membrane: Phospholipid bilayer with essential enzymes.
  • Mesosomes: Inward foldings involved in cell division.
  • Cytoplasm: Contains ribosomes, DNA, plasmids and storage granules.
  • Spores: Endospores and exospores formed during harsh conditions.

Study of Fungi

Fungi are eukaryotic heterotrophic organisms lacking chlorophyll. Their body is called a mycelium, made of hyphae. They grow in moist environments and may be saprophytic, parasitic or symbiotic.

Classification

  • Myxomycophyta: Slime moulds
  • Eumycophyta: Includes phycomycetes, ascomycetes, basidiomycetes, deuteromycetes

Reproduction in Fungi

Fungi reproduce by both asexual and sexual methods.

Asexual Methods

  • Fragmentation
  • Budding (yeast)
  • Various spores: sporangiospores, conidiospores, uredospores, teliospores

Sexual Methods

  • Gamete fusion, plasmogamy, karyogamy (varies by fungal group)
  • Formation of fruiting bodies like ascocarp and basidiocarp

Study of Viruses

Viruses are acellular particles containing DNA or RNA within a protein coat called a capsid. They are obligate intracellular parasites, meaning they survive only inside living host cells.

Key Features

  • Very small (30–970 nm)
  • Shapes include helical, icosahedral, spherical and complex
  • Some viruses have envelopes and spikes for host cell attachment

Classification (Baltimore Classification)

  • dsDNA viruses
  • ssDNA viruses
  • dsRNA viruses
  • Positive-sense ssRNA viruses
  • Negative-sense ssRNA viruses
  • RNA viruses with reverse transcriptase

Virus Cultivation

  • Fertile eggs
  • Living animals
  • Tissue culture (most common method)

Life Cycle of Viruses

  • Lytic cycle: Attachment → DNA/RNA entry → replication → assembly → lysis of host cell
  • Lysogenic cycle: Viral genome integrates into host chromosome and replicates silently before activation

Study of Rickettsiae

Rickettsiae are gram-negative, obligate intracellular bacteria transmitted by ticks, lice, mites and fleas. They cause diseases such as typhus fever, scrub typhus and Rocky Mountain spotted fever.

  • Non-motile, pleomorphic
  • Reproduce only inside host cells
  • Genera include Rickettsia, Ehrlichia, Orientia, Coxiella

Study of Spirochetes

Spirochetes are long, thin, flexible bacteria with spiral or helical shape. They move using axial filaments located between the cell wall layers.

  • Gram-negative, 3–500 µm long
  • Motile due to internal axial fibres
  • Observed using dark-field or phase-contrast microscopy
  • Cause diseases like leptospirosis, lyme disease, relapsing fever and syphilis

Key Points

  • Microbes can be classified using shape, structure, nutrition, genetics and staining properties.
  • Bacteria are prokaryotic, fungi and protozoa are eukaryotic, while viruses are acellular.
  • Rickettsiae and spirochetes are specialized groups with unique properties and disease patterns.
  • Understanding classification helps in proper identification and treatment of microbial diseases.

Detailed Notes:

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