Structure and Classification of Microbes

Dr. Parag Chauhan

Introduction

  • Microbes (microorganisms) are tiny living organisms that cannot be seen with the naked eye.

  • They are present everywhere in nature such as air, water, soil, food, and inside the human body.

  • Some microbes are beneficial (normal flora, fermentation, biotechnology), while others cause diseases.

  • Understanding the structure and classification of microbes is essential for microbiology, pathology, medicine, and laboratory diagnosis.

What are Microbes?

  • Microbes are organisms that can be observed only using:

    • Light microscope

    • Electron microscope

  • Major groups of microbes include:

    • Bacteria

    • Viruses

    • Fungi

    • Protozoa

    • Helminths (parasitic worms)

    • Algae (some are microscopic)

    • Prions (infectious proteins)

 

Basic Structure of Microbes

  • Microbes may contain the following structures depending on their type:

    • Cell wall

    • Cell membrane

    • Cytoplasm

    • Genetic material (DNA/RNA)

    • Ribosomes

    • Nucleus (in eukaryotic microbes)

    • Organelles (in eukaryotic microbes)

    • Surface structures such as capsule, pili, and flagella

 

Structure of Bacteria

  • Bacteria are prokaryotic organisms.

  • They do not have:

    • True nucleus

    • Membrane-bound organelles

  • Their genetic material is present as:

    • Single circular chromosome

    • Plasmids (extra DNA)

Main Components of the Bacterial Cell

Cell Wall

  • Gives bacteria:

    • Shape

    • Strength

    • Protection

  • Made mainly of peptidoglycan.

  • Important for:

    • Gram staining

    • Antibiotic action

Cell Membrane

  • Controls entry and exit of substances.

  • Contains enzymes for:

    • Respiration

    • Transport mechanisms

Cytoplasm

  • Semi-fluid part of cell containing:

    • Enzymes

    • Nutrients

    • Ribosomes

    • Inclusion bodies

Ribosomes

  • Type: 70S ribosomes

  • Function: protein synthesis

Nucleoid

  • Region containing bacterial DNA

  • No nuclear membrane

Capsule

  • Outer protective layer in some bacteria

  • Helps in:

    • Preventing phagocytosis

    • Increasing virulence

  • Example: Streptococcus pneumoniae

Flagella

  • Thread-like structures used for movement

  • Helps bacteria in motility

  • Different arrangements:

    • Single flagellum

    • Tuft at one end

    • Flagella at both ends

    • Flagella all around

Pili and Fimbriae

  • Hair-like structures

  • Functions:

    • Attachment to host tissues

    • Conjugation (DNA transfer)

Endospores

  • Dormant resistant structures produced by some bacteria

  • Highly resistant to:

    • Heat

    • Chemicals

    • Dryness

  • Examples:

    • Bacillus

    • Clostridium

 

Classification of Bacteria

Classification Based on Shape

  • Cocci (round)

  • Bacilli (rod-shaped)

  • Spiral forms:

    • Vibrio (comma shaped)

    • Spirillum (rigid spiral)

    • Spirochetes (flexible spiral)

Classification Based on Arrangement

  • Cocci:

    • Pairs (diplococci)

    • Chains (streptococci)

    • Clusters (staphylococci)

    • Groups of four (tetrads)

  • Bacilli:

    • Single rods

    • Chains (streptobacilli)

    • Short rods (coccobacilli)

Classification Based on Gram Staining

  • Gram-positive bacteria:

    • Thick peptidoglycan layer

    • Purple in Gram stain

  • Gram-negative bacteria:

    • Thin peptidoglycan layer

    • Outer membrane present

    • Pink/red in Gram stain

Classification Based on Oxygen Requirement

  • Aerobic bacteria (require oxygen)

  • Anaerobic bacteria (cannot survive in oxygen)

  • Facultative anaerobes (grow with or without oxygen)

  • Microaerophilic bacteria (need low oxygen)

  • Capnophilic bacteria (need increased CO₂)

Classification Based on Spore Formation

  • Spore-forming bacteria:

    • Bacillus

    • Clostridium

  • Non-spore forming bacteria:

    • E. coli

    • Staphylococcus

Structure of Viruses

  • Viruses are acellular (non-cellular).

  • They are smaller than bacteria.

  • They require living host cells to multiply.

  • They are called obligate intracellular parasites.

 

Main Components of Viruses

Genetic Material

  • Viruses contain either:

    • DNA

    • RNA

  • They never contain both together.

Capsid

  • Protein coat covering the nucleic acid

  • Protects viral genetic material

Envelope

  • Present in some viruses

  • Lipid layer derived from host cell membrane

  • Helps in viral entry into host cells

 

Classification of Viruses

Classification Based on Genetic Material

  • DNA viruses

  • RNA viruses

Classification Based on Envelope

  • Enveloped viruses

  • Non-enveloped viruses

Structure of Fungi

  • Fungi are eukaryotic microbes.

  • They have:

    • True nucleus

    • Membrane-bound organelles

  • Their cell wall is made of:

    • Chitin

 

Forms of Fungi

Yeast

  • Unicellular fungi

  • Reproduce by budding

  • Example: Candida

Mould

  • Multicellular fungi

  • Form hyphae and mycelium

  • Example: Aspergillus

Dimorphic Fungi

  • Exist as:

    • Yeast form in human body

    • Mould form in environment

  • Example: Histoplasma

 

Structure of Protozoa

  • Protozoa are unicellular eukaryotes.

  • They have:

    • Nucleus

    • Cytoplasm

    • Organelles

  • Many protozoa are parasitic.

Types of Protozoa Based on Locomotion

  • Amoebae (pseudopodia)

  • Flagellates (flagella)

  • Ciliates (cilia)

  • Sporozoa (non-motile)

Helminths (Parasitic Worms)

  • Helminths are multicellular parasites.

  • They are larger than typical microbes but studied in microbiology due to their disease-causing role.

  • Types include:

    • Nematodes (roundworms)

    • Cestodes (tapeworms)

    • Trematodes (flukes)

Prions

  • Prions are infectious protein particles.

  • They contain:

    • No DNA

    • No RNA

  • They cause fatal brain diseases.

  • Example:

    • Creutzfeldt–Jakob disease

 

Overall Classification of Microbes

Prokaryotic Microbes

  • Bacteria

Eukaryotic Microbes

  • Fungi

  • Protozoa

  • Helminths

  • Algae

Acellular Infectious Agents

  • Viruses

  • Prions

Importance of Classification of Microbes

  • Helps in:

    • Identification of microbes in laboratory

    • Selecting correct treatment

    • Understanding mode of transmission

    • Studying epidemiology

    • Preventing infections

    • Vaccine development