Bacterial Classification and Structure

Dr. Neha Sharma

Introduction

Bacteria are prokaryotic, unicellular microorganisms that play essential roles in health, disease, environment, and biotechnology. They are widely distributed in soil, water, air, and living organisms. Understanding bacterial classification, morphology, structure, chemical composition, and ecological importance is fundamental in microbiology and medical sciences.

This blog discusses:

  • Classification of bacteria

  • Morphology (Cocci and Bacilli – Gram positive and Gram negative)

  • Bacterial structure

  • Chemical composition

  • Plasmids

  • Microsomes

  • Cytoplasmic inclusions

  • Growth and reproduction

  • Microbial ecology

 

Classification of Bacteria

Bacteria can be classified based on various criteria:

A. Based on Shape

  1. Cocci – Spherical

  2. Bacilli – Rod-shaped

  3. Spiral forms – Spirilla, spirochetes

  4. Vibrios – Comma-shaped

B. Based on Gram Staining

Developed by Hans Christian Gram

  • Gram-positive bacteria – Retain crystal violet (purple)

  • Gram-negative bacteria – Take counterstain (pink/red)

C. Based on Oxygen Requirement

  • Obligate aerobes

  • Obligate anaerobes

  • Facultative anaerobes

  • Microaerophilic

D. Based on Spore Formation

  • Spore-forming bacteria

  • Non-spore-forming bacteria

 

Morphology of Bacteria

A. Cocci

Cocci are spherical bacteria.

Arrangement of Cocci

  • Diplococci – Pairs

  • Streptococci – Chains

  • Staphylococci – Clusters

  • Tetrads – Groups of four

  • Sarcinae – Cubical packets

1. Gram-Positive Cocci

Examples:

  • Staphylococcus aureus

  • Streptococcus pyogenes

  • Enterococcus faecalis

Characteristics:

  • Thick peptidoglycan layer

  • Teichoic acids present

  • No outer membrane

2. Gram-Negative Cocci

Examples:

  • Neisseria gonorrhoeae

  • Neisseria meningitidis

Characteristics:

  • Thin peptidoglycan layer

  • Outer membrane present

  • Lipopolysaccharide (LPS)

B. Bacilli (Rod-Shaped Bacteria)

https://www.researchgate.net/publication/277205153/figure/fig1/AS%3A294510841090048%401447228264193/a-Spores-and-bacteria-of-Clostridium-tetani-with-a-typical-drum-stick-shape-isolated.png

Bacilli are rod-shaped bacteria.

1. Gram-Positive Bacilli

Examples:

  • Bacillus anthracis

  • Clostridium tetani

  • Corynebacterium diphtheriae

Characteristics:

  • Thick peptidoglycan

  • Some form spores

  • No outer membrane

2. Gram-Negative Bacilli

Examples:

  • Escherichia coli

  • Salmonella typhi

  • Pseudomonas aeruginosa

Characteristics:

  • Thin peptidoglycan

  • Outer membrane

  • Endotoxin (LPS)

 

Structure of Bacterial Cell

Bacteria are prokaryotes and lack a true nucleus.

Main Components

  1. Cell wall – Provides shape and rigidity

  2. Plasma membrane – Selective permeability

  3. Cytoplasm – Contains enzymes and ribosomes

  4. Nucleoid – Circular DNA

  5. Ribosomes (70S) – Protein synthesis

  6. Capsule – Protection from phagocytosis

  7. Flagella – Motility

  8. Pili/Fimbriae – Attachment

 

Chemical Composition of Bacteria

Bacterial cells contain:

  • Water (70–80%)

  • Proteins

  • Lipids

  • Carbohydrates

  • Nucleic acids

  • Inorganic salts

Cell Wall Composition

  • Gram-positive: Thick peptidoglycan + teichoic acid

  • Gram-negative: Thin peptidoglycan + outer membrane + LPS

 

Plasmids

Plasmids are:

  • Small circular extrachromosomal DNA

  • Capable of independent replication

Functions:

  • Antibiotic resistance (R plasmids)

  • Toxin production

  • Virulence factors

Plasmids can transfer between bacteria via conjugation.

Microsomes

Bacteria do not have true microsomes like eukaryotic cells. However:

  • Mesosomes (membrane infoldings) were previously described

  • Involved in respiration and cell division

  • Now considered artifacts in electron microscopy

 

Cytoplasmic Inclusions

These are storage materials in bacterial cytoplasm:

  1. Glycogen granules

  2. Polyphosphate granules

  3. Sulfur granules

  4. Lipid inclusions

Functions:

  • Energy storage

  • Nutrient reserve

 

Growth and Reproduction

A. Mode of Reproduction

Bacteria reproduce by binary fission:

  1. DNA replication

  2. Septum formation

  3. Division into two daughter cells

Generation time varies from 20 minutes (e.g., Escherichia coli) to several hours.

B. Growth Curve

Bacterial growth in culture follows four phases:

  1. Lag phase

  2. Log (exponential) phase

  3. Stationary phase

  4. Death phase

C. Factors Affecting Growth

  • Temperature

  • pH

  • Oxygen

  • Nutrients

  • Moisture

 

Microbial Ecology

Microbial ecology studies the interaction of microorganisms with their environment.

A. Normal Flora

  • Skin

  • Gastrointestinal tract

  • Respiratory tract

Example: Escherichia coli in intestine

B. Pathogens

Cause disease under favorable conditions.

C. Environmental Role

  • Decomposition

  • Nitrogen fixation

  • Biogeochemical cycles