General Characteristics of Bacteria

Mr. Parag Chauhan

Introduction

  • Bacteria are ancient and highly adaptable microorganisms that inhabit almost every environment on Earth, from soil and water to the human body.

  • They possess a simple prokaryotic cell structure but show tremendous diversity in shape, size, and physiological capabilities.

  • Their rapid growth and metabolic versatility allow them to survive in extreme conditions where many other organisms cannot.

  • In medical microbiology, bacteria play a dual role—some act as beneficial commensals while others are major causes of infectious diseases.

  • Understanding their general characteristics is essential for identifying pathogens, designing antibiotics, and studying disease mechanisms.

  • Knowledge of bacterial structure, nutrition, reproduction, and genetics provides a fundamental base for clinical diagnostics and laboratory practices.

 

Cellular Organization

Bacteria possess a prokaryotic cellular architecture, characterized by:

Cell Envelope Structure

The cell envelope is a critical determinant of bacterial identity and pathogenicity.

  • Gram-positive bacteria

    • Thick peptidoglycan layer

    • Teichoic and lipoteichoic acids

    • Retain crystal violet stain (purple)

    • High susceptibility to β-lactam antibiotics

  • Gram-negative bacteria

    • Thin peptidoglycan layer

    • Outer membrane rich in lipopolysaccharide (LPS)

    • Porins for nutrient transport

    • Do not retain crystal violet (pink/red with Safranin)

    • Higher antibiotic resistance due to outer membrane barrier

Cytoplasmic Components

  • Nucleoid containing circular, double-stranded DNA without a nuclear membrane

  • 70S ribosomes (30S + 50S subunits), target for many antibiotics

  • Inclusion bodies like glycogen, polyphosphate granules, PHB granules

  • Plasmids carrying accessory genes (antibiotic resistance, virulence factors)

Surface Structures

  • Flagella for motility (monotrichous, lophotrichous, peritrichous, amphitrichous)

  • Fimbriae and pili for adhesion, conjugation, biofilm formation

  • Capsules and slime layers providing protection from phagocytosis and desiccation

 

General Characteristics of Bacteria

  1. Cell Structure: Bacteria are prokaryotic organisms lacking membrane-bound nuclei and organelles. They have a simple structure with a cell wall made of peptidoglycan.
  2. Shape: Common shapes include cocci (spherical), bacilli (rod-shaped), and spirilla (spiral).
  3. Reproduction: Bacteria primarily reproduce asexually through binary fission.
  4. Metabolism: They exhibit diverse metabolic pathways, including autotrophy (producing food) and heterotrophy (absorbing nutrients).
  5. Size: Typically small, ranging from 0.2 to 10 micrometres in diameter.
  6. Genetic Material: Bacteria usually have a single circular chromosome and may also contain plasmids, which are small, circular DNA molecules.
  7. Movement: Some bacteria are motile, using flagella or pili for locomotion.

Classification:

  • Gram Staining: Based on their cell wall composition, bacteria are classified into Gram-positive (thick peptidoglycan layer) and Gram-negative (thin layer with an outer membrane).
  • Oxygen Requirements:
    • Aerobic: Requires oxygen.
    • Anaerobic: Do not require oxygen and may be harmed by it.
    • Facultative: Can survive with or without oxygen.
  • Metabolic Type:
    • Autotrophic: Include photoautotrophs and chemoautotrophs.
    • Heterotrophic: Include saprophytic and parasitic bacteria.
  • Shape and Arrangement: Cocci (singular or in chains), bacilli (single or in clusters), etc.

Importance of Bacteria:

  • Decomposers: Bacteria are crucial in breaking down organic matter and recycling ecosystem nutrients.
  • Symbiosis: Many bacteria form beneficial relationships with plants and animals (e.g., gut bacteria aiding in digestion).
  • Pathogens: Some bacteria can cause diseases in humans, animals, and plants (e.g., streptococcus, E. coli).
  • Biotechnology: Bacteria are used in various applications, including fermentation (e.g., yoghurt production) and genetic engineering.

General Characteristics of Fungi

  1. Cell Structure: Fungi are eukaryotic organisms with a defined nucleus and organelles. Their cell walls contain chitin.
  2. Morphology: They can exist as single-celled organisms (yeasts) or multicellular (molds and mushrooms). Fungi often have a filamentous structure known as mycelium.
  3. Reproduction: Fungi can reproduce asexually (e.g., budding, fragmentation) and sexually (spore formation).
  4. Metabolism: Fungi are heterotrophic, obtaining nutrients through absorption. They are decomposers, breaking down organic material.
  5. Size: Varies widely, from microscopic yeasts to large mushrooms.

Classification:

  • Chytridiomycota: Aquatic fungi with flagellated spores.
  • Zygomycota: Includes molds; known for zygospore formation.
  • Ascomycota: The largest group, includes yeasts and morels; characterized by ascus (sac-like structure).
  • Basidiomycota: Includes mushrooms and puffballs; characterized by basidium (club-shaped structure).
  • Glomeromycota: Forms arbuscular mycorrhizal associations with plant roots.

Importance of Fungi:

  • Decomposers: Fungi play a crucial role in nutrient cycling by breaking down dead organic matter and returning nutrients to the ecosystem.
  • Food Sources: Many fungi are edible (e.g., mushrooms) and are used in food production (e.g., yeast in bread and fermentation).
  • Medicinal Uses: Some fungi produce antibiotics (e.g., penicillin) and other pharmaceuticals.
  • Pathogens: Certain fungi can cause plant, animal, and human diseases.

 

MCQs

  1. Bacteria belong to which domain of life?
    A. Eukarya
    B. Archaea
    C. Bacteria
    D. Protista

  2. The cell wall of most bacteria is primarily composed of:
    A. Chitin
    B. Peptidoglycan
    C. Cellulose
    D. Pectin

  3. Which of the following structures is found in Gram-negative bacteria but NOT in Gram-positive bacteria?
    A. Thick peptidoglycan
    B. Outer membrane
    C. Teichoic acid
    D. Capsule

  4. Which bacterial shape describes spherical bacteria?
    A. Bacilli
    B. Cocci
    C. Spirilla
    D. Vibrio

  5. The structure responsible for motility in many bacteria is:
    A. Capsule
    B. Pili
    C. Flagella
    D. Endospore

  6. Which of the following is a function of the bacterial capsule?
    A. Protein synthesis
    B. Protection from phagocytosis
    C. ATP production
    D. DNA replication

  7. The space between the cell membrane and outer membrane in Gram-negative bacteria is known as:
    A. Cytoplasm
    B. Nucleoid
    C. Periplasmic space
    D. Mesosome

  8. Which of the following is TRUE about bacterial nucleoid?
    A. Surrounded by nuclear membrane
    B. Contains linear DNA
    C. Contains circular, double-stranded DNA
    D. Found in the cytoplasmic membrane

  9. Fimbriae are mainly used for:
    A. Motility
    B. Attachment to surfaces
    C. DNA replication
    D. Toxin secretion

  10. Which structure helps bacteria exchange genetic material via conjugation?
    A. Capsule
    B. Sex pilus
    C. Flagella
    D. Ribosome

  11. Bacteria reproduce mainly by:
    A. Binary fission
    B. Budding
    C. Fragmentation
    D. Sporulation

  12. Which of the following is NOT a bacterial arrangement?
    A. Diplococci
    B. Streptococci
    C. Tetrads
    D. Filaments with cross walls

  13. Gram staining differentiates bacteria based on:
    A. Lipid content
    B. Cell wall composition
    C. Capsule thickness
    D. Spore formation

  14. The component responsible for endotoxin activity is:
    A. Lipid A
    B. Flagellin
    C. Teichoic acid
    D. Peptidoglycan

  15. Which of the following bacteria lack a cell wall?
    A. Mycobacterium
    B. Mycoplasma
    C. Clostridium
    D. Bacillus

  16. The cytoplasmic membrane of bacteria lacks:
    A. Proteins
    B. Carbohydrates
    C. Sterols
    D. Phospholipids

  17. Plasmids in bacteria are:
    A. Essential genes
    B. Extra-chromosomal DNA
    C. Viral DNA
    D. Membrane components

  18. Endospores are produced by:
    A. Escherichia coli
    B. Neisseria
    C. Bacillus
    D. Vibrio

  19. Endospores are resistant due to presence of:
    A. High RNA content
    B. Dipicolinic acid
    C. Thick capsule
    D. Flagella

  20. Which term describes bacteria that require oxygen for growth?
    A. Obligate anaerobes
    B. Obligate aerobes
    C. Facultative anaerobes
    D. Aerotolerant anaerobes

  21. Bacteria that grow best at moderate temperatures (25–40°C) are called:
    A. Thermophiles
    B. Mesophiles
    C. Psychrophiles
    D. Hyperthermophiles

  22. The rigid structure that maintains bacterial shape is:
    A. Cytoplasm
    B. Cell membrane
    C. Cell wall
    D. Ribosome

  23. Which of the following is a spiral-shaped bacterium?
    A. Streptococcus
    B. Staphylococcus
    C. Treponema
    D. Lactobacillus

  24. Which enzyme helps bacteria detoxify hydrogen peroxide?
    A. DNA polymerase
    B. Catalase
    C. Peptidase
    D. Lipase

  25. Peptidoglycan is made up of:
    A. NAG and NAM
    B. Glucose and fructose
    C. Cellulose and lignin
    D. Amino acids only

  26. Which of the following is not a function of the cell membrane?
    A. Electron transport
    B. Osmotic regulation
    C. Energy production
    D. Maintenance of capsule

  27. The minimum concentration of a substance required to inhibit bacterial growth is:
    A. MIC
    B. MBC
    C. LD50
    D. ED50

  28. The term ‘pleomorphism’ refers to:
    A. Ability to form spores
    B. Ability to change shape
    C. Ability to invade host
    D. Ability to produce toxin

  29. Which bacteria are known for acid-fast property?
    A. Streptococcus
    B. Bacillus
    C. Mycobacterium
    D. Vibrio

  30. Bacterial ribosomes are of which type?
    A. 60S
    B. 80S
    C. 70S
    D. 50S

  31. The term ‘prokaryote’ means:
    A. True nucleus
    B. Without nucleus
    C. With organelles
    D. With chloroplast

  32. Which of the following is a storage granule in bacteria?
    A. Lipopolysaccharide
    B. Volutin granule
    C. Peptidoglycan
    D. Ribosome

  33. Which layer is responsible for Gram-positive bacteria retaining crystal violet stain?
    A. Thin peptidoglycan
    B. Thick peptidoglycan
    C. LPS
    D. Capsule

  34. Which of the following bacteria exhibit swarming motility?
    A. Staphylococcus
    B. E. coli
    C. Proteus
    D. Shigella

  35. The time required for a bacterial population to double is called:
    A. Stationary phase
    B. Generation time
    C. Lag phase
    D. Growth rate

Answer Key

  1. C

  2. B

  3. B

  4. B

  5. C

  6. B

  7. C

  8. C

  9. B

  10. B

  11. A

  12. D

  13. B

  14. A

  15. B

  16. C

  17. B

  18. C

  19. B

  20. B

  21. B

  22. C

  23. C

  24. B

  25. A

  26. D

  27. A

  28. B

  29. C

  30. C

  31. B

  32. B

  33. B

  34. C

  35. B