Structure of Bacterial Cell and Classification of Microbes One Shot Notes and MCQs

Cucture of Bacterial Cell and Classification of Microbes

1. Structure of Bacterial Cell

Bacteria are unicellular microorganisms that lack a true nucleus and membrane-bound organelles. They exhibit a variety of shapes such as cocci (spherical), bacilli (rod-shaped), and spirilla (spiral-shaped). Understanding the structural components of bacterial cells is essential for microbiology and pharmacology.

Key Structural Components of a Bacterial Cell

Cell Wall
- Made up of peptidoglycan (murein), providing rigidity and maintaining the cell’s shape.
- Gram-positive bacteria: Thick peptidoglycan layer with teichoic acids.
- Gram-negative bacteria: Thin peptidoglycan layer with an outer membrane containing lipopolysaccharides (LPS).
Plasma Membrane
- Phospholipid bilayer with embedded proteins, involved in nutrient transport and respiration.
Cytoplasm
- Contains ribosomes, enzymes, and genetic material. It is the site of various metabolic processes.
Nucleoid
- Region where the circular DNA is located, as bacteria lack a nucleus.
Ribosomes
- 70S type (smaller than eukaryotic ribosomes), responsible for protein synthesis.
Capsule or Slime Layer
- Composed of polysaccharides or polypeptides. Protects against phagocytosis and desiccation.
Flagella
- Long, whip-like structures used for motility.
Pili or Fimbriae
- Hair-like structures on the surface, helping in attachment to surfaces and in conjugation (transfer of genetic material).
Endospores
- Formed by some bacteria (e.g., Bacillus and Clostridium) under harsh conditions. Endospores are highly resistant to heat, radiation, and chemicals.

2. Classification of Microbes and Their Taxonomy

Microbes are classified into several groups based on their structure, metabolism, and genetics. Proper classification helps in identifying microorganisms and understanding their role in health, disease, and the environment.

Major Groups of Microbes

Actinomycetes
- Gram-positive, filamentous bacteria resembling fungi.
- Important for producing antibiotics (e.g., Streptomyces produces streptomycin).
Bacteria
- Prokaryotic microorganisms classified based on shape, Gram staining, and oxygen requirements.
- Examples:
  - Escherichia coli (Gram-negative)
  - Staphylococcus aureus (Gram-positive)
Rickettsiae
- Obligate intracellular parasites transmitted by arthropods (e.g., ticks and lice).
- Cause diseases like typhus and Rocky Mountain spotted fever.
Spirochetes
- Spiral-shaped bacteria with a flexible cell wall and axial filaments for motility.
- Examples:
  - Treponema pallidum (causes syphilis)
  - Borrelia burgdorferi (causes Lyme disease)
Viruses
- Acellular entities that require a host cell to replicate.
- Consist of genetic material (DNA or RNA) enclosed in a protein coat (capsid).
- Examples:
  - HIV (causes AIDS)
  - Influenza virus (causes flu)

3. Taxonomy of Microbes

Microbial taxonomy involves the classification of microorganisms into a hierarchical structure:

Domain: Bacteria, Archaea, and Eukarya
Kingdom: Eubacteria, Archaebacteria, Protista, Fungi, etc.
Phylum: E.g., Proteobacteria (in bacteria)
Class: E.g., Gammaproteobacteria
Order: E.g., Enterobacterales
Family: E.g., Enterobacteriaceae
Genus: E.g., Escherichia
Species: E.g., Escherichia coli

Key Differences Between Microbial Groups

Feature	Bacteria	Actinomycetes	Rickettsiae	Spirochetes	Viruses
Cell type	Prokaryotic	Prokaryotic	Prokaryotic	Prokaryotic	Acellular
Genetic Material	DNA (circular)	DNA	DNA	DNA	DNA or RNA
Cell Wall	Peptidoglycan	Peptidoglycan	Peptidoglycan	Peptidoglycan	None
Motility	Flagella or non-motile	Non-motile	Non-motile	Motile (axial filaments)	None
Example	E. coli	Streptomyces	Rickettsia rickettsii	Treponema pallidum	HIV, Influenza virus

MCQs on Bacterial Cell Structure

What is the major component of bacterial cell walls?
a) Cellulose
b) Chitin
c) Peptidoglycan
d) Lipopolysaccharide
Answer: c) Peptidoglycan
Which type of bacteria has a thick peptidoglycan layer?
a) Gram-negative
b) Gram-positive
c) Acid-fast bacteria
d) Mycoplasma
Answer: b) Gram-positive
What structure provides motility to bacteria?
a) Fimbriae
b) Pili
c) Flagella
d) Capsule
Answer: c) Flagella
Which structure protects bacteria from phagocytosis?
a) Capsule
b) Cell membrane
c) Plasmid
d) Ribosome
Answer: a) Capsule
What is the function of ribosomes in bacteria?
a) ATP production
b) Protein synthesis
c) DNA replication
d) Nutrient storage
Answer: b) Protein synthesis
What is the size of bacterial ribosomes?
a) 70S
b) 80S
c) 60S
d) 90S
Answer: a) 70S
Which of the following structures is absent in bacteria?
a) Nucleus
b) Plasma membrane
c) Ribosomes
d) Cell wall
Answer: a) Nucleus
Endospores are resistant to heat because of the presence of:
a) Calcium dipicolinate
b) Lipopolysaccharide
c) Mycolic acid
d) Capsule
Answer: a) Calcium dipicolinate
Plasmids in bacteria are involved in:
a) Protein synthesis
b) Antibiotic resistance
c) Cell division
d) Endospore formation
Answer: b) Antibiotic resistance
Which of the following bacteria can produce endospores?
a) Escherichia coli
b) Bacillus subtilis
c) Staphylococcus aureus
d) Mycoplasma pneumoniae
Answer: b) Bacillus subtilis
Which of the following is a Gram-negative bacterium?
a) Staphylococcus aureus
b) Escherichia coli
c) Bacillus cereus
d) Clostridium botulinum
Answer: b) Escherichia coli
What is the characteristic shape of Treponema pallidum?
a) Rod
b) Cocci
c) Spiral
d) Filamentous
Answer: c) Spiral
Actinomycetes resemble fungi because they:
a) Are multicellular
b) Have a nucleus
c) Produce branching filaments
d) Lack a cell wall
Answer: c) Produce branching filaments
Which of the following microbes is an obligate intracellular parasite?
a) Escherichia coli
b) Rickettsia rickettsii
c) Streptomyces
d) Lactobacillus
Answer: b) Rickettsia rickettsii
Which disease is caused by Borrelia burgdorferi?
a) Syphilis
b) Tuberculosis
c) Lyme disease
d) Malaria
Answer: c) Lyme disease
Which group of microorganisms lacks a cell wall?
a) Viruses
b) Actinomycetes
c) Mycoplasma
d) Rickettsiae
Answer: c) Mycoplasma
Which of the following produces streptomycin?
a) Escherichia coli
b) Streptomyces griseus
c) Clostridium tetani
d) Lactobacillus acidophilus
Answer: b) Streptomyces griseus
Which microorganism is used in the production of antibiotics?
a) Streptomyces
b) Escherichia coli
c) Treponema
d) Rickettsia
Answer: a) Streptomyces
What type of nucleic acid is found in viruses?
a) Only DNA
b) Only RNA
c) Either DNA or RNA
d) Both DNA and RNA
Answer: c) Either DNA or RNA
What is the main component of the outer membrane of Gram-negative bacteria?
a) Teichoic acid
b) Lipopolysaccharide (LPS)
c) Peptidoglycan
d) Mycolic acid
Answer: b) Lipopolysaccharide (LPS)
Which bacterium causes syphilis?
a) Treponema pallidum
b) Mycobacterium tuberculosis
c) Borrelia burgdorferi
d) Clostridium tetani
Answer: a) Treponema pallidum
Which of the following microbes is filamentous in nature?
a) Escherichia coli
b) Streptomyces
c) Staphylococcus aureus
d) Rickettsia
Answer: b) Streptomyces
The causative agent of Rocky Mountain spotted fever is:
a) Borrelia
b) Rickettsia rickettsii
c) Treponema pallidum
d) Staphylococcus aureus
Answer: b) Rickettsia rickettsii
Which of the following is NOT classified as a prokaryote?
a) Bacteria
b) Actinomycetes
c) Mycoplasma
d) Viruses
Answer: d) Viruses
Which of the following bacteria lack a cell wall?
a) Escherichia coli
b) Mycoplasma pneumoniae
c) Streptococcus pyogenes
d) Lactobacillus acidophilus
Answer: b) Mycoplasma pneumonia
Which type of microorganism requires a host cell for replication?
a) Bacteria
b) Viruses
c) Actinomycetes
d) Spirochetes
Answer: b) Viruses
Gram staining differentiates bacteria based on:
a) Cell shape
b) Peptidoglycan thickness
c) Presence of flagella
d) Ability to form spores
Answer: b) Peptidoglycan thickness
Which of the following is an example of Gram-positive bacteria?
a) Escherichia coli
b) Staphylococcus aureus
c) Salmonella typhi
d) Vibrio cholerae
Answer: b) Staphylococcus aureus
Which bacterium is responsible for tetanus?
a) Clostridium botulinum
b) Clostridium tetani
c) Mycobacterium tuberculosis
d) Treponema pallidum
Answer: b) Clostridium tetani
Which of the following microbes is used in vaccine production?
a) Mycobacterium tuberculosis
b) Streptomyces
c) Bacillus anthracis
d) Rickettsia
Answer: c) Bacillus anthracis