6.4 Structure: Shape and Size of Bacteria

Introduction

Bacteria are prokaryotic microorganisms that exhibit a wide variety of shapes and sizes. Their structure is relatively simple compared to eukaryotic cells, but they possess all the necessary components for survival, growth, and reproduction. Understanding the structure of bacteria is essential for understanding their functions, classification, and roles in nature and disease.

Definition

Bacterial Structure: The structure of a bacterium includes its cell wall, cell membrane, cytoplasm, genetic material, and specialized structures outside the cell wall.

In simple words: The structure of bacteria refers to the parts that make up a bacterial cell.

Core Concept

• A typical bacterium consists of:
• The cell wall is the outermost component common to all bacteria.
• Cell wall is absent in Mycoplasma species, which are bounded by cell membrane.
• Some bacteria have surface features external to the cell wall such as:
• Capsule
• Flagella
• Pili

Detailed Explanation

General Structure of a Bacterium

Cell Wall

• The cell wall is the outermost component common to all bacteria.
• It is a multilayered structure located external to the cytoplasmic membrane.
• It is composed of:
• An inner layer of peptidoglycan
• An outer membrane that varies in thickness and chemical composition depending on the bacterial type.
• The peptidoglycan provides structural support and maintains the characteristic shape of the cell.
• Cell wall is absent in Mycoplasma species, which are bounded by cell membrane.
• Some bacteria have surface features external to the cell wall such as:
• Capsule
• Flagella
• Pili

• Peptidoglycan is a complex interwoven network that surrounds the entire cell.
• It is composed of a singly covalently linked macromolecule.
• It is found only in bacterial cell walls.
• The term peptidoglycan is derived from the peptide and sugars (glycan) that make up the molecule.
• Synonyms for peptidoglycan are murein and mucopeptide.

• Sugar chains (glycan) cross-linked by peptides.
• S = Sugar
• AA = Amino acid

Capsule

• The capsule is a gelatinous layer covering the entire bacterium.
• It is composed of polysaccharide, except in the Anthrax bacillus, which has a capsule of polymerized D-glutamic acid.
• The sugar components of the polysaccharide vary from one species of bacteria to another.
• The capsule may play a role in the adherence of bacteria to human tissues.

Shape and Size of Bacteria

• Spherical (Cocci)
• Rod-shaped (Bacilli)
• Spiral (Spirochetes)

1. Spherical Bacteria (Cocci)

Definition: A coccus (kokus) is a spherical bacterium.

Arrangements of Cocci:

• Streptococcus pneumoniae
• Neisseria meningitidis

2. Rod-Shaped Bacteria (Bacilli)

Definition: Rod-shaped bacteria are called bacilli.

Arrangements of Bacilli:

• Escherichia coli
• Pseudomonas

3. Spiral Bacteria (Spirochetes)

Definition: Spirochetes are spiral bacteria that usually occur singly and seldom form colonies.

• They are thin-walled flexible spiral rods.
• Relatively, spirochetes are large and flexible.
• Example: Treponema pallidum

Size of Bacteria

- Bacteria range in size about 0.1 to 600 μm over a single dimension.

Summary Table of Bacterial Shapes

Summary Table of Bacterial Size

Intermediate Level

General Structure in Detail

Cell Wall in Detail

Capsule in Detail

Shapes of Bacteria in Detail

Size of Bacteria in Detail

Important Points

• 1. A typical bacterium consists of a cell wall, cell membrane, cytoplasm, genetic material, and specialized structures outside the cell wall.
• 2. The cell wall is the outermost component common to all bacteria.
• 3. Cell wall is absent in Mycoplasma species, which are bounded by cell membrane.
• 4. Some bacteria have surface features external to the cell wall such as capsule, flagella, and pili.
• 5. The cell wall is a multilayered structure located external to the cytoplasmic membrane.
• 6. It is composed of an inner layer of peptidoglycan and an outer membrane that varies in thickness and chemical composition depending upon the bacterial type.
• 7. The peptidoglycan provides structural support and maintains the characteristic shape of the cell.
• 8. Peptidoglycan is a complex interwoven network that surrounds the entire cell and is composed of a singly covalently linked macromolecule.
• 9. It is found only in bacterial cell walls.
• 10. The term peptidoglycan is derived from the peptide and sugars (glycan) that make up the molecule.
• 11. Synonyms for peptidoglycan are murein and mucopeptide.
• 12. The capsule is a gelatinous layer covering the entire bacterium.
• 13. It is composed of polysaccharide, except in the Anthrax bacillus, which has a capsule of polymerized D-glutamic acid.
• 14. The sugar components of the polysaccharide vary from one species of bacteria to another.
• 15. The capsule may play a role in the adherence of bacteria to human tissues.
• 16. Bacteria have three main shapes: spherical, rod-shaped, and spiral.
• 17. Spherical: A coccus (kokus) is a spherical bacterium. Cocci (koksi) generally appear in groups:
• - Diplococci (two cells)
• - Streptococci (long chain)
• - Staphylococci (irregular clumps)
• - Tetrad (packet of 4 cells)
• - Octate or sarcina (packet of 8 cells)
• 18. Examples of cocci include Streptococcus pneumoniae, Neisseria meningitidis.
• 19. Rod-shaped: Rod-shaped bacteria are called Bacilli. They are found in:
• - Pairs, called Diplobacillus
• - Very short and ovoid, called Coccobacilli
• - Curved into a form resembling comma, called Vibrio
• - Look like stack of coin, called Palisade
• 20. Examples of rod-shaped bacteria are Escherichia coli, Pseudomonas.
• 21. Spiral: Spirochetes are spiral bacteria that usually occur singly, seldom form colonies.
• 22. They are thin-walled flexible spiral rods.
• 23. Relatively spirochetes are large and flexible e.g., Treponema pallidum.
• 24. Size of Bacteria: Bacteria range in size about 0.1 to 600 μm over a single dimension.

Remember

• - Bacterial structure = cell wall, cell membrane, cytoplasm, genetic material, specialized structures
• - Cell wall = outermost layer; peptidoglycan
• - Cell wall absent in Mycoplasma
• - Peptidoglycan = murein, mucopeptide
• - Capsule = gelatinous layer; polysaccharide
• - Three shapes: Cocci (spherical), Bacilli (rod-shaped), Spirochetes (spiral)
• - Cocci arrangements: Diplococci, Streptococci, Staphylococci, Tetrad, Octate
• - Bacilli arrangements: Diplobacillus, Coccobacilli, Vibrio, Palisade
• - Spirochetes = spiral; singly; Treponema pallidum
• - **Size range** = 0.1 to 600 μm

Common Mistakes

• 1. Thinking all bacteria are rod-shaped – Bacteria have three main shapes: cocci, bacilli, spirochetes.
• 2. Confusing cocci and bacilli – Cocci = spherical; Bacilli = rod-shaped.
• 3. Confusing streptococci and staphylococci – Streptococci = chains; Staphylococci = clusters.
• 4. Thinking all bacteria have a capsule – Only some bacteria have a capsule.
• 5. Thinking the capsule is made of protein – The capsule is made of polysaccharide (except Anthrax bacillus).
• 6. Confusing peptidoglycan and capsule – Peptidoglycan = cell wall; Capsule = gelatinous layer outside the cell wall.
• 7. Thinking the cell wall is present in all bacteria – The cell wall is absent in Mycoplasma.
• 8. Confusing diplococci and tetrad – Diplococci = 2 cells; Tetrad = 4 cells.
• 9. Forgetting the examples – Streptococcus pneumoniae, Neisseria meningitidis, Escherichia coli, Pseudomonas, Treponema pallidum.
• 10. Thinking spirochetes form colonies – Spirochetes usually occur singly.
• 11. Confusing murein and mucopeptide – Both are synonyms for peptidoglycan.
• 12. Thinking bacteria are all the same size – Bacteria range in size from 0.1 to 600 μm.

Quick Revision

• A typical bacterium consists of a cell wall, cell membrane, cytoplasm, genetic material, and specialized structures outside the cell wall.
• The cell wall is the outermost component common to all bacteria.
• Cell wall is absent in Mycoplasma species, which are bounded by cell membrane.
• Some bacteria have surface features external to the cell wall such as capsule, flagella and pili.
• The cell wall is a multilayered structure located external to the cytoplasmic membrane.
• It is composed of an inner layer of peptidoglycan and an outer membrane that varies in thickness and chemical composition depending upon the bacterial type.
• The peptidoglycan provides structural support and maintains the characteristic shape of the cell.
• Peptidoglycan is a complex interwoven network that surrounds the entire cell and is composed of a singly covalently linked macromolecule.
• It is found only in bacterial cell walls.
• The term peptidoglycan is derived from the peptide and sugars (glycan) that make up the molecule.
• Synonyms for peptidoglycan are murein and mucopeptide.
• The capsule is a gelatinous layer covering the entire bacterium.
• It is composed of polysaccharide, except in the Anthrax bacillus, which has a capsule of polymerized D-glutamic acid.
• The sugar components of the polysaccharide vary from one species of bacteria to another.
• The capsule may play a role in the adherence of bacteria to human tissues.
• Bacteria have three main shapes: spherical, rod shaped and spiral.
• Spherical: A coccus (kokus) is a spherical bacterium. Cocci (koksi) generally appear in groups:
• (1) the groups consisting of two cells are called diplococci,
• (2) in the long chain called streptococci,
• (3) in irregular clumps that look like bunches of grapes called staphylococci,
• (4) Cocci may form packet of 4 cells called tetrad and
• (5) packet of eight cells called octate or sarcina.
• Examples of cocci include Streptococcus pneumoniae, Neisseria meningitidis.
• Rod shaped: Rod shaped bacteria are called Bacilli. They are found in:
• (1) pairs, called Diplobacillus,
• (2) very short and ovoid, called Coccobacilli,
• (3) curved into a form resembling comma called Vibrio,
• (4) look like stack of coin called palisade.
• Examples of rod shaped bacteria are Escherichia coli, Pseudomonas.
• Spiral: Spirochetes are spiral bacteria that usually occur singly, seldom form colonies. They are thin walled flexible spiral rods. Relatively spirochetes are large and flexible e.g., Treponema pallidum.
• Size of Bacteria: Bacteria range in size about 0.1 to 600 μm over a single dimension.

60-Second Revision

• Bacterial structure = cell wall, cell membrane, cytoplasm, genetic material, specialized structures
• Cell wall = outermost layer; peptidoglycan
• Cell wall absent in Mycoplasma
• Peptidoglycan = murein, mucopeptide
• Capsule = gelatinous layer; polysaccharide
• Three shapes: Cocci (spherical), Bacilli (rod-shaped), Spirochetes (spiral)
• Cocci arrangements: Diplococci, Streptococci, Staphylococci, Tetrad, Octate
• Bacilli arrangements: Diplobacillus, Coccobacilli, Vibrio, Palisade
• Spirochetes = spiral; singly; Treponema pallidum
• **Size range** = 0.1 to 600 μm
• Warning: Bacteria have three main shapes, not one. Cocci are spherical; bacilli are rod-shaped; spirochetes are spiral. Streptococci are chains; staphylococci are clusters. Not all bacteria have a capsule. The capsule is made of polysaccharide, not protein. Cell wall is absent in Mycoplasma. Peptidoglycan is found only in bacterial cell walls. Spirochetes usually occur singly. Bacteria range in size from 0.1 to 600 μm.