Chapter 2.1 – Structure of Bacteria
General Characteristics of Bacteria
Let’s start with the basics.
What are bacteria?
Bacteria are tiny living organisms — so small that you need a microscope to see them. They are unicellular, which means each bacterium is made up of just one cell. But don’t let their size fool you — they are found almost everywhere on Earth, from soil to water, and even inside our bodies!
Prokaryotic Organization
Bacteria belong to a group called prokaryotes. The word “prokaryote” means “before nucleus.” So, these cells do not have a true nucleus or any membrane-bound organelles (like mitochondria or Golgi bodies). Everything in the cell floats around freely in the cytoplasm.
Because of this, bacteria have the simplest type of cell structure among living organisms.
Cell Components of Bacteria
Now, let’s go deeper and look at the different parts of a bacterial cell.
1. Cell Wall
Structure
The cell wall is a hard, outer covering that surrounds the plasma membrane. Think of it like a protective armor that gives the cell its shape and keeps it safe from bursting or drying out.
The main building material of the bacterial cell wall is a special chemical compound called peptidoglycan (also known as murein).
- This compound is made up of long chains of sugars (glycans).
- These sugar chains are connected with short protein fragments (peptides).
- This combination forms a strong net-like structure.
Chemical Composition
- Peptidoglycan (murein): A mixture of sugars and proteins.
- Lipids (fats): Some bacteria also have lipids attached to the peptidoglycan.
Variations in Cell Walls
Not all bacterial cell walls are the same. Scientists divide bacteria into two main types based on their cell wall structure, using a technique called the Gram stain test.
a) Gram-positive Bacteria
- They have a thick layer of peptidoglycan.
- They contain less fat (lipids).
- Their thick wall absorbs the purple stain, so they appear purple under the microscope.
b) Gram-negative Bacteria
- They have a thin layer of peptidoglycan.
- They have an outer membrane made of lipopolysaccharides (sugars + fats) and lipoproteins (proteins + fats).
- They also have more space between the cell wall and the cell membrane, called periplasmic space.
- These bacteria do not retain the purple stain, and appear pink or red.
Special Features of Gram-negative Bacteria
- The outer membrane acts like a shield, making them resistant to many antibiotics.
- They contain porin proteins, which act like gates or channels that allow certain molecules to pass in and out of the cell.
Additional Structure: Capsule
Some bacteria have an extra layer outside the cell wall called a capsule.
- It’s gel-like and sticky.
- It helps bacteria to stick to surfaces, form colonies, and protect themselves from the immune system or drying out.