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5 different structures of microbes

This article explains the structure of different microbes - bacteria, fungi, protozoa, and viruses, and they differ from each other.
© BSAC

There are many different types of microbes. Each microbe differs from the next one in various ways, with one difference being structure.

This article will point out important parts of each microbes structure and how you can differentiate between them.

1. Bacterium

Bacteria are prokaryotic cells that have some unique and important structures that aid their survival and spread. Specific organelles within the bacterium include:

  • Capsule – is a sugar-based layer called glycocalyx which is situated outside of the cell wall. The glycocalyx layer can vary from a loose bound, slimy coat to a thick capsule. One role of the capsule is to protect the bacteria from the human immune system.
  • Ribosomes – are required for protein translation but differ from eukaryotic ribosomes in their size and structure.
  • Cell wall – is a thick rigid layer that maintains the integrity of the cell and determines its shape. Cell death can occur if the cell wall is destabilised.
  • Plasma membrane – selective passages in and out of the cell occur through the plasma membrane. Metabolic enzymes of nutrients and the production of energy are associated with the plasma membrane.
  • Nucleoid – is the genetic material (DNA) of the bacterium, which is often circular and anchored to the cell wall.
  • Flagellum – a tail-like structure that is used for motility.
  • Inclusion body – act as food reserves for the bacterium.
  • Fimbriae/pili – these hair-like structures are used to attach bacterium to surfaces. Can also be used to transfer genetic material from one bacterium to another, horizontal gene transfer (HGT).

Below shows a typical bacterium structure with all the mentioned organelles highlighted.

diagram of the bacteria structure, with labels showing the organelles.

2. Fungus

Fungi are eukaryotic cells, which contains similar organelles to human cells, these include:

  • Ribosomes – the same function are prokaryotic ribosomes but differ in size and structure.
  • Mitochondria – produce energy molecules for use in the metabolism.
  • Endoplasmic reticulum – involved in protein synthesis and protein transport.
  • Nucleus – a membrane-bound structure that encloses the genetic material of the cell.
  • Nucleolus – an area in the nucleus where ribosomes are assembled.
  • Cell wall – contains chitin and provides strength and structure to the cell.
  • Cell membrane – this is significantly different to the human cell membrane. It lies under the cell wall and encases the membrane.
  • Golgi apparatus – an organelle involved in targeting newly synthesized substances and proteins.
  • Storage vacuole – membrane-covered spaces within cells that store nutrients and waste products.
  • Centrioles – has a role in organizing the skeletal system of the cell.

Below shows an image of a fungal cell with the described organelles highlighted.

diagram of fungus structure, with labels showing the organelles.

3. Protozoan

Protozoa are eukaryotes which possess many of the organelles similar to fungi. However, unlike fungi, protozoa do not have cell walls. Protozoa have a pellicle, which is a very thin protein layer that protects the cell membrane.

The image below shows a typical protozoan structure which many of the organelles highlight that we have previously mentioned.

diagram of protozoa structure, with labels showing the organelles.

5. Virus

Viral structures are the most different to other microbe structures. Viruses can be naked and composed of the capsid and genetic material or be enclosed by an envelope. The particular virus structures are:

  • Capsid – a protein coat that encapsulates the nucleic acid of the virus.
  • Nucleic acid – the genetic material of the virus, can either be RNA or DNA and can also be single or double-stranded.
  • Envelope – external membrane which is typically a phospholipid bilayer and contains glycoproteins.
  • Spike – proteins on the outer surface of the envelope which can have multiple functions including adhering to surfaces.

Below an image shows the structure of viruses. This shows just have different viruses are in structure compared to other microbes.

diagram of virus structure, with labels showing the parts inside

© BSAC
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Introduction to Practical Microbiology

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