Types of Pathogenic Microorganisms

Types of Pathogenic Microorganisms

The average human body contains about 10 trillion cells. Imagine how much that is! If our population was 1400 times greater in the entire world, then we still would not be more than the number of cells in the entire body. Amazing isn’t it?

But what if I tell you the gut alone, contains 100 trillion microorganisms living within it this very minute? And hence the picture above, our world is really a microorganism’s world, we are simply the ones large enough to be seen.

And thus we see the importance of microbiology, how exactly are these microorganisms affecting our lives? 

Most of these microorganisms are actually beneficial to our body, for example, by aiding in the process of digestion, however, there are microorganisms that are damaging to their host, either by the production of toxic products, or direct infection, and these microorganisms are termed pathogenic. 

To have an idea of thislet us talk about the types of microorganisms, and the pathogenic ones in each type, that is, the one that can give us a disease.


Microbes that Cause Diseases

Microbes that cause diseases can be divided into 5 groups of organisms:

  1. Bacteria
  2. Fungi
  3. Protozoa
  4. Helminths and Rotifiers
  5. Viruses

There is also a recently discovered type of microbe that can cause a disease, known as a prion.

Of these microbes, we can classify them in several different ways.

Classification of Microbes:

Firstly, it is important to consider the status of prions and viruses. Technically, these “microbes” are not living. Prions are simply misfolded proteins, and viruses are only “alive” when they infect an organism. Thus, both prions and viruses have their own classifications.

As for the other organisms, we can classify them in several ways:

  • Eukaryote vs Prokaryote
    • In this classification scheme, all bacteria are prokaryotes, and fungi, protozoa, helminths and rotifers are eukaryotes
      • The prokaryotes are further subdivided into eubacteria and archaebacteria. Eubacteria are the medically important bacteria, while archaebacteria are a group of evolutionarily distinct bacteria.

Differences between Eukaryotes and Prokaryotes:

  • General Size
    • Eukaryotes are much larger than Prokaryotes, being about 10-100mm in diameter.
    • Prokaryotes are much smaller, being about only 0.2-2mm in diameter.
  • Nucleus vs Nucleoid:
    • Eukaryotic cells contain a true nucleus, with multiple chromosomes, linear DNA, and a nuclear membrane, using mitotic apparatus to ensure chromosomes are equally distributed to the daughter cells.
    • Prokaryotic cells contain a nucleoid, which is an area of loosely organized, circular supercondensed DNA, lacking nuclear membrane and mitotic apparatus.
  • Organelles:
    • Eukaryotes, unlike Prokaryotes, contain membrane-bound organelles such as mitochondria, endoplasmic reticulum and Golgi Apparatus. They also contain lysosomes, peroxisomes, and a microtubular network. Furthermore, they contain the larger (remember this by thinking: Eukaryotes are larger) 80S ribosomes. Membrane bound organelles such as mitochondria however, contain 70S ribosomes, which are smaller.
    • Prokaryotes contain no membrane-bound organelles, no lysosomes, peroxisomes and no (except in rare circumstances) microtubules, and contain the smaller 70S ribosomes.
  • Cell Wall
    • Eukaryotes do not usually contain a cell wall. Eukaryotes classified under Plantae, contain cell walls, and Fungi contain cell walls, made of a substance called chitin. Thus, these cell walls do not contain peptidoglycan.
    • Prokaryotes usually have an external cell wall that contains a substance known as peptidoglycan.
  • Cell Membrane
    • The Eukaryotic cell membranes contain sterols within the cell membrane.
    • Prokaryotes do not contain sterols within the cell membrane, with the exception of Mycoplasma, which contains no cell wall.
  • Classification Based on Motility
    • Some microorganisms are motile, while others are not.
    • Protozoa, Helminths, Rotifiers and Bacteria are motile.
      • Protozoa move by use of a flagella, cilia, or pseudopods.
      • Bacteria move by use of only a flagella.
    • Fungi and Viruses are nonmotile.
    • Prions are not able to move by themselves, but are able to move, thus they are not classified under motility.
  • Kingdom Classification
    • Bacteria are classified as Prokaryotes, further subdivided into eubacteria and archaebacteria as already discussed.
    • Fungi and Protozoa are classified as Protists.
    • Helminths and Rotifiers are classified as Animalia.

Two of the classification schemes can be well summarized using this diagram:


Brief Mechanism of Infectious Diseases

Infectious diseases triggered by most of these organisms begin by a process known as colonization, whereby the proliferating organism becomes established on the skin or mucous membranes, before it spreads through the body and causes infection. However, there are some exceptions, as some microorganisms begin infection internally, from internal organs, or by introduction directly into the bloodstream. In either case, once a microorganism begins colonization, there can only be 3 outcomes:

  1. Elimination of the microorganism without affecting the host, or incorporation into normal bodily flora.
  2. Infection, whereby organisms multiply and produce an immune reaction.
  3. A transient “carrier” state, where there appear to be no symptoms or immune response, but organisms may multiply.

Infectious disease occurs when the microorganism causes some impairment of bodily function, or tissue damage. The pathogenicity of a microorganism refers to its ability to produce an infection disease.


(Courtesy Lipincott’s Microbiology, 3rd Ed.)

Features of Microbes

Before we begin discussing each type of microbe, let us quickly understand why all organisms except Prions and Viruses are considered cellular, and living.

There are 3 main reasons why Prions and Viruses are not considered cellular organisms:

  1. Structure: Eukaryotic cells have a nucleus, while prokaryotic cells have a nucleoid, which contains DNA, surrounded by cytoplasm that contains cellular machinery to synthesize proteins. On the contrary, viruses have an inner core of genetic matter that is either DNA or RNA, but no cytoplasm and no cellular machinery for protein synthesis, and thus depend on the host cell to provide this for them. Prions, being misfolded proteins, do not need genetic material or cytoplasm, and lack any form of cellular structure.
  2. Method of Replication: Cells divide either by binary fission or mitosis, in where one cell produces 2 identical daughter cells. Prokaryotic cells carry out binary fission, while eukaryotic cells carry out mitosis.
    In contrast, viruses actually disassemble, make several copies of their proteins and nucleic acids, and then reassemble, and they can only do this if they are within a host cell and have access to its cellular machinery. Note that there are a few exceptions with regards to the protozoa: specifically, rickettsiae and chlamydiae also require cellular machinery of the host, but they have a structure, unlike viruses.
    All Prions do not divide at all, instead, they are like little zombies. When they come into contact with normal proteins, they misfold them and they then become prions themselves. Remember that, prions are zombies.
  3. Nature of the Nucleic Acid: While cells contain both RNA and DNA, viruses contain either RNA or DNA. Prions contain neither, since they are misfolded proteins.


As discussed already, bacteria can be divided into eubacteria and archaebacteria.


Most bacteria have shapes that can be defined either as a rod, sphere or corkscrew.


These are the typical bacteria that have features expected of bacteria. Nearly all bacteria, with the exception of the bacteria, Mycoplasma, discussed above, have cell walls, that contain a substance known as peptidoglycan. Furthermore, this cell wall determines a further classification of eubacteria, whether the bacteria is gram-positive, or gram-negative. 

Briefly, a gram-positive bacteria contains a very thick, outer peptidoglycan layer and appears violet-purple, while a gram-negative bacteria contains a very thin, inner peptidoglycan layer and appears pink/red. This will be discussed in more detail when we discuss the Structure of a Bacterium.


Outside the Eubacteria, a flagella, pili and capsule may be present. The flagella is the tail-like structure of the bacteria that allows motility. It may or may not be present.
Pili are the hair-like projections on the bacteria that allow bacterial conjugation, a method of genetic exchange within bacteria.
An outer capsule covers the cell wall and serves as the outermost protection of the bacterial cell.



Atypical bacteria include Mycoplasma, Chlamydia and Rickettsia. We discussed the irregular features of these three above.

Mycoplasma contains no cell wall, instead a sterol-imbued cell membrane.

Chlamydia and Rickettsia require a host in order to carry out protein synthesis and obtain cellular machinery for protein synthesis.


Fungi, although often confused for plants, are not plants. They are not capable of photosynthesis, and are instead generally saprophytic (feed on dead and decaying matter). Some fungi are filamentous and are called molds, while other fungi are unicellular, such as yeasts.

Fungal reproduction may be sexual, asexual, or even both, but the certain factor is that all fungi reproduce by spores. 

This topic will be discussed further under “Microbiology of Fungi”.


Protozoa are nonphotosynthetic, mostly motile, unicellular organisms, and are among the most widespread microorganisms. Members of this group can infect almost every tissue within the body.

They can function either as intracellular parasites, or extracellular parasites in the blood, urogenital region, or intestine. Transmission is generally by ingestion of infected food, or by an insect bite.

Helminths and Rotifers

Helminths are a group of multicellular, complex worms that live as parasites. They receive their nutrition by ingesting body fluids or tissues, or feeding on digested matter in the gastrointestinal system. They can be divided into 3 types:

  1. Cestodes (eg. Tapeworms)
  2. Trematodes (eg. Flukes)
  3. Nematodes (eg. Roundworms)

Rotifers are a unique species. They are a group of freshwater dwellers who are very commonly found in the thin films of water within moist soil. They are multicellular and their cavities are lined by mesoderm, which forms the stomach and an advanced digestive system of the rotifer. Because of this, they are also classified as animals, despite being microscopic.

Here are how Rotifers appear above and below:

Here is a brief summary of the characteristics of all the types of microbes:

And here’s a visual depiction of the size of each of the microbe types:

That’s all guys! This would be an introduction to Microbiology, and I will go into detail into each and every one of the fields, and then into individual bacteria and their pathogenesis over time. For now, I hope you enjoyed it! As usual, some questions for you to try out:


1) Which one of the following contains DNA not surrounded by a nuclear membrane?

A. Bacteria

B. Molds

C. Protozoa

D. Yeasts

2) Which one of the following contains either DNA or RNA, but not both?

A. E. Coli

B. Tapeworms

C. Herpes Virus

D. Yeast

3) Which of the following is a difference between a virus and a bacteria?

A. Viruses do not have mitochondria but bacteria do.

B. Viruses do not have a nucleolus while Bacteria do.

C. Viruses do not have ribosomes but bacteria do.

D. Viruses are prokaryotic while Bacteria are Eukaryotic.


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