Difference Between Bacteria and Protozoa

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In this post, we will know about the key differences between Bacteria and Protozoa.

After reading this post, you can perfectly differentiate a bacteria from a Protozoa and vice-versa.

The main difference between Bacteria and Protozoans is that the Bacteria are unicellular prokaryotic organisms whereas, Protozoans are unicellular eukaryotic organisms. Bacteria are the most primitive first formed organisms to come on earth whereas, Protozoans are formed from Bacteria over the course of evolution.

Bacteria fall under the Kingdom Monera of the 5 Kingdom Classification system by R.H. Whittaker (1969). The Kingdom of Kingdom Monera is further subdivided into Archaea and Bacteria. This sub-division of Archaea and Bacteria is called Domains of Life.

The structure of the bacteria is very simple but they are very complex in their behavior. Compared to other organisms, bacteria show the most extensive metabolic diversity.

Protozoans fall under the Kingdom Protista of the 5 Kingdom Classification system by R.H. Whittaker (1969). Protozoans are further classified into Amoeboid, Flagellated, Ciliated, and Sporozoans.

Protozoans are the single-celled eukaryotes but the boundaries of this Sub-kingdom are still not well-defined. The protozoans body contains a well-defined nucleus and other membrane-bound organelles.


Flow chart of 5 Kingdom Classification system by R.H. Whittaker (1969). Here, the Kingdom Kingdom Monera is further sub-divided into Archaea and Bacteria. And, Kingdom Protista is divided into Sub-Kingdom Protozoa which is further sub-divided into Amoeboid, Flagellated, Ciliated, and Sporozoans.
Flow chart of 5 Kingdom Classification system by R.H. Whittaker (1969). Here, the Kingdom Monera is further sub-divided into Archaea and Bacteria. And, Kingdom Protista is divided into Sub-Kingdom Protozoa which is further sub-divided into Amoeboid, Flagellated, Ciliated, and Sporozoans.

Comparison Chart: Bacteria Vs. Protozoa

Cell Structure Prokaryotic & Unicellular & Microscopic Eukaryotic & Unicellular & Microscopic
Structure is the most simple amongst the prokaryotes with very very sub-cellular organelles like Single circular chromosome, ribosomes, plasmid, pilus, and fagellum. Structure is the most simple amongst the eukaryotes with subcellular organelles like mitochondria, Endoplasmic Reticulum, Chroloplast, Nucleus, flagellum, mitochondria, plasma membrane, etc.
Bacterias may or may not have cell wall Protozoans don’t have cell wall
Nutrition Bacterias are phototropic and some are heterotropic Majority protozoans are heterotropic. Only a few are photoautotrophs.
Bacterias need only a nutrition source on a surface of organic matter to allow it to colonize and live in groups either outside the living body or inside it. Protozoans can stay in our body or outside as a parasite occasionally. They can only cause diseases if they release any bacteria or viruses they carry.
Bacteria eat by absorbing nutrients from their cell wall with the help of pili and other mechanisms. Protozoans eat by enveloping their prey within small pockets called vacuoles and absorb the nutrients for the prey.
Origination Bacteria are the first organisms to come on earth Protozoans are evolved from bacteria
Reproduction & Life Cycle  Bacteria reproduce asexually by binary fission. They can also exchange genetic information with each other using a pilus which is called conjugation. Asexual reproduction is universal and sexual reproduction is rare
Bacterial growth cycles in a growth curve consist of four phases: lag, exponential (log), stationary, and death. Life cycle with more than one generation. It generally passes through several stages that differ in structure and activity.
Variation Form of individuals can vary even in the same species. Form of bodies is definite for all the members of a species.
Shapes They are classified into 5 main groups. These are Spherical (cocci), Rod (bacilli), Spiral (spirilla), Comma (vibrios), or Corkscrew Bacteria (spirochaetes) Protozoans are classified into 4 distinct shapes (Amoeboid, Flagellated, Ciliated, Sporozoans)
Motility Moves by use of Pili and Flagella Moves using pseudopodia, flagella, and cilia.
Derive energy from Bacteria obtain energy from sugars, proteins, and fats They are the photoautotrophs (derive energy by photosynthesis), photoheterotrophs (both photosynthetic and heterotrophic), and mostly heterotrophic.
Disease caused by them Tuberculosis, rabies, leprosy, tetanus, diphtheria, strep throat, leprosy, pertussis, cholera, etc Malaria, Leishmaniasis, African trypanosomiasis, Cryptosporidiosis, Chagas disease, Sleeping sickness, Giardiasis, etc
Examples Cyanobacteria, Streptococcus pneumoniae, Burkholderia cepacia, Mycobacterium tuberculosis, etc Entamoeba histolytica, Giardia lamblia, Trichomonas vaginalis, Acanthamoeba, etc

Definition of Bacteria

Bacteria are very simple, single-celled prokaryotes that can survive in various types of environments ranging from oceans to thermal vents to oceans and even inside the human guts as well.

They are the simplest living things known to date that are capable of independent growth and reproduction. In very simple words, they are the most primitive and simples living prokaryotes known.

The cell structure is simpler than that of other organisms as there is no nucleus or membrane-bound organelles. They have a single circular DNA loop that is the one and only genetic haploid chromosome.

Although with some exceptions, present-day scientists have discovered that most bacteria have one or two circular chromosomes as well.

Some bacteria have an extra circle of genetic material called a plasmid. Genomic DNA is the main central circular DNA of bacteria that is part of an organism’s chromosomes. A Plasmid is a small loop of DNA that is separate from the genomic DNA.

Bacteria are classified into five groups on the basis of their shapes. These are spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios), or corkscrew (spirochaetes). They can exist as single cells, in pairs, chains, or clusters.

Some of the bacteria are autotrophic, i.e. they can produce their own food using photosynthesis. They may also be photosynthetic autotrophic or chemosynthetic autotrophic. The vast majority of them are autotrophs as well.

Some of the bacterias can live in the harshest habitats like salty areas, hot springs, marshy areas, soil, rock, oceans, and even arctic snow. Some live in or on other organisms including plants and animals including humans.

They are characterized by the presence of a rigid type of cell wall, and if motile they do have a flagellum. They are often seen to form bloom in polluted areas as well.

Some of them have chlorophyll so can be even photosynthetic in acquiring nutrition. Some are chemosynthetic autotrophic in nature, so oxidizes various inorganic substances to produce ATP energy.

For Example:

  • Streptococcus pneumoniae (S. pneumoniae), which causes pneumonia in humans
  • Escherichia coli (E. coli) is a bacterium that is commonly found in the gut of humans and warm-blooded animals
  • Lactobacillus which grow on milk causes the formation of milk to curd

Definition of Protozoa

Protozoans are a bit complex than the bacterias and are composed of unicellular, eukaryotic, microscopic animals. They do not have any type of tissue.

Protozoans are of many different shapes and sizes. These include the unevenly (fluid) shaped ones like the Amoeba which can change their shape, to those of the Paramecium having fixed shape and complex structure.

With only one cell (can also be referred to as acellular with some exceptions), protozoans’ body is a complete animal performing all the vital functions of life such as nutrition, excretion, respiration, reproduction, etc.

These are the first and the most primitive animals included in the 5 Kingdom Classification by R.H. Wittaker under the Kingdom Protista. More than 31,000 species that live solitary or colonial are included in this kingdom.

The cell organelles are well specialized with membrane-bound organelles like mitochondria, Golgi bodies, chloroplast, Endoplasmic Reticulum, Nucleus, lysozyme. There can be single or many nuclei in each organism depending on the species type.

Nutrition is holozoic, holophytic, and saprozoic and they can be free-living, commensal, symbiotic, or parasitic in nature. They can be seen in Freshwater, Marine, or Moist Terrestrial habitat.

They are motile and can move by:

1. Cilia: Tiny hair-like structures that cover the outside of the microbe. Cilia help in movement because of the interactions of a set of microtubules inside the cilia parts.

2. Flagella: Long thread-like structures that extend from the cell surface. It’s like a propeller that moves the protozoa’s body.

3. Amoeboid Movement: The organism moves by sending out pseudopodia or false feet. It’s like a temporary protrusion of the surface of an amoeboid cell for movement and feeding.

Protozoans are able to reproduce asexually using the reproduction methods of Binary Fission, Plasmotomy, Budding, Multiple Fission, and Plasmogamy.

Some protozoans can even reproduce sexually using the reproduction methods of Syngamy, Anisogamy, and Congujagation.

While some are even, parthenogenetic in nature. Meaning that reproduction occurs without fertilization, especially as a normal process.

For Example:

  • Entamoeba histolytica causes Amoebic dysentery (amoebiasis)
  • Giardia intestinalis causes Diarrhoea (loose bowels) in humans
  • Trichomonas vaginalis causes vagina and urethral infection in humans

Key Differences Between Bacteria and Protozoa

The following points are significant to understand the difference between Bacteria and Protozoa:

1. Bacteria are Prokaryotes whereas, Protozoans are Eukaryotes.

2. Both protozoa and bacteria are one-celled organisms. But the thing to be noted is that the protozoans are actually bigger than bacteria and the protozoans contain a nucleus along with various other cell structures. Protozoans are also considered to have both plant and animal cells.

3. Bacteria find their application both in Zoology and Botany whereas, on the other hand, Protozoans mostly find their application in Zoology and are highly regarded as the smallest living animals on the earth.

4. Bacteria don’t have a nucleus or nuclear membrane itself, and so, the single circular chromosomes can stay free in the center of the prokaryotic cell. Protozoans do have a nucleus with a well-defined nuclear membrane that boundaries the genetic chromosomal materials indie the cell.

5. Bacteria are extremely smaller than Protozoans. The protozoans act like a vector that transmits the bacteria from one location to another. The bacteria are so small that they can enter the body of the protozoa and complete their life cycle there or, can even get transmitted to another host with the help of the protozoa.


So, it can be seen that bacteria are the most primitive and simple living organisms that are present everywhere. Protozoans are also the same and they are present everywhere but, not at the harshest habitats where the bacteria can be present.

We all know that living organisms are evolved from inorganic non-living substances. So, present-day living organisms can be divided into prokaryotes and eukaryotes.

This can be well understood with the case of Bacteria and Protozoans. The Bacteria are the simplest living prokaryotes on earth, which over the course of evolution have mutated themselves to form the Protozoans which are the simplest living eukaryotes on earth.

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