Bacteria vs Amoeba

Team Biology at
Created by: Team Biology at, Last Updated: May 24, 2024

Bacteria vs Amoeba

Bacteria and amoebas are two distinct types of single-celled organisms with significant roles in various ecosystems. Bacteria are prokaryotic, lacking a defined nucleus, and known for their diversity and adaptability. Amoebas are eukaryotic, with complex structures and a well-defined nucleus. This article explores their fundamental differences, highlighting unique characteristics, structures, and functions to better understand microbial life.

What Are Bacteria?

Bacteria are single-celled microorganisms that lack a nucleus and other membrane-bound organelles, making them prokaryotes. They are incredibly diverse and can be found in nearly every environment on Earth, including soil, water, acidic hot springs, radioactive waste, and even in and on the human body.

Structure of Bacteria

Bacteria have a simple cell structure, which includes:

  • Cell Wall: Provides shape and protection. It is made of peptidoglycan in most bacteria.
  • Plasma Membrane: A phospholipid bilayer that controls the movement of substances in and out of the cell.
  • Cytoplasm: A gel-like substance inside the cell where cellular processes occur.
  • Ribosomes: The site of protein synthesis.
  • Nucleoid: A region containing the bacterial DNA, which is typically a single circular chromosome.
  • Plasmids: Small, circular DNA molecules that are separate from the chromosomal DNA and can carry additional genes.
  • Flagella: Tail-like structures used for movement.
  • Pili: Hair-like structures that help in attachment to surfaces and in DNA transfer between bacteria.

Types of Bacteria

Bacteria can be classified based on several criteria:

  • Shape:
    • Cocci: Spherical bacteria (e.g., Staphylococcus aureus).
    • Bacilli: Rod-shaped bacteria (e.g., Escherichia coli).
    • Spirilla: Spiral-shaped bacteria (e.g., Spirillum volutans).
  • Gram Staining:
    • Gram-Positive: Bacteria with thick peptidoglycan cell walls that retain the crystal violet stain (e.g., Bacillus subtilis).
    • Gram-Negative: Bacteria with thin cell walls and an outer membrane, do not retain the crystal violet stain but take up the counterstain (e.g., Escherichia coli).
  • Oxygen Requirement:
    • Aerobic: Require oxygen for growth (e.g., Mycobacterium tuberculosis).
    • Anaerobic: Do not require oxygen and may even be killed by it (e.g., Clostridium botulinum).
    • Facultative Anaerobes: Can grow with or without oxygen (e.g., Escherichia coli).

Roles and Importance of Bacteria

Bacteria play crucial roles in various ecological and industrial processes:

  • Decomposition: Bacteria break down dead organic matter, recycling nutrients in ecosystems.
  • Nitrogen Fixation: Some bacteria convert atmospheric nitrogen into forms usable by plants (e.g., Rhizobium).
  • Digestion: Human gut bacteria aid in digestion and synthesize vitamins.
  • Disease: Some bacteria cause diseases in humans, animals, and plants (e.g., Streptococcus pneumoniae causes pneumonia).
  • Biotechnology: Bacteria are used in the production of antibiotics, enzymes, and other biochemicals. They are also employed in genetic engineering and bioremediation.

Examples of Bacteria

  • Beneficial Bacteria:
    • Lactobacillus: Used in yogurt production and as probiotics.
    • Nitrosomonas: Involved in nitrogen cycling in soil.
  • Pathogenic Bacteria:
    • Salmonella typhi: Causes typhoid fever.
    • Mycobacterium leprae: Causes leprosy.

What is an Amoeba?

An amoeba is a type of single-celled organism belonging to the kingdom Protista. Amoebas are characterized by their ability to change shape due to the flexible cell membrane and the formation of pseudopodia (temporary projections of cytoplasm). They are eukaryotic microorganisms, meaning they have a nucleus and other membrane-bound organelles.

Structure of an Amoeba

Amoebas have a simple but highly adaptable structure, which includes:

  • Plasma Membrane: A flexible cell membrane that encloses the cell and allows it to change shape.
  • Cytoplasm: Divided into two parts:
    • Ectoplasm: The clear, gel-like outer layer.
    • Endoplasm: The granular, fluid inner layer containing organelles.
  • Nucleus: A membrane-bound structure that contains the cell’s genetic material.
  • Pseudopodia: Temporary, foot-like extensions of the cell used for movement and feeding.
  • Contractile Vacuole: A structure that expels excess water from the cell, maintaining osmotic balance.
  • Food Vacuoles: Membrane-bound structures where ingested food is digested.

Movement and Feeding

Amoebas move and feed using pseudopodia, which they extend and retract to creep along surfaces and engulf food particles. This process, known as phagocytosis, involves the amoeba surrounding its prey (such as bacteria or smaller protists) with its pseudopodia, enclosing it within a food vacuole where digestion takes place.


Amoebas are found in a variety of habitats, including:

  • Freshwater environments (ponds, lakes, streams)
  • Marine environments (oceans, seas)
  • Soil
  • Inside other organisms (as parasites)

Types of Amoebas

There are many species of amoebas, but they can be broadly classified into two main groups:

  • Free-Living Amoebas: These amoebas live independently in various environments. Examples include:
    • Amoeba proteus: A well-known species often studied in laboratories.
    • Dictyostelium discoideum: A social amoeba that can aggregate to form multicellular structures.
  • Parasitic Amoebas: These amoebas live inside host organisms and can cause diseases. Examples include:
    • Entamoeba histolytica: Causes amoebic dysentery in humans.
    • Naegleria fowleri: Known as the “brain-eating amoeba,” it can cause a rare but often fatal brain infection.

Roles and Importance of Amoebas

Amoebas play several significant roles in the environment and in scientific research:

  • Ecological Role: Amoebas contribute to nutrient cycling by feeding on bacteria, algae, and other microorganisms.
  • Research: Amoebas, particularly Amoeba proteus, are used as model organisms in cell biology to study cell motility, cytoplasmic streaming, and other cellular processes.
  • Disease: Parasitic amoebas are important in medical research due to their role in human diseases, leading to advancements in understanding and treating these infections.

Examples of Amoebas

  • Beneficial Amoebas:
    • Amoeba proteus: Widely used in educational and research settings.
    • Dictyostelium discoideum: Studied for its unique life cycle and social behavior.
  • Pathogenic Amoebas:
    • Entamoeba histolytica: Causes amoebic dysentery, a severe form of diarrhea.
    • Naegleria fowleri: Causes primary amoebic meningoencephalitis (PAM), a rare but often fatal brain infection.

Differences between Bacteria and Amoeba

KingdomMonera (Prokaryota)Protista
DomainBacteria or ArchaeaEukarya
Cell TypeProkaryotic (no nucleus)Eukaryotic (with nucleus)
Cell StructureSimple, lack membrane-bound organellesComplex, with membrane-bound organelles
SizeGenerally 0.2 – 2.0 micrometersGenerally 10 – 600 micrometers
ShapeVarious shapes: cocci (spherical), bacilli (rod-shaped), spirilla (spiral)Variable, often irregular with pseudopodia
Mode of NutritionAutotrophic (photosynthesis, chemosynthesis) or heterotrophic (absorption)Heterotrophic (phagocytosis, engulfing particles)
ReproductionAsexual (binary fission)Asexual (binary fission) and sexual (rarely)
MovementSome have flagella or pili for movementMoves using pseudopodia (false feet)
Genetic MaterialSingle, circular DNA moleculeLinear DNA organized into chromosomes
Cell WallPresent in most, made of peptidoglycanAbsent
Ribosomes70S ribosomes80S ribosomes
Metabolic DiversityHigh; can be aerobic, anaerobic, or facultative anaerobesLess diverse; mainly aerobic with some facultative anaerobes
HabitatUbiquitous: soil, water, extreme environmentsAquatic environments, moist soil, host organisms
PathogenicitySome species are pathogenic (cause diseases)Some species can cause diseases (e.g., amoebic dysentery)
ExamplesEscherichia coli, StreptococcusAmoeba proteus, Entamoeba histolytica
SporesSome form endospores for survivalDo not form spores
PlasmidsOften contain plasmids (extra-chromosomal DNA)Rarely contain plasmids
OsmoregulationNo specialized structures for osmoregulationContractile vacuoles to expel excess water
Sensitivity to AntibioticsGenerally sensitive to antibioticsNot affected by antibiotics targeting prokaryotes
SymbiosisCan form symbiotic relationships (mutualistic, commensal, parasitic)Often free-living, can be parasitic
FlagellaPresent in some, simple structureAbsent
MitochondriaAbsentPresent, used for energy production
ChloroplastsAbsentSome amoebae have symbiotic algae

Similarities Between Bacteria and Amoeba

Cell TypeProkaryoticEukaryotic
Number of CellsUnicellularUnicellular
Reproduction MethodAsexual (Binary Fission)Asexual (Binary and Multiple Fission)
MotilityFlagella, CiliaPseudopodia
Nutrient AcquisitionAbsorption, DecompositionPhagocytosis
HabitatsDiverse (Soil, Water, Host)Diverse (Soil, Water, Host)
Role in EcosystemDecomposers, Nitrogen FixationControl Bacterial Populations
Presence in Human BodyNormal Flora, PathogensPathogens

What are the main differences between bacteria and amoeba?

Bacteria are single-celled prokaryotes without a nucleus, while amoebas are single-celled eukaryotes with a nucleus.

How do bacteria reproduce?

Bacteria reproduce asexually through binary fission, dividing into two identical cells.

How do amoebas move?

Amoebas move using pseudopodia, temporary projections of their cell membrane and cytoplasm.

What is the size difference between bacteria and amoeba?

Bacteria are typically 0.2-2.0 micrometers in size, while amoebas can be up to 1 millimeter.

Can bacteria and amoeba cause diseases?

Yes, both can cause diseases. Bacteria cause infections like strep throat, while amoebas can cause amoebic dysentery.

How do bacteria obtain nutrients?

Bacteria absorb nutrients from their environment through their cell walls.

How do amoebas obtain nutrients?

Amoebas engulf food particles through phagocytosis, forming food vacuoles.

What environments do bacteria thrive in?

Bacteria thrive in diverse environments, including soil, water, and extreme conditions like hot springs.

Where are amoebas commonly found?

Amoebas are commonly found in freshwater environments, soil, and as parasites in host organisms.

Do bacteria and amoebas have cell walls?

Bacteria have rigid cell walls made of peptidoglycan. Amoebas lack cell walls but have flexible cell membranes.

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