A bacterium is a living prokaryotic cell, whereas a virus is an acellular infectious agent that can replicate only inside a host cell. Bacteria have a plasma membrane, cytoplasm, ribosomes and DNA, allowing them to perform metabolism and reproduce by binary fission. Viruses consist principally of genetic material enclosed in a protein capsid, sometimes with a lipid envelope, and lack the cellular machinery required for independent reproduction.
This distinction explains why bacteria can grow on suitable nutrient media while viruses require living cells, why antibiotics can treat certain bacterial infections but not viral infections, and why describing a virus as a very small bacterium is biologically incorrect. For broader connections to cells, membranes and microscopy, use the topic-wide IB Biology Cell Biology Explained: Exam-Focused Guide.
Virus vs bacteria at a glance
The singular term is bacterium and the plural is bacteria. A virus particle outside a host cell is called a virion.
| Feature | Bacterium | Virus |
|---|---|---|
| Biological organization | Living, unicellular prokaryote | Acellular infectious agent |
| Typical structure | Plasma membrane, cytoplasm, 70S ribosomes, DNA and usually a cell wall | Nucleic acid enclosed in a protein capsid; some have a lipid envelope |
| Genetic material | DNA genome, with RNA produced during gene expression; plasmids may also occur | A genome made of either DNA or RNA, which may be single- or double-stranded |
| Cytoplasm | Present | Absent |
| Ribosomes | Present | Absent |
| Metabolism | Carries out metabolic reactions using its own enzymes and cellular structures | No independent metabolism or ATP production |
| Reproduction or replication | Usually reproduces asexually by binary fission | Viral components are produced and assembled inside a host cell |
| Growth | Cell grows before dividing | A virion does not grow before producing new virions |
| Dependence on host cells | Can generally live and reproduce independently in a suitable environment, although some bacteria are obligate intracellular parasites | Always requires a host cell for replication |
| Treatment | Some bacterial infections can be treated with appropriate antibiotics | Antibiotics do not work; some viral infections have specific antiviral treatments |
| Examples | Escherichia coli, Staphylococcus aureus, Bacillus species | Influenza virus, HIV, coronaviruses, bacteriophage lambda |
Size is a useful supporting distinction, but it should not be your main definition. Most viruses are smaller than most bacteria, yet the size ranges overlap because unusually large viruses and unusually small bacteria exist. Structure and reproductive dependence provide more reliable comparisons.
Why a bacterium is a cell but a virus is not
A bacterium is a prokaryotic cell. It has no nucleus or membrane-bound organelles, but it possesses the fundamental components needed for cellular life: a plasma membrane, cytoplasm, genetic material and ribosomes. These features allow the cell to regulate exchanges, express genes, synthesize proteins and carry out metabolic reactions.
In the current IB Biology course, bacterial organization is studied through A2.2 Cell structure, which applies to both SL and HL. The required model emphasizes a cell wall, plasma membrane, cytoplasm, naked DNA in a loop and 70S ribosomes, while recognizing that bacterial structures vary. RevisionDojo’s A2.2 Cell Structure notes and Cell Structure topic hub place these structures in their wider syllabus context.
Many bacteria possess additional structures:
- Plasmids, which are small DNA molecules separate from the main chromosome
- A capsule or slime layer outside the cell wall
- Flagella used for movement
- Pili, which can support attachment or DNA transfer
These are not universal, so an exam answer should not imply that every bacterium has them. Similarly, most bacterial cell walls contain peptidoglycan, but some bacteria lack a conventional cell wall.
A virus is not a cell because it has no cytoplasm, ribosomes or complete metabolic system. Its essential structure is a genome enclosed by a protein capsid. Some viruses also possess an outer lipid envelope, usually derived from host-cell membrane during viral release, with viral proteins embedded in it.
The current IB syllabus places A2.3 Viruses at HL only. The official IB Biology roadmap identifies A2.3 within Theme A, Unity and diversity, and the official subject brief marks viruses as content taught only to HL students. SL students may still encounter viruses in a broader biological or data-based context, but the dedicated A2.3 syllabus content is additional higher level material.
Structural differences in more detail
Genetic material
Bacteria use double-stranded DNA as their cellular genome, usually arranged as a circular chromosome in a nucleoid region rather than inside a nucleus. They also contain RNA because transcription and translation occur within the cell. A bacterium therefore contains both DNA and RNA, although DNA stores its main hereditary information.
A virus has a genome consisting of either DNA or RNA. Depending on the virus, that genome may be single-stranded or double-stranded, linear or circular, and divided into one or several segments. Saying that every virus contains RNA, or that every virus contains DNA, is therefore incorrect.
Protein synthesis
Bacterial 70S ribosomes translate messenger RNA into polypeptides. A bacterium can consequently synthesize its own enzymes and structural proteins, provided it has suitable nutrients and environmental conditions.
Viruses have no ribosomes. Viral genes can direct protein production only after the virus enters a suitable host cell and uses the host’s translation machinery. This is one reason viruses are described as obligate intracellular parasites.
Cell boundaries
A bacterial plasma membrane is a selectively permeable boundary surrounding living cytoplasm. In addition to controlling transport, it supports important metabolic processes. A bacterial cell wall outside the membrane provides support and helps prevent osmotic lysis.
A viral envelope is not equivalent to a bacterial plasma membrane. It surrounds some virions but does not enclose metabolically active cytoplasm. Many viruses are non-enveloped and consist of a genome and capsid without an external lipid membrane.
How bacterial reproduction differs from viral replication
The sharpest difference between virus and bacterium is how each produces new individuals or particles. A bacterium reproduces through division of an existing cell, whereas a virus redirects a host cell to manufacture and assemble viral components.
Bacterial binary fission
Most bacteria reproduce asexually by binary fission:
- The bacterial chromosome is replicated.
- The cell grows and the DNA copies move apart.
- The plasma membrane and cell wall grow inward.
- A septum forms between the DNA copies.
- The parent cell divides into two daughter cells.
The daughter cells are usually genetically very similar, but they are not guaranteed to remain identical. Mutations can arise, and bacteria can acquire genetic material through processes such as conjugation, transformation and transduction. These processes increase variation, but they are not the same as reproduction by binary fission.
Viral replication
A virus does not divide into two. The general replication sequence is:
- Attachment to specific receptors on a host cell
- Entry or penetration of the virion or its genetic material
- Uncoating, where applicable, to release the viral genome
- Genome replication and protein synthesis using host-cell resources
- Assembly of new viral components into virions
- Release by cell lysis, budding or another mechanism
New virions are therefore assembled from newly produced parts rather than formed by enlargement and division of a parent virion. The specificity of viral attachment proteins and host receptors also explains why a virus may infect only particular species, tissues or cell types.
For bacteriophages, IB HL students distinguish the lytic cycle from the lysogenic cycle. In the lytic cycle, viral genes direct rapid production of virions and the host cell is lysed. In the lysogenic cycle, viral DNA becomes integrated into the host genome or is otherwise maintained with it, allowing the viral genetic material to be copied when the bacterium divides before a later switch to lytic replication.
Metabolism and the question of whether viruses are alive
Bacteria meet standard cellular characteristics of life. They use energy, maintain an internal environment, synthesize biological molecules, respond to environmental conditions, grow and reproduce. Even a dormant bacterial cell remains a cellular entity with the potential to resume metabolism under appropriate conditions.
Viruses occupy a more difficult conceptual position. They contain genetic information, mutate, evolve by natural selection and produce descendants inside hosts, but they lack cellular organization, independent metabolism and autonomous reproduction. For school and IB Biology purposes, it is safest to describe viruses as acellular and generally not classified as living organisms, rather than claiming that they are simply dead.
The distinction matters because biological definitions should be based on evidence. A virus is inactive outside a host, but its genome can participate in complex replication and evolutionary processes once it enters a compatible cell. The disagreement is therefore partly about which characteristics should be required in a definition of life.
Why antibiotics affect bacteria but not viruses
Antibiotics act on susceptible bacterial structures or processes. Different drugs may inhibit peptidoglycan cell-wall synthesis, interfere with bacterial ribosomes, disrupt particular metabolic pathways or affect bacterial nucleic-acid processes. Viruses do not have peptidoglycan walls, bacterial ribosomes or independent metabolism, so these targets are absent.
The CDC states clearly that antibiotics do not work against viruses. They may be appropriate for certain bacterial infections, but they are not automatically needed for every illness caused by bacteria. Unnecessary or incorrect antibiotic use also contributes to selection for antibiotic-resistant bacteria.
Antiviral drugs work differently. Because viruses rely heavily on host cells, antiviral treatments usually target a virus-specific stage, protein or enzyme, such as viral entry, genome replication or release. An antiviral effective against one virus may have no effect on another, so “antiviral” does not describe one universal treatment.
Vaccines are also distinct from antibiotics and antivirals. A vaccine prepares the immune system to recognize particular antigens and can prevent disease or reduce its severity. Vaccines exist for several viral and bacterial diseases, but they do not directly kill every pathogen already present in the body.
Similarities between viruses and bacteria
A strong comparison includes similarities as well as differences. Both viruses and bacteria:
- Contain genetic material and can undergo genetic change
- Can evolve through natural selection
- May be transmitted between hosts
- Can cause infectious disease, although many bacteria are harmless or beneficial
- Have surface molecules that interact with cells or immune-system components
- Can stimulate immune responses
- Can show variation that reduces the effectiveness of medicines or immunity
Do not write that all bacteria cause disease. Human-associated bacterial communities contribute to digestion, nutrient production, competition against pathogens and normal ecosystem processes. Viruses can also infect bacteria: these viruses are called bacteriophages.
How to answer a virus vs bacteria IB exam question
For a compare question, organize your response by paired features rather than writing one paragraph about viruses followed by a separate paragraph about bacteria. RevisionDojo’s IB Biology command terms guide explains that comparison requires both similarities and differences.
A precise short answer might state:
A bacterium is a prokaryotic cell with cytoplasm, a plasma membrane, 70S ribosomes and DNA, whereas a virus is acellular and consists of DNA or RNA inside a protein capsid, sometimes with an envelope. Bacteria carry out metabolism and usually reproduce by binary fission. Viruses lack ribosomes and independent metabolism, so viral components can be produced and assembled only inside host cells.
Common mistakes include:
- Calling a virus a prokaryote
- Saying bacteria have no DNA because they lack a nucleus
- Saying all viruses contain both DNA and RNA
- Claiming viruses reproduce by binary fission
- Describing a viral envelope as a cell membrane around cytoplasm
- Assuming all bacteria are harmful
- Stating that antibiotics kill viruses
- Using size as the only distinction
After learning the comparison, test it through the A2.2 Cell Structure Questionbank. The Cell Structure flashcards can reinforce terminology, while the wider IB Biology resource hub supports mixed-topic revision. Jojo AI can help identify whether a written comparison is missing paired points or uses inaccurate terms.
Conclusion
The fundamental difference between a virus and a bacterium is cellular independence. A bacterium is a living prokaryotic cell with cytoplasm, ribosomes, metabolism and the capacity to reproduce by binary fission. A virus is an acellular package of DNA or RNA that must use a host cell to replicate and assemble new virions.
For IB Biology, connect this distinction to cell structure, protein synthesis, reproduction, disease and evolution rather than memorizing an isolated list. RevisionDojo Study Notes and Flashcards can secure the terminology, followed by targeted Questionbank practice and Jojo AI feedback to improve exam comparisons.
Sources and referenced URLs
- Official IB Biology subject brief, first assessment 2025
- Official IB DP Biology guide roadmap
- Official IB Biology curriculum page
- NCBI Medical Microbiology: Structure and Classification of Viruses
- NCBI Medical Microbiology: Bacterial Structure
- OpenStax: Structure of Prokaryotes
- OpenStax Microbiology: How Microbes Grow
- CDC: Antibiotic use and viruses versus bacteria
- RevisionDojo: IB Biology Cell Biology Explained
- RevisionDojo: A2.2 Cell Structure topic hub
- RevisionDojo: A2.2 Cell Structure notes
- RevisionDojo: A2.2 Cell Structure Questionbank
- RevisionDojo: A2.2 Cell Structure flashcards
- RevisionDojo: IB Biology command terms
- RevisionDojo: IB Biology resources
