How to Know the Difference Between Bacteria and Viruses

Medically reviewed by Mandolin S. Ziadie, MD

Last Updated: June 2, 2022 Approved

Studying for a biology exam? Stuck in bed with the flu and curious to find out what sort of microorganisms have made you so sick? While bacteria and viruses can both make you sick in similar ways, they are actually very different organisms with a wide range of differing qualities. Learning these differences can help you stay informed about any medical treatments you're undergoing and give you a better understanding of the complex biology that's going on inside you all the time. You can learn how to tell the difference between bacteria and viruses not only by learning the basics about them but also by examining them through a microscope and discovering more about their makeup and functions.

Part 1

Part 1 of 2:

Learning the Differences

1
Learn the basic differences. There are key differences between bacteria and viruses in size, origins, and effects on the body.^{[1]
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- Viruses are the smallest and simplest life form; they are 10 to 100 times smaller than bacteria.
- Bacteria are single-celled organisms. Because bacteria have all the cellular machinery need to replicate themselves, they can survive without a cellular host.^{[2]
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  Research source} Viruses, on the other hand, are only intracellular organisms, meaning that they infiltrate the host cell and live inside the cell. Viruses change the host cell's genetic material from its normal function to producing the virus itself.
- Antibiotics cannot kill viruses, but can kill most bacteria, with the exception of bacteria that have become resistant to the antibiotic.^{[3]
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  Research source} Misuse and overuse of antibiotics have led to antibiotic resistance. Antibiotics are becoming less effective against potentially harmful bacteria. ^{[4]
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  US Food and Drug Administration
  U.S. government agency responsible for promoting public health
  
  Go to source} Gram-negative bacteria are highly resistant to treatment with antibiotics, but can be killed by some.^{[5]
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  Centers for Disease Control and Prevention
  Main public health institute for the US, run by the Dept. of Health and Human Services
  
  Go to source}
2
Recognize the differences in reproduction. Viruses need a living host cell in order to multiply themselves, such as a plant or animal. Meanwhile, most bacteria can grow on non-living surfaces.^{[6]
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- Bacteria have all the "machinery" (enzymes) needed for their growth and multiplication and reproduce asexually via a process called "binary fission."
- By contrast, viruses generally carry information - for example, DNA or RNA, packaged in a protein and/or membranous coat. They need another cell's machinery to reproduce. The virus attach onto the surface of the host cell and then the genetic material contained inside the virus is injected into the cell. Put differently, viruses are not alive, but are essentially information (DNA or RNA) that float around until they encounter a sufficient host.
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3
Determine whether the organism has a beneficial effect in the body. Though it may seem hard to believe, many, many tiny organisms live within (but are distinct from) our bodies. In fact, the ratio of bacterial cells to human cells in most people is between 3:1 and 1:1.^{[7]
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PubMed Central
Journal archive from the U.S. National Institutes of Health

Go to source} Many bacteria exist peacefully with our bodies; some even perform very important tasks, like making vitamins, breaking down waste, and making oxygen.^{[8]
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- For example, much of the process of digestion is done by types of bacteria called "gut flora." These bacteria also help maintain pH balance in the body.^{[9]
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- While people are familiar with "good bacteria" (such as gut flora), there are also "good" viruses, such as bacteriophages, that "hijack" the bacteria's cellular mechanisms and cause cell death.^{[10]
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  Research source} Researchers from Yale have designed a virus that may help defeat brain tumors. Most viruses, however, have not been proven to perform any functions that are beneficial to humans. They typically only cause harm.
4
Determine whether the organism meets the criteria for life. Though there is no precise, formal definition of what constitutes life, scientists agree that bacteria are undoubtedly alive. On the other hand, viruses are a bit more like zombies: they're not dead, but they're definitely non-living. For example, viruses have some characteristics of life, like having genetic material, evolving over time through natural selection, and being able to reproduce by creating multiple copies of themselves. However, viruses don't have a cellular structure or their own metabolism; they need a host cell to reproduce. In other respects, viruses are basically non-living. Consider the following:
- When they have not invaded another organism's cell, viruses are essentially dormant in every way. No biological processes occur within them. They can’t metabolize nutrients, produce or excrete wastes, or move around on their own. In other words, they are very similar to inanimate material. They can stay in this "non-living" state for long periods of time.^{[11]
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- When the virus comes in contact with a cell that it can invade, it latches on and an enzyme dissolves part of the cell membrane so that it can inject its genetic material into the cell. At this point, as it hijacks the cell to make copies of itself, it starts to show one important characteristic of life: the ability to move its genetic material into future generations, producing more organisms that are like itself.^{[12]
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5
Identify the bacterial and viral causes of common illnesses. If you are suffering from a disease and you know what it is, figuring out whether you are being affected by bacteria or a virus can be as simple as looking up information about your illness. Common illnesses caused by bacteria and viruses include:
- Bacteria: pneumonia, food poisoning (commonly caused by E. coli), bacterial meningitis, strep throat, ear infections, wound infections, gonorrhea.^{[13]
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- Viruses: influenza, chickenpox, the common cold, hepatitis B, rubella, SARS, measles, Ebola, HPV, herpes, rabies, HIV (the virus that causes AIDS).
- Note that some illnesses, like diarrhea and "head colds," can be caused by either type of organism.
- If you don't know what your illness is exactly, it is harder to tell the difference between bacteria and a virus because the symptoms for each can be difficult to distinguish. Both bacteria and viruses can cause nausea, vomiting, elevated temperatures, fatigue, and general malaise. The best (and sometimes only) way to determine whether you have a bacterial or viral infection is to see your doctor. Your doctor will perform lab tests to determine what kind of infection you have.
- One way to confirm whether you have a virus or bacteria is to assess whether your current antibiotic treatments are effective. Antibiotics such as penicillin will only help if you have a bacterial infection, as opposed to a viral infection. This is why you should not take antibiotics unless your doctor has prescribed them.
- Most viral infections and diseases, including the common cold, do not have cures, but there are anti-viral drugs that can often help manage or limit the symptoms and severity.
6
Use this simple chart to learn the basic differences between bacteria and viruses. ^{[14]
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- Though there are more differences than the ones listed here, these are among the most important.
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*Biological Differences Between Bacteria and Viruses*
Organism	Size	Structure	Reproductive Method	Treatments	Alive?
Bacteria	Larger (about 1000 nanometers)	One cell: peptidoglycan/polysaccharide cell wall; cell membrane; ribosomes; DNA/RNA floating freely	Asexual. Duplicates DNA and reproduces by fission (splitting apart).	Antibiotics; antibacterial cleaners for external sterilizing	Yes
Viruses	Smaller (20-400 nanometers)	No cells: simple protein structure; no cell wall or membrane; no ribosomes, DNA/RNA enclosed in protein coat	Hijacks a host cell, forcing it to make copies of viral DNA/RNA; new viruses released from host cell.	No known cures. Vaccines can prevent illness; symptoms may be treatable.	Unknown; do not meet all traditional standards for life.^{[15] X Research source}

Part 2

Part 2 of 2:

Analyzing Microscopic Features

1
Look for the presence of a cell. In terms of structure, bacteria are more complex than viruses. Bacteria are what is known as unicellular. This means that each bacteria is made up of only one cell. By contrast, the human body contains many trillions of cells.^{[16]
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- Viruses, on the other hand, do not have any cells. Viruses are made up of a protein structure called a capsid.^{[17]
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  Research source} Though this capsid contains the virus's genetic material, it lacks the features of a true cell, such as transport proteins, cytoplasm, organelles, and so on.^{[18]
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  Le, T. (2012). Microbiology. In First Aid for the USMLE Step 1 2012. New York: McGraw-Hill Medical.}
- In other words, if you see a cell through the microscope, you know you're looking at bacteria and not a virus.
2
Check the organism's size. One of the quickest ways to tell the difference between a bacterium and a virus is to determine whether you can see it with a regular microscope. If you can see it, it is not a virus. The average virus is about 10 to 100 times smaller than run-of-the-mill bacteria. They are so small that you cannot see a virus under a normal light microscope, only its effects on cells. You need an electron or other extremely high-powered microscope to see viruses.^{[19]
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- Bacteria are nearly always far bigger than viruses. In fact, the very biggest viruses are only just as big as the very smallest bacteria.^{[20]
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- Bacteria tend to have dimensions of one to several micrometers (1000+ nanometers).^{[21]
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  Research source} By contrast, most viruses have sizes of less than 200 nanometers, which means you won't be able to see them with most everyday microscopes.
3
Monitor the organism's reproductive cycle. Bacteria and viruses are very different from the organisms you can see in your daily life. They don't need to have sex or exchange genetic information with other organisms of the same species to reproduce. However, this is not to say that bacteria and viruses have the same reproductive strategies.
- Bacteria practice asexual reproduction. To reproduce, a bacteria replicates its own DNA, elongates, and splits into two daughter cells. Each daughter cell gets one copy of the DNA, making them clones (exact copies). You can usually watch this process occur under a microscope.^{[22]
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  Research source} Each daughter cell will grow and eventually divide into two more cells. Depending on the species of bacteria and the external conditions, bacteria can multiply very rapidly this way.
- Viruses, in contrast, can't reproduce on their own. Instead, they invade other cells and use the host's internal machinery to make new viruses.^{[23]
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  Research source} Eventually, so many viruses are made that the invaded cell bursts open and dies, releasing the new viruses.
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Question

What is the structure that surrounds and protects a virus?

Mandolin S. Ziadie, MD
Board Certified Pathologist
Dr. Ziadie is a board certified Pathologist in South Florida. She specializes in renal, transplant, and pediatric Pathology and has over 12 years of experience. She earned her medical degree from the University of Miami School of Medicine in 2004 and completed her fellowship in Pediatric Pathology at Children’s Medical Center in 2010.

Mandolin S. Ziadie, MD

Board Certified Pathologist

Expert Answer

Some viruses have membrane coats or envelopes. These coatings are similar to a cell membrane. However, not all viruses have these coatings.

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Question

What kind of coating do bacteria have?

Mandolin S. Ziadie, MD
Board Certified Pathologist
Dr. Ziadie is a board certified Pathologist in South Florida. She specializes in renal, transplant, and pediatric Pathology and has over 12 years of experience. She earned her medical degree from the University of Miami School of Medicine in 2004 and completed her fellowship in Pediatric Pathology at Children’s Medical Center in 2010.

Mandolin S. Ziadie, MD

Board Certified Pathologist

Expert Answer

Bacteria have cell walls just like the cells of the human body and other animals. Then, there is also a capsule made of polysaccharides (complex carbohydrate structures) that surrounds the cell walls for protection. Some bacteria also have a third protective layer made of lipids (fats).

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