Fact Checked

What Are Harmful Bacteria?

Brendan McGuigan
Brendan McGuigan

Bacteria are a domain of prokaryotes, the earliest type of life on earth. These microorganisms can be traced back billions of years, and there are an unknown, but massive, amount of them on the planet today. The unifying quality of all bacteria is that they are single-celled, and the vast majority are so small they can only be viewed under a microscope, although there are a rare few that can just barely be seen with the naked eye. Most bacteria are harmless, but harmful bacteria, also known as pathogenic bacteria, can cause incredible damage to a person's body, including death.

The majority of bacteria are known as nonpathogenic bacteria, and they comprise more than two-thirds of all those found on Earth. Nonpathogenic bacteria are found on and in everything, from skin, to water, to human stomachs. They actually are necessary to the healthy functioning of any life form, assisting with vital functions like digestion. These bacteria are constantly moving through the human body's systems, and cause no harm, except in very rare circumstances.

Microscopic view of bacteria.
Microscopic view of bacteria.

The term pathogenic means disease carrying, and pathogenic or harmful bacteria are often referred to simply as germs. They are generally classified based on their shape, with three main groupings: coccus, bacillus, and spirillium. The cocci can be further sub-divided into three groups: streptococci, diplococci, and staphylococci.

Each of these three main groupings of bacteria are associated with their own set of symptoms and ailments. The cocci, for example, produce pus and grow in larger groupings. They cause all sorts of pustules and boils in the body, and so are quite distinctive when they infect a wound or region of flesh. Bacilli are responsible for a number of much more serious diseases, most notably tuberculosis. Spirilla cause other serious diseases, including syphilis.

Bacteria in a petri dish.
Bacteria in a petri dish.

As single-celled organisms, harmful bacteria are able to reproduce quite rapidly. Through a process called mitosis, they split into two identical copies of themselves. These copies can then further split into two more copies, and so on, quickly enlarging a population to staggering numbers. An average bacterium can, within a 24 hour period, create many millions of copies of itself, overrunning an organism. Bacteria’s ability to reproduce on its own is one of the key differences between them and harmful viruses. Viruses are also pathogenic, but as they are much, much smaller than bacteria, they need some sort of a host to reproduce, making them slower and more awkward at reproduction.

Most bacteria can only be viewed under a microscope.
Most bacteria can only be viewed under a microscope.

There are many types of harmful bacteria, some of which are absolutely deadly, while others only cause minor illness. One of the most dangerous is Clostridium botulinum, which causes botulism; it can be found in food and can cause death even in incredibly small amounts. Another is Yersinia pestis, which caused the bubonic plague, and which still afflicts small portions of the world’s population. A less-dangerous, but still inconvenient, form is Campylobacter, which is responsible for most cases of food poisoning, causing severe intestinal discomfort and often vomiting. Other bacteria may be an inconvenience if treated, but deadly if left on their own, such as the different types of Streptococcus that are responsible for strep throat and pneumonia.

Frequently Asked Questions

What are the most widespread types of harmful bacteria?

Escherichia coli, Salmonella, Shigella, Staphylococcus aureus, Clostridium botulinum, and Clostridium perfringens are the harmful bacteria that are most often seen. 

Escherichia coli may be found in raw or undercooked meat, poultry, and eggs, and when consumed, it can result in food poisoning. 

Salmonella, which may result in foodborne disease, is often found in raw or undercooked chicken, eggs, and shellfish. 

Shigella may cause diarrhea, fever, and cramps and is commonly found in contaminated water, food, and surfaces. 

Staphylococcus aureus may cause skin infections, pneumonia, and toxic shock syndrome. It is often found on the skin, in the nose, and on medical equipment. 

Foods that have been canned or vacuum-packed can contain the bacteria Clostridium botulinum, which may lead to the uncommon but serious disease of botulism. 

Food poisoning may be brought on by Clostridium perfringens, which is present in cooked meats, poultry, and gravies.

How are harmful bacteria spread?

Antibiotics are used to treat certain bacterial infections like strep throat and syphilis.
Antibiotics are used to treat certain bacterial infections like strep throat and syphilis.

By contact with ill people and animals, contaminated surfaces, food, or water, harmful bacteria may be spread. By consuming contaminated food, drinking polluted water, touching contaminated surfaces, or coming into contact with an infected person or animal, people may get hazardous germs. When an infected person coughs, sneezes, or speaks, bacteria may also be dispersed via the air.

What symptoms indicate an illness brought on by harmful bacteria?

Coccus bacteria cause a number of common infections, including strep throat.
Coccus bacteria cause a number of common infections, including strep throat.

Depending on the kind of bacteria, different infections might show different symptoms. Fever, chills, cramping in the abdomen, nausea, vomiting, diarrhea, and exhaustion are typical symptoms. A skin rash, joint discomfort, disorientation, and breathing difficulties are possible additional symptoms.

How can people protect themselves from harmful bacteria?

Bacterial food poisoning often occurs when food is prepared or handled at improper temperatures or in a way that's unsanitary.
Bacterial food poisoning often occurs when food is prepared or handled at improper temperatures or in a way that's unsanitary.

People can protect themselves from harmful bacteria by regularly and thoroughly washing their hands with soap and water, avoiding contact with people who are ill, avoiding contact with animals that may carry harmful bacteria, and avoiding contact with contaminated surfaces and objects. People should also cook food thoroughly, avoid eating raw or undercooked food, and avoid drinking contaminated water.

How are infections caused by harmful bacteria treated?

Infections caused by harmful bacteria can be treated with antibiotics. Antibiotics are medications that kill or stop the growth of bacteria. It is essential to take all prescribed antibiotics as directed and finish the full course of treatment, even if the symptoms go away. If a person does not complete the full course of antibiotics, the bacteria may not be killed entirely, and this can cause the infection to come back.

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Discussion Comments


@anisha34: I don't think you'll need to see a doctor. I would suggest getting a good antibacterial soap and washing your hands well in hot water. Even if a little smell remains, you will know your hands are clean. You can also get some hand sanitizer that doesn't require water to use and you can use it to make sure the smell goes away.

Next time you wash really nasty dishes, wear gloves. I always do.


Please give me a solution. I am very afraid. Do I wash my hands with hot water?


I have a problem and need help solving it. My girlfriend used to give me different types of tasty food whenever she cooked, but carrying the food to my home is dangerous because of my mom. So, I took it in my own room very quietly and ate it, and kept the boxes in my study desk unwashed. I was a little bit busy for several weekends and I forgot to wash those boxes. After a while, when I opened them up, they had yucky stuff in them and a very bad smell. Bacteria, I guess. So then I took them to my bathroom and tried to wash them but, they stunk so much that I threw the boxes out of my window.

My problem is that I had a very bad smell on my hands.

I washed them a lot with good soap, but the smell is not coming off my hands. I am afraid it might cause me some real trouble or a severe sickness.

Please tell me what I should and will it cause me any problems. Do I consult a doctor?


i had a problem,please do solved it friends.

my girlfriend used to give me different types of tasty food whenever she cooked. but carrying those stuffs to my home its so much dangerous because of my mom. so i take it in my own room very silently and ate it,and keep the tiffing boxes in my study desk un-washed. for some weekends m a little bit busy in my study so that i forgot to wash those boxes. after a while when i opened it up,it grew with some yucky stuffs i mean to say very bad smell bacteria i guess. so then i take it to my bathroom and try to wash it,but it stinks too much that i thrown the box out of my window.

MY PROBLEM IS THAT FRIENDS, i had a very pathetic types of smell in my hands,i washed it a lot with dettol,good soaps but the smell are not getting rid of my hands. i had feared that if it causes me some real trouble or a severe sickness..

please tell me what have now i do and is it causing me a great problem? do i consult a doctor?


@anamur-- No. They might cause some annoying side effects such as flatulence and gas, but they're not going to be harmful.

For example, we have a lot of good bacteria in our intestines that feed on the protein found in fiber. They allow for regular bowel movements and protect us from intestinal cancer. Of course, they also cause flatulence and excess gas when they're at work. Some people might hate that, but that doesn't mean they're harmful. We need them!


Is it possible for good bacteria to become harmful? For example if there is too many of them?


@EarlyForest-- I'm not a doctor but as far as I know, we all ingest bad as well as good bacteria. The important part is not getting any stomach bacteria but keeping the amount of good bacteria higher than the amount of bad bacteria. So I think that you're right.


I've heard that the same bacteria that cause you to develop a urinary tract infection can lead to a kidney infection. That's why it's so important to see your doctor when you start having symptoms of a UTI.

I used to get the bladder cramps and frequent urges to pee every few months, and I would always have to go for antibiotics. Then, my doctor told me I could avoid infections just by drinking plenty of water and a glass of cranberry juice a day. I tried it, and he was right!


I had food poisoning after eating at a new restaurant, and after that, I was so scared of harmful bacteria that I wouldn't eat out for months. I didn't know which places I could trust.

I had a terrible time with it. I vomited several times, and I also had diarrhea at the same time. I nearly dehydrated, because it was so hard to keep water down.

Any time you eat out, you take a risk with your health. You have no way of knowing how the food is handled in the kitchen and what bacteria might be lurking in it.


@healthy4life – A strep infection can wind up damaging your kidneys, your joints, and even your heart if you don't see a doctor to stop it before it gets to that point. It is scary that something that starts out as a sore throat could lead to this, but if you ever get strep, you will understand its severity.

It's unlike any other type of sore throat. Once you start to feel that your throat is sore, within a few hours, it will swell so much that you can barely swallow your saliva. You'll also have a fever and maybe even swollen glands in your neck.

Doctors can tell by swabbing your throat whether or not you have a strep infection. They can also tell by seeing if you have white patches in the back of your throat. They prescribe antibiotics, and you need to take them all, even if you feel better after a few days.


I had no idea that strep bacteria could be so dangerous! How can you tell if you have strep throat as opposed to just a regular sore throat caused by a cold? Also, what can leaving strep throat untreated do to your body?


@anon123161-- Bacilli are divided into: single bacilli, diplobacilli and streptobacilli.


@googlefanz-- I'm not an expert but I think different kinds of bacteria can survive in different kinds of environments. So while some bacteria would die entering the acidic environment of the stomach, others are durable to that environment and can continue to live.

So different bacteria are capable of living in different parts of the body. Also, don't forget that we have antibodies that are fighting harmful bacteria. There might be a lot of bacteria entering our body, but it doesn't mean that all of them will make us sick. Most actually are destroyed by the immune system before they can cause disease.


If bacteria are either nonpathogenic or pathogenic, how do some bacteria cause disease in some and not others?

I was treated for a stomach bacteria called helicobacter pylori several years ago. It was making me sick and giving me all sorts of gastritis symptoms. The doctor said that most of the population carries this bacteria in their digestive system but not everyone gets sick from it. Only some people react badly.

How is this possible? How can a pathogenic bacteria be harmful to some people and not others?


Actually, bacteria reproduce by binary fission because they lack nuclei.


thank you so much. i have to prepare for presentation on harmful aspects of bacteria and this article really helped me to prepare myself.


what groups can the bacilli and spirilla be divided into?


I have a question. I am pretty rigorous when it comes to taking my prebiotic and probiotic supplements, so I know that I have a good supply of probiotic bacteria in my gut.

So would this mean that I would have a lesser chance of getting bad bacteria, since the good bacteria could kill it off, or am I just as prone to infection as anyone else?

Thanks for the information.


So where do these harmful bacteria usually live? Are most of them found in the intestine once you're infected, or do they rather travel about? I know that the easiest way to be infected is through contact with fecal matter, but does that necessarily mean that the bacteria live in the intestine?


Thanks so much for this article -- my daughter has an essay topic entitled "What are some harmful bacteria, and their effects?" and this really helped us out! Thanks so much! Now we just have to find an article on good bacteria for question two...

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