Bacterial Infections

Bacterial infections occur when harmful forms of bacteria multiply inside the body. These infections range from mild to severe. Although they include such deadly diseases as plague, tuberculosis, and cholera, these and many other bacterial diseases can be prevented with good sanitation and cured by antibiotics.

What Are Bacteria?

Bacteria (singular: Bacterium) are microorganisms * and they exist everywhere: in the air, in the water, and in every animal and person. Bacteria are by far the most numerous organisms on Earth.

Most bacteria are harmless. Often bacteria are beneficial, even essential, to plant and animal life. Bacteria decompose (break down) dead plants and animals. This process allows a chemical element such as carbon to return to the soil to be used again. In addition, some bacteria enable plants to get nitrogen from the soil and some enable plants to use atmospheric nitrogen. Without bacteria, plants could not grow. In the human body, bacteria help keep the digestive tract working properly.

Bacteria can also cause disease. Like viruses * , bacteria can cause hundreds of illnesses. Some bacterial infections are common in childhood, such as strep throat (caused by streptococcus [strep-toe-KOKus] bacteria) and ear infections. Others cause diseases, such as tuberculosis, plague, syphilis, and cholera. The infection may be localized (limited to a small area), as when a surgical wound gets infected with a staphylococcus (staf-i-lo-KOK-us) bacteria. It may involve an internal organ, as in bacterial pneumonia (infection of the lungs) or bacterial meningitis (infection of the membranes covering the brain and spinal cord).

Anatomy of an individual bacterium. Its DNA (genetic material) is in the nucleoid area, but it is not enclosed within a membrane. The vacuole is enclosed within a membrane and stores everything from water and food to waste products.

Anatomy of an individual bacterium. Its DNA (genetic material) is in the nucleoid area, but it is not enclosed within a membrane. The vacuole is enclosed within a membrane and stores everything from water and food to waste products. This bacterium uses its flagellum (tail) to move around.
Illustration by Frank Forney. © 2016 Cengage Learning®.
* is weakened, however, these bacteria can grow out of control and start doing damage. Such illnesses are called opportunistic infections. Through the beginning of the 21st century, they became more common, in part because AIDS * , organ transplants, and other medical treatments left more people living with weakened immune systems.

Staphylococcus aureus bacteria.

Staphylococcus aureus bacteria.

How Are Bacteria Different from Viruses?

Unlike other living cells, a bacterium does not have a membrane that encloses its nucleus, the part of the cell containing DNA, which is the cell's genetic material. Unlike viruses, most bacteria are complete cells that can reproduce on their own without having to invade a plant or animal cell. Some bacteria need to live inside another cell just as viruses do.

How Do Bacteria Spread?

People get bacteria in the following ways:

How Do Bacteria Cause Illness?

Bacteria can cause illness in several ways. Some destroy tissue directly. Some become so numerous that affected tissue cannot work normally. And some produce toxins * that kill cells. Exotoxins are toxins that are actively secreted by bacteria into the surrounding medium. Endotoxins are part of the bacterial architecture. They are usually part of the cell walls of bacteria and can be extremely toxic to people. Most bacterial toxins are exotoxins.

Bacteria that frequently cause infections in humans include:

How Are Bacterial Infections Diagnosed and Treated?

The symptoms of bacterial infections vary widely but often include fever.

Bacterial infections and mode of transmission

Bacterial infections and mode of transmission
SOURCE: Adapted from data at © 2016 Cengage Learning®.
Food Poisoning

Food poisoning is often the result of bacterial contamination. Important precautions for preventing food poisoning include:

Flesh-Eating Bacteria

A virulent strain of Streptococcus A bacteria caused illness in 117 people in Texas between December 1997 and March 1998. Of those 117 infections, 26 (17 adults and 9 children) resulted in death. Described in media coverage as “flesh-eating bacteria,” these pathogens induced hemolysis (he-MOL-y-sis), or the lysis (dissolution) of red blood cells. The Texas outbreak was short-lived, and experts remained uncertain about how it started.


Doctors may test the patient's blood, sputum * , wounds, or urine for evidence of harmful bacteria. If a lung infection is suspected, the doctor may take a chest x-ray or collect a sputum sample to be examined. If meningitis is suspected, the doctor may do a spinal tap, using a needle to extract a sample of the fluid that surrounds the spinal cord for testing so that the sample can then be studied.


Most bacterial infections can be cured by antibiotics, which were one of the great medical success stories of the 20th century. These drugs either kill the bacteria or prevent them from reproducing. According to several studies, antibiotic prescribing rates in the United States are high. Results from a study reported in 2015 revealed that for every 1,000 adults, 789 antibiotic prescriptions were written. For children, the antibiotic prescriptions rates were even higher, with 889 prescriptions written for every 1,000 children. Penicillin, the first antibiotic, was used in the 21st century to treat some infections. Other widely used antibiotics included azithromycin, amoxicillin, bacitracin, erythromycin, and tetracycline. Examples of antibiotic classes include cephalosporins (sef-a-LO-spor-ins) and fluoroquinolones (FLOR-o-kwin-o-LONES). Sometimes antitoxins may be given to patients to counter the effects of bacterial toxins, as in the case of tetanus or botulism toxins.

How Are Bacterial Infections Prevented?

Most young children are vaccinated against diphtheria, whooping cough, tetanus, and Haemophilus influenza B, which are all bacterial infections. In addition, there are vaccines for cholera, meningococcal and pneumococcal infections, plague, and typhoid fever.

For many bacterial infections, good living conditions are the best prevention. That means clean water supplies, modern sewage systems, well-ventilated housing that is not overcrowded, and prompt medical treatment for people who do get sick.

Other precautions include:

How Do Bacteria Become Drug Resistant?

Virtually every kind of bacteria can be killed by specific antibiotics. However, some bacteria develop resistance to the antibiotics that are used to kill them. This is one of the big problems in controlling infectious diseases.

How Does Resistance Occur?

In some cases, resistance occurs by chance. As bacteria reproduce, mutations (variations) in their genes occur.

One of these chance mutations may happen to make a single bacterium in a person's body less vulnerable to a drug. This bacterium multiplies along with other bacteria. Whereas other bacteria are killed off by the drug, the mutated or resistant bacterium and its descendants thrive, and they can spread to other people.

Why Does Resistance Occur More Often in the 21st Century?

The use of antibiotics made the phenomenon of bacterial resistance far more prevalent than it would have been in their absence. The problem of resistance was made worse by people taking antibiotics that were not needed or by choosing to stop taking them too soon.

Here is an example of how the use of an antibiotic can increase resistance: A man has a bacterial infection. He takes an antibiotic, feels better, and stops taking the drug in five days, even though the doctor instructed him to take it for 10 days. Inside the man's body, the drug may have killed 80 percent of the bacteria. That was enough to make him feel better. But the bacteria still alive are the 20 percent that were strongest, the ones best able to survive the drug. If the man had kept taking the antibiotic for the remaining five days, the toughest bacteria might have been killed. But now, having survived the medication, they start to multiply. Soon the man feels sick again. But this time, all the bacteria in his body are the stronger kind. A scientist would say that the man's behavior had “selected” the most resistant bacteria for survival.

A similar process occurs when doctors prescribe antibiotics that are not needed. Let us say that a teenage girl has a cough and fever. The doctor prescribes antibiotics on the chance that the teen has a bacterial infection. But she actually has a virus, and viruses cannot be treated with antibiotics. The teen's immune system, her natural defense system, fights off the virus as it would have without the drug. Meanwhile, the antibiotic kills off some bacteria that usually live harmlessly in her throat. The bacteria in her throat that survive are those that are better at resisting the antibiotic. Later, if those bacteria get into her ears, her lungs, or some other part of her body where they can cause illness, the antibiotic may not work as well against them.

If events like these happen repeatedly among many people, eventually strains of bacteria may arise that partially or completely resist a drug that is used to kill them.

Several Resistant Infections

Clostridium difficile Vancomycin-resistant Staphylococcus aureus

Vancomycin-resistant Staphylococcus aureus (VRSA) is a specific type of bacteria that are resistant to the antibiotic vancomycin. The bacterium Staphylococcus aureus is found on the skin and in the nose of about 30 percent of people. Usually, the presence of these bacteria does not cause any problems, but if the bacteria get into the body through a cut or scrape, a serious infection can develop.

A blood test will be done to determine what medication is needed to treat the Staphylococcus aureus infection. The best way to prevent the development of this type of infection is to perform good hand hygiene by:

Methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is another bacterium resistant to many antibiotics. These bacteria cause skin infections and, if present in an ill person, can lead to life-threatening infections and pneumonia.

The easiest way to get MRSA is through having direct contact with an infected wound or sharing items, such as a towel or razor, with a person who has an infected wound. The risk for this infection increases when there is close skin-to-skin contact, such as in locker rooms, day-care centers, and school dormitories.

Symptoms of MRSA include a red, swollen, painful area on the skin that is warm to the touch and full of pus. A fever might also occur. The person might think that this is a spider bite. Medical attention is needed to drain the wound. Antibiotics might also be prescribed to heal the infected skin area.

MRSA can be prevented by:

See also AIDS and HIV Infection • Antibiotic Resistance • Cholera • Deafness and Hearing Loss • Diphtheria • Food Poisoning • Helicobacter pylori Infection • Infection • Legionnaires' Disease • Meningitis • Pneumonia • Rheumatic Fever • Salmonellosis • Sexually Transmitted Infections (STIs): Overview • Staphyloccocal Infections • Streptococcal Infections • Syphilis • Tetanus (Lockjaw) • Tick-Borne Illnesses: Overview • Toxic Shock Syndrome • Tuberculosis • Typhoid Fever • Typhus • Whooping Cough (Pertussis) • Zoonoses: Overview


Books and Articles

Bennett, John E., Raphael Dolin, and Martin J. Blaser, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 8th ed. Philadelphia: Saunders, 2015.


Centers for Disease Control and Prevention. “Clostridium Difficile Infection.” Healthcare-Associated Infections (HAIs). . (accessed March 19, 2016).

Centers for Disease Control and Prevention. “General Information about VISA/VRSA.” Healthcare-Associated Infections (HAIs). . (accessed March 19, 2016).

Centers for Disease Control and Prevention. “Methicillin-Resistant Staphylococcus Aureus (MRSA) Infections.” . (accessed March 19, 2016).


Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-232-4636. Website: (accessed March 19, 2016).

Food and Drug Administration, Center for Food Safety and Applied Nutrition. 10903 New Hampshire Ave., Silver Spring, MD 20993. Toll-free: 888-463-6332. Website: (accessed March 19, 2016).

* microorganisms are tiny organisms that can be seen only by using a microscope. Types of microorganisms include fungi, bacteria, and viruses.

* viruses (VY-rus-sez) are tiny infectious agents that can cause infectious diseases. Viruses can reproduce only within the cells they infect.

* immune system (im-YOON SIS-tem) is the system of the body, composed of specialized cells and the substances they produce, that helps protect the body against disease-causing germs.

* AIDS or acquired immunodeficiency (ih-myoo-no-dih-FIH-shensee) syndrome, is an infection that severely weakens the immune system. It is caused by the human immunodeficiency virus (HIV).

* toxins are substances that cause harm to the body.

* sputum (SPYOO-tum) is a substance that contains mucus and other matter coughed out from the lungs, bronchi, and trachea.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)