Bacteria are single-celled organisms. Depending on the species, they can be harmful, neutral, or beneficial to humans. When harmful bacteria invade the body, a person is said to have a bacterial infection or sepsis.
In order to cause an infection, bacteria must penetrate the body. For example, they can enter through a break in the skin, by ingesting contaminated food or water, through inhaling the bacteria into the airways, or through sexual activity. The body has many defenses against invading bacteria. Sweat and oil gland secretions help kill bacteria on the skin. Mucus in the mouth and throat helps to trap inhaled
bacteria before they can reach the lungs. Highly acidic stomach acid kills many bacteria found in food and water.
Most bacteria that succeed in entering the body are quickly killed soon after they arrive. If, however, the body is unable to destroy the bacteria, an infection occurs. Acute infections develop suddenly, and they disappear completely when a person receives treatment or the body's immune system responds and destroys the bacteria. Chronic infections develop if the body has only limited control over the bacteria. In chronic infections, the body is never completely free of the bacteria, but symptoms disappear temporarily and then flare up later in response to physical or emotional stress or when the body is weakened by another illness.
Many bacterial diseases begin as local infections in places such as the skin, throat, or bladder. If bacterial infections are left untreated, bacteria can spread by entering the bloodstream and infect other, more distant, organs. When bacteria are found in the bloodstream, the condition is called septicemia or, more commonly, blood poisoning. Sepsis is a condition in which organs of the body are overwhelmed by infection.
It is a leading cause of death among the elderly and those whose immune systems are compromised.
Bacteria damage the body by adhering to or invading cells, altering cellular metabolism, or releasing toxic (poisonous) substances. When bacteria begin reproducing, the body's immune system has a set series of responses that attempt to control the spread of infection. The immune system consists of the spleen, thymus, lymph nodes, and white blood cells. Its reactions take time and are not always successful. Many of the symptoms associated with bacterial infections such as fever, redness, and swelling, are actually caused by the immune system's response to the invading bacteria rather than the bacteria themselves. Other reactions, such as vomiting and diarrhea caused by gastrointestinal infections, are a reaction to toxins the bacteria release as a byproduct of their metabolism.
Anyone can develop a bacterial infection. In general, the people most at risk are the very young, whose immune systems are immature, and the very old, whose immune systems respond less vigorously to infection. People who have diseases that compromise the immune system, such as AIDS , or those receiving chemotherapy or drugs that suppress the immune system (e.g., transplant CA-Bacterial patients) are also at high risk. Since most bacterial infections are contagious, living in a communal setting such as a nursing home, dormitory, or prison also increases the likelihood of acquiring a bacterial disease. People with diabetes and those with spinal cord injuries, both of whom lose feeling in their legs and feet, are at high risk of developing bacterial infections through open wounds or bedsores because they cannot feel when their skin is damaged. Secondary bacterial infections are infections that occur after the body has been weakened by another illness. Pneumonia is a common secondary infection in the elderly following influenza or a severe cold.
Causes and symptoms
A wide range of symptoms is associated with bacterial infections. Common symptoms include:
- redness and warmth at the site of the infection. This is caused by the body's response to the chemical histamine released by wounded cells.
- swelling at the infection site. This occurs because capillaries (tiny blood vessels) become “leaky,” allowing fluid and infection-fighting white blood cells to accumulate at the infection site.
- pus accumulation. Pus is a mixture of dead WBCs, cellular debris, and fluid.
- fever. Elevation in body temperature is a protective response to infection that occurs in response to the release of the chemical interferon by infected cells. Increased body temperature often slows bacterial reproduction.
- nausea, vomiting, diarrhea. This is a common response to toxins produced by bacteria.
- headache, muscle ache, chills, and swollen lymph nodes. All common symptoms of infection. Other symptoms can include itching and burning sensations.
Bacterial infections are initially diagnosed on the basis of symptoms and confirmed by additional testing. If a person has an infection, a complete blood CA-Bacterial count of MRSA were recognized—hospital-acquired MRSA (HA-MRSA) and community-acquired MRSA (will usually reveal an abnormally high number of white blood cells. This, however, only tells the physician that the body is fighting an infection. It does not indicate the cause (bacterial, viral, fungal) of the infection. To determine the cause of the infection, a sample of material (blood, cerebrospinal fluid, mucus, etc.) from the infection site is cultured (grown) in the laboratory and examined under the microscope, so that the species of bacteria can be determined. Cultures can also be tested to see if they are sensitive to specific antibiotics , as not all antibiotics kill all types of bacteria.
QUESTIONS TO ASK YOUR DOCTOR
- Is my infection viral or bacterial?
- What is the normal course of this illness? When can I expect to feel better?
- Can I transmit this infection to others?
- What are the potential side effects for the drug you have prescribed?
- What complications might occur from this type of infection? Can I do anything to prevent them?
Many infections, especially infections of skin wounds, can be cured by cleaning and application of an antibiotic ointment. More serious infections require oral (by mouth) or intravenous (IV, directly into a vein) antibiotic therapy. Different classes of antibiotics affect different species of bacteria. If the type of bacteria has not been determined, the physician may prescribe a broad-spectrum antibiotic that kills several of the most common types of bacteria. Symptoms often improve within a few days after antibiotic therapy is begun, but patients should always take the entire amount of antibiotic prescribed. Stopping the antibiotic early increases the chance of the infection reasserting itself within a few days.
The development of mutated (genetically changed as the result of natural selection) antibiotic-resistant bacteria is of increasing concern to healthcare professionals.
Methicillin-resistant Staphylococcus aureus. (MRSA) is a strain of staphylococcal bacteria that is resistant to the antibiotic methicillin and other common antibiotics. infections. In 2008, two forms of MRSA were recognized—hospital-acquired MRSA (HA-MRSA) and community-acquired MRSA (CA MRSA). Symptoms of a MRSA infection are similar to those of other infections, only MRSA is much more dangerous and has a much higher mortality rate because treatment with common antibiotics does not kill the bacterium. Hospital-acquired MSRA is of particular concern to the hospitalized elderly who make up a large, vulnerable population. The incidence of drug-resistant tuberculosis is also rising. Overuse of antibiotics accelerates the development of these drug-resistant bacteria.
Nutrition and diet only indirectly affect bacterial infections. A healthy, well-balanced diet helps to keep the immune system strong, allowing a faster, stronger response to infection.
The outcome of a bacterial infection depends on the species of bacterium causing the infection, the health of the individual who is infected, and the type of treatment they receive. Treatment with the proper antibiotic greatly improves the chance that the individual will recover from the infection without complications. Some bacterial infections, such as bacterial meningitis , are fatal if not treated and have about a 75% recovery rate even if treated promptly. Others, such as strep throat, are almost 100% treatable with antibiotics. If left untreated, most people recover from a strep infection on their own, but permanent damage can be done to the heart (rheumatic heart disease ) that may not become apparent until years later.
Preventing bacterial infections begins with practicing good hygiene. This includes:
- avoiding exposure to infected people.
- washing hands well and frequently, especially after nose blowing or sneezing and before food handling
- not sharing food and eating utensils with anyone
- avoiding potentially contaminated food and water.
- keeping immunizations against bacterial diseases up to date
- taking the entire amount of prescribed antibiotic, even if symptoms improve.
Caregivers need to practice good hygiene and ensure that their charges do likewise. Beyond this, care givers need to recognize the symptoms of bacterial infection and see that their charges receive appropriate medical care. Caregivers can reinforce the fact that it is important for the individual to take all the antibiotic prescribed to cure an infection, even if the individual begins to feel better and symptoms disappear.
Bacterial meningitis —A serious bacterial disease in which the membranes lining the brain and spinal cord (the meninges) become infected.
Cerebrospinal fluid (CSF) —A clear fluid that fills the hollow cavity inside the brain and spinal cord. The cerebrospinal fluid has several functions, including providing a cushion for the brain against shock or impact, and removing waste products from the brain.
Complete blood count (CBC) —A blood test to check the numbers of red blood cells, white blood cells, and platelets in the blood.
Lymph nodes —Small, bean-shaped masses of tissue scattered along the lymphatic system that act as filters and immune monitors, removing fluids, bacteria, or cancer cells that travel through the lymph system. Cancer cells in the lymph nodes are a sign that the cancer has spread, and that it might recur.
Secondary or opportunistic infection —An infection by a microbe that occurs because the body is weakened by a primary infection caused by a different kind of microbe.
Fauci, Anthony, et al., eds. Harrison's Principles of Internal Medicine, 17th ed. New York, NY: McGraw-Hill, 2008.
Goldman, Lee and Dennis Ausiello., eds. Cecil Textbook of Medicine., 23rd ed. Philadelphia Saunders Elsevier, 2008.
ICON Health Publications. Bacterial Infections: A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet References. San Diego, CA: ICON Health Publications, 2004.
“Bacterial Infections” MedlinePlus. April 2, 2008 [cited April 5, 2008]. http://www.nlm.nih.gov/medlineplus/bacterialinfections.html.
“Group A Streptococcal Infections.” National Institute of Allergy and Infectious Diseases. September 17, 2007 [cited April 1, 2008]. http://www3.niaid.nih.gov/healthscience/healthtopics/streptococcal/default.htm.
“MRSA Infection.”Mayo Clinic. Nov 9, 2008 [cited April 1, 2008]. http://www.mayoclinic.com/health/mrsa/DS00735.
“Staphylococcal Infections.” Medline Plus. January 18, 2008 [cited April 1, 2008]. http://www.nlm.nih.gov/medlineplus/staphylococcalinfections.html.
“Streptococcal Infections” MedlinePlus. March 20, 2008 [cited April 1, 2008]. http://www.nlm.nih.gov/medlineplus/streptococcalinfections.html.
National Institute of Allergy and Infectious Diseases Office of Communications and Government Relations, 6610 Rockledge Drive, MSC 6612, Bethesda, MD, 20892–6612, (301) 496-5717, (866) 284-4107 or TDD: (800) 877-8339 (for hearing impaired), (301) 402-3573, http://www3.niaid.nih.gov.
Bacterial infections are illnesses that occur when harmful forms of bacteria multiply inside the body. They range from mild to severe. Although they include such deadly diseases as plague, tuberculosis, and cholera, these and many other bacterial infections can be prevented by good sanitation or cured by antibiotics.
for searching the Internet and other reference sources
- * microorganisms
- are living organisms that can only be seen using a microscope. Examples of microorganisms are bacteria, fungi, and viruses.
Most bacteria are either harmless, or helpful, or even essential to life. Bacteria break down (decompose) dead plants and animals. This allows chemical elements like carbon to return to the earth to be used again. In addition, some bacteria help plants get nitrogen. Without them, plants could not grow. In the human body, bacteria help keep the digestive tract working properly.
Like viruses, however, bacteria can cause hundreds of illnesses. Some bacterial infections are common in childhood, such as strep throat and ear infections. Others cause major 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 bacterium called Staphylococcus (staf-i-lo-KOK-us). It may involve an internal organ, as in bacterial pneumonia (infection of the lungs) or bacterial meningitis (infection of the membrane covering the brain and spinal cord).
Some bacteria, such as pneumococcus (noo-mo-KOK-us), which is also called Streptococcus pneumoniae, almost always cause illness if they get into the body. Others, such as Escherichia coli, usually called by the short form E. coli, often are present without doing harm. If the immune system is weakened, however, these bacteria can grow out of control and start doing damage. Such illnesses are called “opportunistic infections.” They have become more common in recent years, in part because AIDS, organ transplants, and other medical treatments have left more people living with weakened immune systems.
Food poisoning is often the result of bacterial contamination. Important precautions for preventing food poisoning include:
- Avoiding raw or undercooked meat, poultry, seafood, and eggs.
- Avoiding non-pasteurized dairy products.
- Throwing out food items that are old or have an “off” smell.
- Keeping foods cold until ready to serve.
- Storing and cooking foods properly.
- Washing knives, cutting boards, cooking utensils, and food preparation areas after every use.
- Washing hands before preparing food, before eating, and after using the bathroom.
Unlike other living cells, bacteria do not have a membrane enclosing their nucleus, the part of the cell containing DNA, or genetic matter. Unlike viruses, most bacteria are complete cells that can reproduce on their own without having to invade a plant or animal cell. Some bacteria, however, do need to live inside another cell just as viruses do.
HOW do bacterial infections spread?
Different bacteria spread in different ways. Examples include:
- through contaminated water (cholera and typhoid fever)
- through contaminated food (botulism, E. coli food poisoning, salmonella food poisoning)
- through sexual contact (syphilis, gonorrhea, chlamydia)
- through the air, when infected people sneeze or cough (tuberculosis)
- through contact with animals (anthrax, cat scratch disease)
- through touching infected people (strep throat)
- from one part of the body, where they are harmless, to another part, where they cause illness (as when E. coli spread from the intestines to the urinary tract).
How do bacteria cause illness?
Bacteria can cause illness in several ways. Some destroy tissue directly. Some become so numerous that the body cannot work normally. And some produce toxins (poisons) that kill cells. Exotoxins are poisons released by live bacteria. Endotoxins are poisons released when the bacteria die.
Symptoms of bacterial infections vary widely but often include fever.
Doctors may test the blood, sputum, or urine for evidence of harmful bacteria. If a lung infection is suspected, the doctor may take a chest x-ray or do a biopsy, taking cells from an infected area to be examined. If meningitis is suspected, the doctor may do a spinal tap, using a needle to extract a sample of the spinal fluid surrounding the spinal cord for testing.
Most bacterial infections can be cured by antibiotic drugs, which were one of the great medical success stories of the twentieth century. These drugs either kill the bacteria or prevent them from reproducing. Penicillin, the first antibiotic, is still used to treat some infections. Other widely used antibiotics include amoxicillin, bacitracin, erythromycin, cephalosporins, fluroquinolones, and tetracycline. Sometimes antitoxins are also given to counter the effects of bacterial toxins, as in the case of tetanus or botulism.
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 deaths. Described in media coverage as “flesh-eating bacteria,” these pathogens caused their damage through a process called hemolysis (he-MOL-y-sis), which causes red blood cells to disintegrate. The Texas outbreak was short lived, and experts still do not know how it started.
Young children commonly get shots of vaccine to prevent diphtheria, whooping cough, tetanus, and Hemophilus influenza B, all bacterial infections. In addition, vaccines are available to help prevent 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, sanitary disposal of human waste, well-ventilated housing that is not overcrowded, and prompt medical treatment for people who do get sick.
Other steps include:
- Washing hands (before handling food; after using the toilet; after touching animals; after having contact with infected people).
- Washing fruits and vegetables before eating.
- Cooking meat thoroughly.
- Abstaining from sexual contact, or using condoms during sexual activity.
Each kind of bacteria can be killed by certain antibiotics and each kind is naturally resistant to others. But in recent years, some bacteria have developed resistance to antibiotics that used to kill them. This is one of the big problems in controlling infectious diseases.
How does resistance occur?
In some cases, it occurs by chance. As bacteria reproduce, mutations (variations) in their genes occur all the time. One of these mutations may happen, by chance, to make one bacterium in a person’s body less vulnerable to a drug. This bacterium multiples along with other bacteria. While other bacteria are killed off by the drug, the mutated—or resistant—bacterium thrives, and eventually spreads from person to person.
Why does resistance occur more often now?
Humans can make drug resistance far more likely to develop than it would simply by chance. They do this when they use antibiotics that are not needed or when they do need antibiotics but stop taking them too soon.
Here is how those practices can increase resistance: Let us say that a man has a bacterial infection. He takes an antibiotic, feels better, and stops taking the drug in five days, even though the doctor said to take it for 10 days. Inside the mans body, the drug may have killed, say, 80 percent of the bacteria. That was enough to make the man feel better. But the bacteria still alive are the 20 percent that were toughest, the ones best able to survive the drug. If the man had kept taking the antibiotic, the toughest bacteria might have been killed on Day 8, 9, or 10. But now, left alone, they start to multiply. Soon the person feels sick again. But this time, all the bacteria in his body are the tougher kind. A scientist would say the person’s behavior “selected” the most resistant bacteria for survival.
A similar process occurs when doctors prescribe antibiotics that are not needed. Let us say that a girl has a cough and fever. The doctor prescribes antibiotics on the chance that the girl has a bacterial infection. But she actually has a virus, and viruses cannot be treated with antibiotics. The girl’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 the girl’s 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 countless times, in countless people, eventually strains of bacteria may arise that partially or completely resist a drug that used to kill them.
Cat Scratch Disease
Rocky Mountain Spotted Fever
Toxic Shock Syndrome
Bacteria get a bad reputation for causing disease when, in reality, very few species of bacteria infect humans. The ones that do, however, are the ones most often written about in magazines and newspapers. These bacteria inhabit the human body because of the constant source of nourishment, moist environment, relatively stable pH and body temperature, and extensive surface area.
WILLIAM, ANNA WESSELS (1863–1954)
Physician and bacteriologist who isolated a strain of the bacterium that causes diphtheria, from which she made an antitoxin that could be used to treat the disease. She also discovered a way to diagnose rabies in a few minutes instead of a few days, thus saving many more lives.
Contamination with bacteria from the environment can lead to colonization, taking up residence on or in the human body. The mixed collection of bacteria that are adapted to the body and reside in it for extended periods of time are called normal flora. Some bacteria inhabit the body only as transients, soon destroyed by human (host) defense mechanisms or removed by cleaning. Bacteria that evade host defenses and cause infection are described as virulent. Under certain circumstances, such as an imbalance in normal flora or lowered host resistance, even normal flora can cause infection.
Infection may proceed to disease if host defenses do not arrest the infection before tissue damage occurs. Bacterial disease can have several outcomes. The immune system may arrest the infection and stop progression of the disease. In other cases, the body may be unable to repair damaged tissues and permanent dysfunction or even death may result. For this reason, treatments are designed to stop the infection before permanent damage has occurred.
KOCH, ROBERT (1843–1910)
German physician who discovered the three bacteria that cause the deadly diseases tuberculosis, cholera, and anthrax, and who won the 1905 Nobel Prize in medicine. His most enduring work was a set of guidelines, called Koch's postulates, for telling which pathogens (bacteria or viruses) cause which diseases.
Most bacterial diseases are treated with antibiotics to kill the organisms. In recent years, more and more bacteria have become resistant to the available antibiotics. This has forced the scientific community to examine the use of antibacterial agents in soaps and cleansers, the use of antibiotics in animal feeds, and the inappropriate prescription of antibiotics. Patients who terminate the treatment prematurely because they feel better, even though the infection is not yet eliminated, compound the problem of antibiotic resistance. All of these situations lead to the killing off of susceptible bacteria while leaving the resistant ones to multiply. The best "medicine" is still prevention of infection.
see also Disease; Eubacteria
Berkow, Robert, et al. The Merck Manual of Diagnosis and Therapy, 17th ed. Rahway, NJ: Merck & Co., 1999.
Levy, Stuart B. "The Challenge of Antibiotic Resistance." Scientific American (March 1998). <www.sciam.com/1998/0398issue/0398levy.html>.
Murray, Patrick R., et al. Medical Microbiology, 3rd ed. St. Louis: Mosby-Year Book, 1998.
Staley, James T., et al. Bergey's Manual of Systematic Bacteriology, 4 vols. Philadelphia, PA: Lippincott, Williams & Wilkins, 1984–1989.