Bacteria are microscopic, single-celled organisms. There are thousands of different kinds, and they live in every conceivable environment all over the world. They live in soil, seawater, and deep within the earth's crust. Some bacteria have been reported even to live in radioactive waste. Some bacteria live in the bodies of people and animals—on the skin and in the airways, mouth, and digestive, reproductive, and urinary tracts—often without causing any harm.

Only a few kinds of bacteria cause disease. They are called pathogens. Sometimes bacteria that normally reside harmlessly in the body cause disease. Bacteria can cause disease by producing harmful substances (toxins), invading tissues, or doing both.

Classification

Bacteria can be classified in several ways:

Bacteria in the Body
The body normally contains several hundred different species of bacteria but many trillions of individual bacteria. The bacteria outnumber the cells of the body by about 10 to 1. Most of these bacteria reside on the skin and teeth, in the spaces between teeth and gums, and in the mucous membranes that line the throat, intestine, and vagina. The species differ at each site, reflecting the different environment at each site. Many of them are anaerobes—that is, they do not require oxygen. Usually, these anaerobes do not cause disease. Many have useful functions, such as helping break down food in the intestine. However, these bacteria can cause disease if the mucous membranes are damaged. Then, bacteria can enter tissues that are usually off-limits to them and that have no defenses against them. The bacteria may infect nearby structures (such as the sinuses, middle ear, lungs, brain, abdomen, pelvis, and skin) or enter the bloodstream and spread.

• Scientific names: Bacteria, like other living things, are classified by genus (based on having one or several similar characteristics) and, within the genus, by species. Their scientific name is genus followed by species (for example, Clostridium botulinum). Within a species, there may be different types, called strains. Strains differ in genetic makeup and chemical components. Sometimes certain drugs and vaccines are effective only against certain strains.
• Staining: Bacteria may be classified by the color they turn after certain chemicals (stains) are applied to them. A commonly used stain is the Gram stain. Some bacteria stain blue. They are called gram-positive. Others stain pink. They are called gram-negative. Gram-positive and gram-negative bacteria stain differently because their cell walls are different. They also cause different types of infections, and different types of antibiotics are effective against them.
• Shapes: All bacteria may be classified as one of three basic shapes: spheres (cocci), rods (bacilli), and spirals or helixes (spirochetes).
• Need for oxygen: Bacteria are also classified by whether they need oxygen to live and grow. Those that need oxygen are called aerobes. Those that have trouble living or growing when oxygen is present are called anaerobes. Some bacteria, called facultative bacteria, can live and grow with or without oxygen.

How Bacteria Shape Up

Bacterial Defenses

Bacteria have many ways of defending themselves.

Biofilm: Some bacteria secrete a substance that helps them attach to other bacteria, cells, or objects. This substance combines with the bacteria to form a sticky layer called biofilm. For example, certain bacteria form a biofilm on teeth (called dental plaque). The biofilm traps food particles, which the bacteria process and use, and in this process, they probably cause tooth decay. Biofilms also help protect bacteria from antibiotics.

Capsules: Some bacteria are enclosed in a protective capsule. This capsule helps prevent white blood cells, which fight infection, from ingesting the bacteria. Such bacteria are described as encapsulated.

Outer Membrane: Under the capsule, gram-negative bacteria have an outer membrane that protects them against certain antibiotics. When disrupted, this membrane releases toxic substances called endotoxins. Endotoxins contribute to the severity of symptoms during infections with gram-negative bacteria.

Spores: Some bacteria produce spores, which are an inactive (dormant) form. Spores can enable bacteria to survive when environmental conditions are difficult. When conditions are favorable, each spore germinates into an active bacterium.

Flagella: Flagella are long, thin filaments that protrude from the cell surface and enable bacteria to move. Bacteria without flagella cannot move on their own.

Antibiotic Resistance: Bacteria develop resistance to drugs because they acquire genes from other bacteria that have become resistant or because their genes mutate. For example, soon after the drug penicillin was introduced in the mid-1940s, a few individual Staphylococcus aureus bacteria acquired genes that made penicillin ineffective against them. The strains that possessed these special genes had a survival advantage when penicillin was commonly used to treat infections. Strains of Staphylococcus aureus that lacked these new genes were killed by penicillin, allowing the remaining penicillin-resistant bacteria to reproduce and over time become dominant. Chemists then altered the penicillin molecule, making a different but similar drug, methicillin, which could kill the penicillin-resistant bacteria. Soon after methicillin was introduced, strains of Staphylococcus aureus developed genes that made them resistant to methicillin and related drugs. These strains are called methicillin-resistant Staphylococcus aureus (MRSA). The genes that encode for drug resistance can be passed to following generations of bacteria or sometimes even to other species of bacteria.

Did You Know... Botulinum toxin, produced by clostridial bacteria, can cause food poisoning and muscle paralysis but can also be used to reduce wrinkles and to treat muscle spasms.

The more often antibiotics are used, the more likely resistant bacteria are to develop. Therefore, doctors try to use antibiotics only when they are necessary. Giving antibiotics to people who probably do not have a bacterial infection, such as those who have cough and cold symptoms, does not make people better but does help create resistant bacteria. Because antibiotics have been so widely used (and misused), many bacteria are resistant to certain antibiotics.

Resistant bacteria can spread from person to person. Because international travel is so common, resistant bacteria can spread to many parts of the world in a short time. Spread of these bacteria in hospitals is a particular concern. Resistant bacteria are common in hospitals because antibiotics are so often necessary and hospital personnel and visitors may spread the bacteria if they do not strictly follow appropriate sanitary procedures. Also, many hospitalized patients have a weakened immune system, making them more susceptible to infection.

Resistant bacteria can also spread to people from animals. Resistant bacteria are common among farm animals because antibiotics are often routinely given to healthy animals to prevent infections that can impair growth or cause illness.

What Are Clostridia?
Clostridia are bacteria that normally reside in the intestine of 3 to 8% of healthy adults and even more newborns. Clostridia also reside in animals, soil, and decaying vegetation. These bacteria do not require oxygen to live. That is, they are anaerobes. Clostridia cause disease in different ways, depending on the species: They may produce a toxin in food, which is then consumed, as occurs in food-borne botulism. They may produce a toxin after they are in the body if circumstances enable them to multiply excessively (overgrow), as occurs in tetanus, Clostridium perfringens food poisoning, and a type of antibiotic-associated diarrhea and colitis called Clostridium difficile—induced diarrhea and colitis. They may produce a toxin and invade tissue, causing infection, as occurs in gas gangrene. Clostridia can infect the gallbladder, colon, and female reproductive organs. Rarely, one species, Clostridium sordellii, causes toxic shock syndrome in women who have infections of the reproductive organs. Clostridia may also spread to the blood (causing bacteremia). Widespread bacteremia (sepsis) can cause fever and serious symptoms such as low blood pressure, jaundice, and anemia. Sepsis can develop after a clostridial infection and be rapidly fatal. Food-borne botulism can develop when people eat food that contains botulinum toxin, produced by Clostridium botulinum. Usually, botulism results from eating uncooked or undercooked food because heat (cooking) destroys the toxin. Botulinum toxin enters the bloodstream from the small intestine and is carried to nerves. This toxin prevents nerves from sending impulses to muscles. About 18 to 36 hours after consuming the toxin, people become tired and dizzy. Their mouth becomes dry. They may feel nauseated and vomit. The abdomen may swell (distend), and constipation may develop. Muscles of the face become slack or paralyzed, causing the eyelids and face to droop and vision to blur. Swallowing and talking become difficult. The muscle weakness then spreads to the upper torso and downward. The muscles involved in breathing may weaken—a problem that may become life threatening. Clostridium perfringens food poisoning can develop when people eat food (usually beef) that contains bacteria (rather than the bacteria's toxin). The bacteria develop from spores, which are inactive (dormant) forms of the bacteria that can survive the heat of cooking. If food that contains spores is not eaten soon after it is cooked, the spores develop into bacteria, which then multiply in the food. If the food is served without adequate reheating, the bacteria are consumed. They multiply in the small intestine and produce a toxin that causes watery diarrhea and abdominal cramping. This type of food poisoning is usually mild but can cause serious problems in older people. Rarely, certain strains of these bacteria produce a toxin that damages the intestine and causes an infection called necrotizing enteritis, which is often fatal. Clostridium difficile —induced diarrhea and colitis (inflammation of the colon) can develop after antibiotics are taken to treat an infection (see Clostridium difficile-Induced Colitis). Antibiotics may destroy some of the bacteria that normally reside in the intestine. If enough are destroyed, Clostridium difficile may overgrow. These bacteria may already be present in the intestine. Or people may get them from other people, pets, or the environment. Being very young or very old, staying in a hospital or nursing home, or having one or more severe disorders increases the risk of this disorder. When the bacteria overgrow, they release two toxins. One causes the intestine to produce fluids and abnormal membranes to form. The other damages the lining of the large intestine.

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Other Bacterial Infections Source Symptoms Treatment Comments Brucellosis (Brucella bacteria) Domestic animals, deer, elk, and buffalo Unpasteurized contaminated milk and other dairy products Fever that may return repeatedly for months to years, night sweats, loss of appetite, weight loss, low back pain, bone and joint pain, and depression Doxycycline, given by mouth, plus streptomycin, injected daily Risk is increased for travelers who consume unpasteurized milk or cheese in areas where brucellosis is common and for laboratory workers, meat packers, veterinarians, farmers, and livestock producers, who may handle infected animal tissue. Sometimes infection develops in the back bones (vertebrae), long bones, joints, or heart valves. The bacteria can be spread through the air (in an aerosol) and thus could be used in biological warfare. Cat-scratch disease (Bartonella henselae) Domestic cats At the site of a cat scratch, a red blister that ruptures and forms a crust Swelling of nearby lymph nodes, which become tender and fill with pus, sometimes after the scratch has healed Occasionally, drainage of pus from the lymph nodes to the skin Application of heat, pain relievers, and sometimes azithromycin Most domestic cats throughout the world are infected, but most show no signs of illness. Cat-scratch disease usually resolves on its own. But if the immune system is weakened, as occurs in people with human immunodeficiency virus (HIV) infection or AIDS, infection can spread throughout the body and be fatal without treatment. Such people can avoid the infection by avoiding domestic cats. Erysipeloid (Erysipelothrix rhusiopathiae) Puncture wound or scrape that occurs while handling animal matter (such as infected carcasses or fish) At the site of injury, a purplish red, hard area, which may itch and burn Penicillin or erythromycin Risk is increased for butchers, farmers, cooks, and fishermen. Rarely, the bacteria spread through the bloodstream and infect the joints or heart valves. Gonorrhea (Neisseria gonorrhoeae) Sexual contact with infected people A discharge from the urethra or vagina and pain during urination A single injection of ceftriaxone into a muscle Occasionally, these bacteria spread through the bloodstream and infect the skin or joints. About half of affected people also have a chlamydial infection that must be treated simultaneously with azithromycin or doxycycline. Relapsing fever (Borrelia species) Body lice and soft-bodied ticks, often carried by rats Sudden chills followed by a high fever (fevers come and go at 1- to 2-week intervals) Severe headache, red eyes, a dry cough, vomiting, muscle and joint pain, a reddish rash on the trunk and limbs, jaundice, an enlarged liver and spleen, and an irregular heart rhythm Tetracycline, erythromycin, doxycycline, or penicillin In the United States, tick-borne infection usually occurs only in the western mountain states, and louse-borne infection is rare. Campers may be bitten by infected ticks. Complications can include a tendency to bleed, eye inflammation (iridocyclitis), and, in pregnant women, miscarriage. Within 2 hours after the first dose of the antibiotic, a dangerous reaction (Jarisch-Herxheimer reaction) may occur, causing sweating, shaking chills, fever, and a fall in blood pressure.

Last full review/revision September 2008 by Matthew E. Levison, MD