Bacteria are microscopic, single-celled organisms. Thousands of different kinds of bacteria live throughout the world. Some live in the environment, and others live on the skin, in the airways, in the mouth, and in the digestive and genitourinary tracts of people and animals. Only a few kinds of bacteria cause disease.

Bacteria are classified in several ways. One way is by their distinctive shapes. Spherical bacteria are cocci, rod-like bacteria are bacilli, and spiral or helical bacteria are spirochetes.

Shapes of Bacteria

Another way bacteria are classified is by their color after a particular chemical stain (Gram stain) is applied. Some bacteria stain blue and are called gram-positive, whereas others stain pink and are called gram-negative. Gram-positive and gram-negative bacteria differ in the kinds of infections they produce and in the kinds of antibiotics that are likely to kill them.

Gram-negative bacteria have a unique outer membrane that prevents many drugs from penetrating them, making gram-negative bacteria generally more resistant to antibiotics than are gram-positive bacteria. The outer membrane of gram-negative bacteria is also rich in molecules called lipopolysaccharides. If gram-negative bacteria enter the bloodstream, their lipopolysaccharides can trigger high fever and a life-threatening drop in blood pressure (see Bacteremia, Sepsis, and Septic Shock: Introduction). For this reason, bacterial lipopolysaccharides are referred to as endotoxins.

Gram-negative bacteria have a great facility for exchanging genetic material (DNA) with other strains of the same species and even with different species. Thus, if gram-negative bacteria undergo a genetic change (mutation) that produces resistance to an antibiotic and then share DNA with another strain of bacteria, the second (recipient) strain becomes resistant as well.

Gram-positive bacteria are usually slow to develop resistance to antibiotics. Some gram-positive bacteria (for example, Bacillus anthracis and Clostridium botulinum) produce potent poisons (toxins) that cause serious illness.

A third way of classifying bacteria is by their use of oxygen. Most bacteria can live and grow in the presence of oxygen; these bacteria are called aerobes. Bacteria that can tolerate only low levels of oxygen, or are poisoned by oxygen, are called anaerobes. Anaerobes thrive in areas of the body that have low levels of oxygen—such as the intestine, decaying tissue, and wounds that are particularly deep and dirty.

Hundreds of species of anaerobes normally live harmlessly on the skin and mucous membranes (such as the lining of the mouth, intestine, and vagina); several hundred billion bacteria may exist in a cubic inch of stool. Most anaerobic infections arise from the body's own pool of bacteria.

Anaerobes tend to invade skin and muscle tissue that has been damaged by injury or surgery—particularly if the tissue has a poor blood supply. Spontaneous infections sometimes develop in people who have certain cancers or a weakened immune system. Also common are infections in the mouth. Anaerobes sometimes cause chronic (but not acute) infections of the sinuses and middle ear. Anaerobic infections tend to form collections of pus (abscesses). Severe anaerobic infections often release gas into the surrounding tissue.

Disease-causing anaerobes include clostridia (which live in the intestinal tract of humans and animals, as well as in dust, soil, and decaying vegetation) and Peptococci and Peptostreptococci—which are part of the normal bacterial population (flora) of the mouth, upper respiratory tract, and large intestine. Other anaerobes include Bacteroides, which is part of the normal flora of the large intestine, and Actinomyces, Prevotella, and Fusobacterium, which are part of the normal flora of the mouth.

What Are Clostridia? Clostridia are toxin-producing anaerobic bacteria that cause a number of serious diseases, including tetanus, botulism, and tissue infections. Clostridia normally inhabit the human intestinal tract, soil, and decaying vegetation. All species of clostridia produce toxins. Some clostridial diseases, such as botulism and the various diarrheal illnesses, result solely from the toxin without any bacterial invasion of tissue. In other clostridial diseases, such as tetanus and clostridial wound infections, there is both tissue invasion and toxin production. The most common toxin-only clostridial diseases are short-lived and relatively mild food poisonings caused by Clostridium perfringens. Sometimes, clostridial food poisoning is more severe, resulting in necrotizing enteritis, an inflammation that destroys the walls of the intestines, producing severe, bloody diarrhea. This infection can occur as an isolated case or in outbreaks caused by eating contaminated meat. People who have taken antibiotics for a long time may have colitis produced by an overgrowth of toxin-producing Clostridium difficile.(see Clostridium difficile-Induced Colitis) Botulism, a disease causing muscle paralysis and sometimes death, occurs from eating food contaminated with a toxin produced by Clostridium botulinum.(see Peripheral Nerve Disorders: Botulism) Clostridia, particularly Clostridium perfringens, also infect wounds. Clostridial wound infections, including skin gangrene, muscle gangrene (clostridial myonecrosis), and tetanus, are relatively rare but may be lethal. These infections are more likely in contaminated wounds, deep puncture wounds, and wounds in which much tissue is crushed. Injecting drug users are more susceptible. The risk of death is high, especially in older people and people with cancer.

Other Bacterial Infections Infection Cause and Source of Infection Symptoms and Treatment Comments Brucellosis Cause: Brucella Source: Domestic animals; buffalo; unpasteurized milk; contaminated dairy products Symptoms: Fever that may return repeatedly for months to years; abdominal pain; vomiting; diarrhea; bone and joint pain Treatment: Oral doxycycline combined with daily injections of streptomycin Meat packers, veterinarians, farmers, and livestock producers are at increased risk Cat-scratch disease Cause: Bartonella henselae Source: Domestic cats Symptoms: Red, crusted blisters at site of a cat scratch; swollen lymph nodes that fill with pus and may drain through the skin Treatment: Heat application; pain relievers; azithromycin may be given Most domestic cats throughout the world are infected (most show no signs of illness) Erysipelothricosis Cause: Erysipelothrix rhusiopathiae Source: Puncture wound that occurs while handling animal matter Symptoms: Purplish red, hard area on skin at site of injury; itching; burning; swelling Treatment: Single penicillin injection or a 1-week course of oral erythromycin; infection usually resolves without treatment Rarely infects joints or heart valves Neisserial infections Cause: Neisseria meningitidis Source: Neisseria meningitidis is part of the resident flora of people Symptoms: Symptoms of meningitis (headache, confusion, lethargy, coma, and death) Treatment: Ceftriaxone Vaccine is available for most types   Cause: Neisseria gonorrhoeae Source: Neisseria gonorrhoeae is sexually transmitted Symptoms: Urethral or vaginal discharge Treatment: Single dose of ceftriaxone or azithromycin   Nocardiosis Cause: Nocardia (usually Nocardia asteroides) Source: Nocardia inhabits decaying matter in soil; lung infection can result from inhalation of contaminated dust; skin infection can result from puncture wounds Symptoms: Cough; general weakness; chills; chest pain; shortness of breath; fever; lung abscesses; skin sores Treatment: Trimethoprim- sulfamethoxazole or imipenem plus amikacin for many months to a year People who are chronically ill or who are receiving drugs that suppress the immune system are at increased risk; infection spreads to the brain in one third of people and causes abscesses; infection is potentially fatal Rat-bite fever Cause: Streptobacillus moniliformis Source: Wild rats or mice; occasionally, dogs, cats, ferrets, weasels, or other carnivores that have fed on infected rodents; food contaminated by rodents Symptoms: Chills; fever that may recur for months; vomiting; headache; back and joint pain; rash on hands and feet; joint swelling Treatment: Penicillin or erythromycin Doctors often give antibiotics to people with rat bites to prevent disease; rat-bite fever caused by Streptobacillus moniliformisis common in the United States. A type of rat-bite fever caused by Spirillum minus is common in Asia. The symptoms are similar, except that the person also has inflammation at the site of the bite, swollen lymph nodes, fatigue, and a rash. Treatment for that type of rat-bite fever is with penicillin or erythromycin Relapsing fever Cause: Borrelia Source: Body lice; soft-bodied ticks Symptoms: Sudden chills followed by high fever (fevers come and go at 1 to 2 week intervals); severe headache; vomiting; muscle and joint pain; reddish rash over trunk, arms, and legs; jaundice; enlargement of liver and spleen; heart inflammation; heart failure Treatment: Tetracycline, erythromycin, or doxycycline In the United States, infection is generally confined to western states; complications can include eye inflammation, eruption of red rash all over the body (erythema multiforme), and miscarriage in pregnant women

Last full review/revision February 2003