Find information on medical topics, symptoms, drugs, procedures, news and more, written in everyday language.

Overview of Bacteria

by Matthew E. Levison, MD

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.


Bacteria can be classified in several ways:

  • 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.


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.


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.


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 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.

Other Bacterial Infections





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.

Resources In This Article

Drugs Mentioned In This Article

  • Generic Name
    Select Brand Names
  • No US brand name