A List of Bad Bacteria. 

Bacteria are microscopic organisms that live in the air, soil and water, on surfaces, and in and on the human body. They are the smallest free-living organisms, meaning they support their own growth and reproduction. These organisms are only about two-thousandth of a millimeter in size. Most bacteria are harmless and, in fact, perform useful functions. Many of the bacteria in the body protect against the harmful effects of other organisms. When they enter internal tissues, however, bacteria can cause disease, discomfort and even death.


Clostridia live harmlessly in soil and the intestines of humans and animals. Some types can infect wounds or cause illness. Clostridium perfringens causes gas gangrene or tissue death. Clostridium difficile can cause antibiotic-induced diarrhea. According to the U.S. National Library of Medicine, Clostridium is associated with gas gangrene and colitis caused by antibiotics.


Streptococcus pyogenes causes strep throat and many other infections and is a major cause of pneumonia and meningitis. According to Merck Manual of Medical Information, Streptococcus and Clostridia can cause necrotizing skin infections, or infections that cause tissue death, though a combination of bacterial strains may also be to blame for these infections.


Staphylococci can cause skin infections, such as boils, pustules and abscesses and infections in bones, joints and wounds. Staphylococcus epidermidis normally lives harmlessly on the skin. If it enters artificial joints or heart valves during surgery, however, it can cause dangerous infections. According to the Cleveland Clinic, Staphylococcus aureus and less-frequently, Staphylococcus pyogenes, cause impetigo, a superficial, extremely contagious skin infection.

Listeria and Bacilli

If Listeria monocytogenes infects a pregnant woman, the fetus or newborn may be affected. According to the U.S. Food and Drug Administration, Listeria and Bacilli have infected people who have eaten contaminated cheese and cold cooked meats. Bacilli can be found in soil and water; animals and insects are most often the carriers. Bacillus cereus can cause food poisoning. Bacillus anthracis causes anthrax, which can cause open sores on the skin.


There are several species of Legionella bacteria. Legionella pneumophila causes Legionnaires’ disease, an often life-threatening form of pneumonia. According to the U.S. National Library of Medicine, these bacteria live and thrive in warm, moist environments such as large air conditioning systems.