Pathogenic bacteria are bacteria which are capable of causing disease. Humans are generally most interested in the species of bacteria which can cause disease in humans, although these bacteria can also infect other animals and plants. Some notable pathogenic bacteria include Streptococcus, Staphylococcus, Tuberculosis, and Escherichia coli, among many others. Worldwide, these bacteria account for many illnesses and disease epidemics.
Intracellular bacteria are pathogenic bacteria which always cause disease when they enter the human body, in contrast with conditional bacteria, which can cause infections and disease in certain circumstances. Many bacterial are conditional, taking advantage of happenstance like an open wound to duplicate themselves and spread disease. Opportunistic bacteria are bacteria which do not normally cause disease, but will if a patient has a compromised immune system.
Many humans actually host large numbers of bacteria at any given time. These bacteria are known as commensal or “good” bacteria, because they perform some vital and useful functions in the human body. Commensal bacteria in the gut, for example, help to break down and digest food.
Pathogenic bacteria can be spread through a human population in a range of ways. Air, water, and soil are all common vectors, and people may also pass bacteria directly to each other through physical contact. Some bacteria are very adept at colonizing locations like door knobs and medical equipment, allowing them to move from person to person with ease, while others are much less virulent, and will die off if they are away from a human host for too long.
Tuberculosis is one of the world's leading killers, making it a pathogenic bacterium of particular interest. Pathogenic bacteria are also responsible for intestinal problems such as chronic diarrhea, and they can cause infections in many parts of the body. Some are deadly, like the Legionella bacterium, while others are relatively benign, especially if treatment can be accessed. One of the biggest problems in the developing world is the proliferation of treatable bacterial infections which run unchecked through populations due to lack of access to medications and medical treatment.
Treatment of an infection with a pathogenic bacterium involves the use of antibiotics, drugs which have been specifically formulated to kill bacteria. Some bacteria have developed antibiotic resistance, which means that they may not respond to many common antibiotics. This forces drug companies to research new antibiotics so that they can stay ahead of bacterial mutations.